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<!-- doc/src/sgml/runtime.sgml -->
<chapter id="runtime">
<title>Server Setup and Operation</title>
<para>
This chapter discusses how to set up and run the database server
and its interactions with the operating system.
</para>
<sect1 id="postgres-user">
<title>The <productname>PostgreSQL</productname> User Account</title>
<indexterm>
<primary>postgres user</primary>
</indexterm>
<para>
As with any server daemon that is accessible to the outside world,
it is advisable to run <productname>PostgreSQL</productname> under a
separate user account. This user account should only own the data
that is managed by the server, and should not be shared with other
daemons. (For example, using the user <literal>nobody</literal> is a bad
idea.) It is not advisable to install executables owned by this
user because compromised systems could then modify their own
binaries.
</para>
<para>
To add a Unix user account to your system, look for a command
<command>useradd</command> or <command>adduser</command>. The user
name <systemitem>postgres</systemitem> is often used, and is assumed
throughout this book, but you can use another name if you like.
</para>
</sect1>
<sect1 id="creating-cluster">
<title>Creating a Database Cluster</title>
<indexterm>
<primary>database cluster</primary>
</indexterm>
<indexterm>
<primary>data area</primary>
<see>database cluster</see>
</indexterm>
<para>
Before you can do anything, you must initialize a database storage
area on disk. We call this a <firstterm>database cluster</firstterm>.
(The <acronym>SQL</acronym> standard uses the term catalog cluster.) A
database cluster is a collection of databases that is managed by a
single instance of a running database server. After initialization, a
database cluster will contain a database named <literal>postgres</literal>,
which is meant as a default database for use by utilities, users and third
party applications. The database server itself does not require the
<literal>postgres</literal> database to exist, but many external utility
programs assume it exists. Another database created within each cluster
during initialization is called
<literal>template1</literal>. As the name suggests, this will be used
as a template for subsequently created databases; it should not be
used for actual work. (See <xref linkend="managing-databases"/> for
information about creating new databases within a cluster.)
</para>
<para>
In file system terms, a database cluster is a single directory
under which all data will be stored. We call this the <firstterm>data
directory</firstterm> or <firstterm>data area</firstterm>. It is
completely up to you where you choose to store your data. There is no
default, although locations such as
<filename>/usr/local/pgsql/data</filename> or
<filename>/var/lib/pgsql/data</filename> are popular. To initialize a
database cluster, use the command <xref
linkend="app-initdb"/>,<indexterm><primary>initdb</primary></indexterm> which is
installed with <productname>PostgreSQL</productname>. The desired
file system location of your database cluster is indicated by the
<option>-D</option> option, for example:
<screen>
<prompt>$</prompt> <userinput>initdb -D /usr/local/pgsql/data</userinput>
</screen>
Note that you must execute this command while logged into the
<productname>PostgreSQL</productname> user account, which is
described in the previous section.
</para>
<tip>
<para>
As an alternative to the <option>-D</option> option, you can set
the environment variable <envar>PGDATA</envar>.
<indexterm><primary><envar>PGDATA</envar></primary></indexterm>
</para>
</tip>
<para>
Alternatively, you can run <command>initdb</command> via
the <xref linkend="app-pg-ctl"/>
program<indexterm><primary>pg_ctl</primary></indexterm> like so:
<screen>
<prompt>$</prompt> <userinput>pg_ctl -D /usr/local/pgsql/data initdb</userinput>
</screen>
This may be more intuitive if you are
using <command>pg_ctl</command> for starting and stopping the
server (see <xref linkend="server-start"/>), so
that <command>pg_ctl</command> would be the sole command you use
for managing the database server instance.
</para>
<para>
<command>initdb</command> will attempt to create the directory you
specify if it does not already exist. Of course, this will fail if
<command>initdb</command> does not have permissions to write in the
parent directory. It's generally recommendable that the
<productname>PostgreSQL</productname> user own not just the data
directory but its parent directory as well, so that this should not
be a problem. If the desired parent directory doesn't exist either,
you will need to create it first, using root privileges if the
grandparent directory isn't writable. So the process might look
like this:
<screen>
root# <userinput>mkdir /usr/local/pgsql</userinput>
root# <userinput>chown postgres /usr/local/pgsql</userinput>
root# <userinput>su postgres</userinput>
postgres$ <userinput>initdb -D /usr/local/pgsql/data</userinput>
</screen>
</para>
<para>
<command>initdb</command> will refuse to run if the data directory
exists and already contains files; this is to prevent accidentally
overwriting an existing installation.
</para>
<para>
Because the data directory contains all the data stored in the
database, it is essential that it be secured from unauthorized
access. <command>initdb</command> therefore revokes access
permissions from everyone but the
<productname>PostgreSQL</productname> user, and optionally, group.
Group access, when enabled, is read-only. This allows an unprivileged
user in the same group as the cluster owner to take a backup of the
cluster data or perform other operations that only require read access.
</para>
<para>
Note that enabling or disabling group access on an existing cluster requires
the cluster to be shut down and the appropriate mode to be set on all
directories and files before restarting
<productname>PostgreSQL</productname>. Otherwise, a mix of modes might
exist in the data directory. For clusters that allow access only by the
owner, the appropriate modes are <literal>0700</literal> for directories
and <literal>0600</literal> for files. For clusters that also allow
reads by the group, the appropriate modes are <literal>0750</literal>
for directories and <literal>0640</literal> for files.
</para>
<para>
However, while the directory contents are secure, the default
client authentication setup allows any local user to connect to the
database and even become the database superuser. If you do not
trust other local users, we recommend you use one of
<command>initdb</command>'s <option>-W</option>, <option>--pwprompt</option>
or <option>--pwfile</option> options to assign a password to the
database superuser.<indexterm>
<primary>password</primary>
<secondary>of the superuser</secondary>
</indexterm>
Also, specify <option>-A md5</option> or
<option>-A password</option> so that the default <literal>trust</literal> authentication
mode is not used; or modify the generated <filename>pg_hba.conf</filename>
file after running <command>initdb</command>, but
<emphasis>before</emphasis> you start the server for the first time. (Other
reasonable approaches include using <literal>peer</literal> authentication
or file system permissions to restrict connections. See <xref
linkend="client-authentication"/> for more information.)
</para>
<para>
<command>initdb</command> also initializes the default
locale<indexterm><primary>locale</primary></indexterm> for the database cluster.
Normally, it will just take the locale settings in the environment
and apply them to the initialized database. It is possible to
specify a different locale for the database; more information about
that can be found in <xref linkend="locale"/>. The default sort order used
within the particular database cluster is set by
<command>initdb</command>, and while you can create new databases using
different sort order, the order used in the template databases that initdb
creates cannot be changed without dropping and recreating them.
There is also a performance impact for using locales
other than <literal>C</literal> or <literal>POSIX</literal>. Therefore, it is
important to make this choice correctly the first time.
</para>
<para>
<command>initdb</command> also sets the default character set encoding
for the database cluster. Normally this should be chosen to match the
locale setting. For details see <xref linkend="multibyte"/>.
</para>
<para>
Non-<literal>C</literal> and non-<literal>POSIX</literal> locales rely on the
operating system's collation library for character set ordering.
This controls the ordering of keys stored in indexes. For this reason,
a cluster cannot switch to an incompatible collation library version,
either through snapshot restore, binary streaming replication, a
different operating system, or an operating system upgrade.
</para>
<sect2 id="creating-cluster-mount-points">
<title>Use of Secondary File Systems</title>
<indexterm zone="creating-cluster-mount-points">
<primary>file system mount points</primary>
</indexterm>
<para>
Many installations create their database clusters on file systems
(volumes) other than the machine's <quote>root</quote> volume. If you
choose to do this, it is not advisable to try to use the secondary
volume's topmost directory (mount point) as the data directory.
Best practice is to create a directory within the mount-point
directory that is owned by the <productname>PostgreSQL</productname>
user, and then create the data directory within that. This avoids
permissions problems, particularly for operations such
as <application>pg_upgrade</application>, and it also ensures clean failures if
the secondary volume is taken offline.
</para>
</sect2>
<sect2 id="creating-cluster-nfs">
<title>Use of Network File Systems</title>
<indexterm zone="creating-cluster-nfs">
<primary>Network File Systems</primary>
</indexterm>
<indexterm><primary><acronym>NFS</acronym></primary><see>Network File Systems</see></indexterm>
<indexterm><primary>Network Attached Storage (<acronym>NAS</acronym>)</primary><see>Network File Systems</see></indexterm>
<para>
Many installations create their database clusters on network file
systems. Sometimes this is done via <acronym>NFS</acronym>, or by using a
Network Attached Storage (<acronym>NAS</acronym>) device that uses
<acronym>NFS</acronym> internally. <productname>PostgreSQL</productname> does nothing
special for <acronym>NFS</acronym> file systems, meaning it assumes
<acronym>NFS</acronym> behaves exactly like locally-connected drives.
If the client or server <acronym>NFS</acronym> implementation does not
provide standard file system semantics, this can
cause reliability problems (see <ulink
url="https://www.time-travellers.org/shane/papers/NFS_considered_harmful.html"></ulink>).
Specifically, delayed (asynchronous) writes to the <acronym>NFS</acronym>
server can cause data corruption problems. If possible, mount the
<acronym>NFS</acronym> file system synchronously (without caching) to avoid
this hazard. Also, soft-mounting the <acronym>NFS</acronym> file system is
not recommended.
</para>
<para>
Storage Area Networks (<acronym>SAN</acronym>) typically use communication
protocols other than <acronym>NFS</acronym>, and may or may not be subject
to hazards of this sort. It's advisable to consult the vendor's
documentation concerning data consistency guarantees.
<productname>PostgreSQL</productname> cannot be more reliable than
the file system it's using.
</para>
</sect2>
</sect1>
<sect1 id="server-start">
<title>Starting the Database Server</title>
<para>
Before anyone can access the database, you must start the database
server. The database server program is called
<command>postgres</command>.<indexterm><primary>postgres</primary></indexterm>
The <command>postgres</command> program must know where to
find the data it is supposed to use. This is done with the
<option>-D</option> option. Thus, the simplest way to start the
server is:
<screen>
$ <userinput>postgres -D /usr/local/pgsql/data</userinput>
</screen>
which will leave the server running in the foreground. This must be
done while logged into the <productname>PostgreSQL</productname> user
account. Without <option>-D</option>, the server will try to use
the data directory named by the environment variable <envar>PGDATA</envar>.
If that variable is not provided either, it will fail.
</para>
<para>
Normally it is better to start <command>postgres</command> in the
background. For this, use the usual Unix shell syntax:
<screen>
$ <userinput>postgres -D /usr/local/pgsql/data &gt;logfile 2&gt;&amp;1 &amp;</userinput>
</screen>
It is important to store the server's <systemitem>stdout</systemitem> and
<systemitem>stderr</systemitem> output somewhere, as shown above. It will help
for auditing purposes and to diagnose problems. (See <xref
linkend="logfile-maintenance"/> for a more thorough discussion of log
file handling.)
</para>
<para>
The <command>postgres</command> program also takes a number of other
command-line options. For more information, see the
<xref linkend="app-postgres"/> reference page
and <xref linkend="runtime-config"/> below.
</para>
<para>
This shell syntax can get tedious quickly. Therefore the wrapper
program
<xref linkend="app-pg-ctl"/><indexterm><primary>pg_ctl</primary></indexterm>
is provided to simplify some tasks. For example:
<programlisting>
pg_ctl start -l logfile
</programlisting>
will start the server in the background and put the output into the
named log file. The <option>-D</option> option has the same meaning
here as for <command>postgres</command>. <command>pg_ctl</command>
is also capable of stopping the server.
</para>
<para>
Normally, you will want to start the database server when the
computer boots.<indexterm>
<primary>booting</primary>
<secondary>starting the server during</secondary>
</indexterm>
Autostart scripts are operating-system-specific.
There are a few distributed with
<productname>PostgreSQL</productname> in the
<filename>contrib/start-scripts</filename> directory. Installing one will require
root privileges.
</para>
<para>
Different systems have different conventions for starting up daemons
at boot time. Many systems have a file
<filename>/etc/rc.local</filename> or
<filename>/etc/rc.d/rc.local</filename>. Others use <filename>init.d</filename> or
<filename>rc.d</filename> directories. Whatever you do, the server must be
run by the <productname>PostgreSQL</productname> user account
<emphasis>and not by root</emphasis> or any other user. Therefore you
probably should form your commands using
<literal>su postgres -c '...'</literal>. For example:
<programlisting>
su postgres -c 'pg_ctl start -D /usr/local/pgsql/data -l serverlog'
</programlisting>
</para>
<para>
Here are a few more operating-system-specific suggestions. (In each
case be sure to use the proper installation directory and user
name where we show generic values.)
<itemizedlist>
<listitem>
<para>
For <productname>FreeBSD</productname>, look at the file
<filename>contrib/start-scripts/freebsd</filename> in the
<productname>PostgreSQL</productname> source distribution.
<indexterm><primary>FreeBSD</primary><secondary>start script</secondary></indexterm>
</para>
</listitem>
<listitem>
<para>
On <productname>OpenBSD</productname>, add the following lines
to the file <filename>/etc/rc.local</filename>:
<indexterm><primary>OpenBSD</primary><secondary>start script</secondary></indexterm>
<programlisting>
if [ -x /usr/local/pgsql/bin/pg_ctl -a -x /usr/local/pgsql/bin/postgres ]; then
su -l postgres -c '/usr/local/pgsql/bin/pg_ctl start -s -l /var/postgresql/log -D /usr/local/pgsql/data'
echo -n ' postgresql'
fi
</programlisting>
</para>
</listitem>
<listitem>
<para>
On <productname>Linux</productname> systems either add
<indexterm><primary>Linux</primary><secondary>start script</secondary></indexterm>
<programlisting>
/usr/local/pgsql/bin/pg_ctl start -l logfile -D /usr/local/pgsql/data
</programlisting>
to <filename>/etc/rc.d/rc.local</filename>
or <filename>/etc/rc.local</filename> or look at the file
<filename>contrib/start-scripts/linux</filename> in the
<productname>PostgreSQL</productname> source distribution.
</para>
<para>
When using <application>systemd</application>, you can use the following
service unit file (e.g.,
at <filename>/etc/systemd/system/postgresql.service</filename>):<indexterm><primary>systemd</primary></indexterm>
<programlisting>
[Unit]
Description=PostgreSQL database server
Documentation=man:postgres(1)
[Service]
Type=notify
User=postgres
ExecStart=/usr/local/pgsql/bin/postgres -D /usr/local/pgsql/data
ExecReload=/bin/kill -HUP $MAINPID
KillMode=mixed
KillSignal=SIGINT
TimeoutSec=0
[Install]
WantedBy=multi-user.target
</programlisting>
Using <literal>Type=notify</literal> requires that the server binary was
built with <literal>configure --with-systemd</literal>.
</para>
<para>
Consider carefully the timeout
setting. <application>systemd</application> has a default timeout of 90
seconds as of this writing and will kill a process that does not notify
readiness within that time. But a <productname>PostgreSQL</productname>
server that might have to perform crash recovery at startup could take
much longer to become ready. The suggested value of 0 disables the
timeout logic.
</para>
</listitem>
<listitem>
<para>
On <productname>NetBSD</productname>, use either the
<productname>FreeBSD</productname> or
<productname>Linux</productname> start scripts, depending on
preference.
<indexterm><primary>NetBSD</primary><secondary>start script</secondary></indexterm>
</para>
</listitem>
<listitem>
<para>
On <productname>Solaris</productname>, create a file called
<filename>/etc/init.d/postgresql</filename> that contains
the following line:
<indexterm><primary>Solaris</primary><secondary>start script</secondary></indexterm>
<programlisting>
su - postgres -c "/usr/local/pgsql/bin/pg_ctl start -l logfile -D /usr/local/pgsql/data"
</programlisting>
Then, create a symbolic link to it in <filename>/etc/rc3.d</filename> as
<filename>S99postgresql</filename>.
</para>
</listitem>
</itemizedlist>
</para>
<para>
While the server is running, its
<acronym>PID</acronym> is stored in the file
<filename>postmaster.pid</filename> in the data directory. This is
used to prevent multiple server instances from
running in the same data directory and can also be used for
shutting down the server.
</para>
<sect2 id="server-start-failures">
<title>Server Start-up Failures</title>
<para>
There are several common reasons the server might fail to
start. Check the server's log file, or start it by hand (without
redirecting standard output or standard error) and see what error
messages appear. Below we explain some of the most common error
messages in more detail.
</para>
<para>
<screen>
LOG: could not bind IPv4 address "127.0.0.1": Address already in use
HINT: Is another postmaster already running on port 5432? If not, wait a few seconds and retry.
FATAL: could not create any TCP/IP sockets
</screen>
This usually means just what it suggests: you tried to start
another server on the same port where one is already running.
However, if the kernel error message is not <computeroutput>Address
already in use</computeroutput> or some variant of that, there might
be a different problem. For example, trying to start a server
on a reserved port number might draw something like:
<screen>
$ <userinput>postgres -p 666</userinput>
LOG: could not bind IPv4 address "127.0.0.1": Permission denied
HINT: Is another postmaster already running on port 666? If not, wait a few seconds and retry.
FATAL: could not create any TCP/IP sockets
</screen>
</para>
<para>
A message like:
<screen>
FATAL: could not create shared memory segment: Invalid argument
DETAIL: Failed system call was shmget(key=5440001, size=4011376640, 03600).
</screen>
probably means your kernel's limit on the size of shared memory is
smaller than the work area <productname>PostgreSQL</productname>
is trying to create (4011376640 bytes in this example). Or it could
mean that you do not have System-V-style shared memory support
configured into your kernel at all. As a temporary workaround, you
can try starting the server with a smaller-than-normal number of
buffers (<xref linkend="guc-shared-buffers"/>). You will eventually want
to reconfigure your kernel to increase the allowed shared memory
size. You might also see this message when trying to start multiple
servers on the same machine, if their total space requested
exceeds the kernel limit.
</para>
<para>
An error like:
<screen>
FATAL: could not create semaphores: No space left on device
DETAIL: Failed system call was semget(5440126, 17, 03600).
</screen>
does <emphasis>not</emphasis> mean you've run out of disk
space. It means your kernel's limit on the number of <systemitem
class="osname">System V</systemitem> semaphores is smaller than the number
<productname>PostgreSQL</productname> wants to create. As above,
you might be able to work around the problem by starting the
server with a reduced number of allowed connections
(<xref linkend="guc-max-connections"/>), but you'll eventually want to
increase the kernel limit.
</para>
<para>
If you get an <quote>illegal system call</quote> error, it is likely that
shared memory or semaphores are not supported in your kernel at
all. In that case your only option is to reconfigure the kernel to
enable these features.
</para>
<para>
Details about configuring <systemitem class="osname">System V</systemitem>
<acronym>IPC</acronym> facilities are given in <xref linkend="sysvipc"/>.
</para>
</sect2>
<sect2 id="client-connection-problems">
<title>Client Connection Problems</title>
<para>
Although the error conditions possible on the client side are quite
varied and application-dependent, a few of them might be directly
related to how the server was started. Conditions other than
those shown below should be documented with the respective client
application.
</para>
<para>
<screen>
psql: could not connect to server: Connection refused
Is the server running on host "server.joe.com" and accepting
TCP/IP connections on port 5432?
</screen>
This is the generic <quote>I couldn't find a server to talk
to</quote> failure. It looks like the above when TCP/IP
communication is attempted. A common mistake is to forget to
configure the server to allow TCP/IP connections.
</para>
<para>
Alternatively, you'll get this when attempting Unix-domain socket
communication to a local server:
<screen>
psql: could not connect to server: No such file or directory
Is the server running locally and accepting
connections on Unix domain socket "/tmp/.s.PGSQL.5432"?
</screen>
</para>
<para>
The last line is useful in verifying that the client is trying to
connect to the right place. If there is in fact no server
running there, the kernel error message will typically be either
<computeroutput>Connection refused</computeroutput> or
<computeroutput>No such file or directory</computeroutput>, as
illustrated. (It is important to realize that
<computeroutput>Connection refused</computeroutput> in this context
does <emphasis>not</emphasis> mean that the server got your
connection request and rejected it. That case will produce a
different message, as shown in <xref
linkend="client-authentication-problems"/>.) Other error messages
such as <computeroutput>Connection timed out</computeroutput> might
indicate more fundamental problems, like lack of network
connectivity.
</para>
</sect2>
</sect1>
<sect1 id="kernel-resources">
<title>Managing Kernel Resources</title>
<para>
<productname>PostgreSQL</productname> can sometimes exhaust various operating system
resource limits, especially when multiple copies of the server are running
on the same system, or in very large installations. This section explains
the kernel resources used by <productname>PostgreSQL</productname> and the steps you
can take to resolve problems related to kernel resource consumption.
</para>
<sect2 id="sysvipc">
<title>Shared Memory and Semaphores</title>
<indexterm zone="sysvipc">
<primary>shared memory</primary>
</indexterm>
<indexterm zone="sysvipc">
<primary>semaphores</primary>
</indexterm>
<para>
<productname>PostgreSQL</productname> requires the operating system to provide
inter-process communication (<acronym>IPC</acronym>) features, specifically
shared memory and semaphores. Unix-derived systems typically provide
<quote><systemitem class="osname">System V</systemitem></quote> <acronym>IPC</acronym>,
<quote><systemitem class="osname">POSIX</systemitem></quote> <acronym>IPC</acronym>, or both.
<systemitem class="osname">Windows</systemitem> has its own implementation of
these features and is not discussed here.
</para>
<para>
The complete lack of these facilities is usually manifested by an
<quote><errorname>Illegal system call</errorname></quote> error upon server
start. In that case there is no alternative but to reconfigure your
kernel. <productname>PostgreSQL</productname> won't work without them.
This situation is rare, however, among modern operating systems.
</para>
<para>
Upon starting the server, <productname>PostgreSQL</productname> normally allocates
a very small amount of System V shared memory, as well as a much larger
amount of POSIX (<function>mmap</function>) shared memory.
In addition a significant number of semaphores, which can be either
System V or POSIX style, are created at server startup. Currently,
POSIX semaphores are used on Linux and FreeBSD systems while other
platforms use System V semaphores.
</para>
<note>
<para>
Prior to <productname>PostgreSQL</productname> 9.3, only System V shared memory
was used, so the amount of System V shared memory required to start the
server was much larger. If you are running an older version of the
server, please consult the documentation for your server version.
</para>
</note>
<para>
System V <acronym>IPC</acronym> features are typically constrained by
system-wide allocation limits.
When <productname>PostgreSQL</productname> exceeds one of these limits,
the server will refuse to start and
should leave an instructive error message describing the problem
and what to do about it. (See also <xref
linkend="server-start-failures"/>.) The relevant kernel
parameters are named consistently across different systems; <xref
linkend="sysvipc-parameters"/> gives an overview. The methods to set
them, however, vary. Suggestions for some platforms are given below.
</para>
<table id="sysvipc-parameters">
<title><systemitem class="osname">System V</systemitem> <acronym>IPC</acronym> Parameters</title>
<tgroup cols="3">
<thead>
<row>
<entry>Name</entry>
<entry>Description</entry>
<entry>Values needed to run one <productname>PostgreSQL</productname> instance</entry>
</row>
</thead>
<tbody>
<row>
<entry><varname>SHMMAX</varname></entry>
<entry>Maximum size of shared memory segment (bytes)</entry>
<entry>at least 1kB, but the default is usually much higher</entry>
</row>
<row>
<entry><varname>SHMMIN</varname></entry>
<entry>Minimum size of shared memory segment (bytes)</entry>
<entry>1</entry>
</row>
<row>
<entry><varname>SHMALL</varname></entry>
<entry>Total amount of shared memory available (bytes or pages)</entry>
<entry>same as <varname>SHMMAX</varname> if bytes,
or <literal>ceil(SHMMAX/PAGE_SIZE)</literal> if pages,
plus room for other applications</entry>
</row>
<row>
<entry><varname>SHMSEG</varname></entry>
<entry>Maximum number of shared memory segments per process</entry>
<entry>only 1 segment is needed, but the default is much higher</entry>
</row>
<row>
<entry><varname>SHMMNI</varname></entry>
<entry>Maximum number of shared memory segments system-wide</entry>
<entry>like <varname>SHMSEG</varname> plus room for other applications</entry>
</row>
<row>
<entry><varname>SEMMNI</varname></entry>
<entry>Maximum number of semaphore identifiers (i.e., sets)</entry>
<entry>at least <literal>ceil((max_connections + autovacuum_max_workers + max_worker_processes + 5) / 16)</literal> plus room for other applications</entry>
</row>
<row>
<entry><varname>SEMMNS</varname></entry>
<entry>Maximum number of semaphores system-wide</entry>
<entry><literal>ceil((max_connections + autovacuum_max_workers + max_worker_processes + 5) / 16) * 17</literal> plus room for other applications</entry>
</row>
<row>
<entry><varname>SEMMSL</varname></entry>
<entry>Maximum number of semaphores per set</entry>
<entry>at least 17</entry>
</row>
<row>
<entry><varname>SEMMAP</varname></entry>
<entry>Number of entries in semaphore map</entry>
<entry>see text</entry>
</row>
<row>
<entry><varname>SEMVMX</varname></entry>
<entry>Maximum value of semaphore</entry>
<entry>at least 1000 (The default is often 32767; do not change unless necessary)</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
<productname>PostgreSQL</productname> requires a few bytes of System V shared memory
(typically 48 bytes, on 64-bit platforms) for each copy of the server.
On most modern operating systems, this amount can easily be allocated.
However, if you are running many copies of the server, or if other
applications are also using System V shared memory, it may be necessary to
increase <varname>SHMALL</varname>, which is the total amount of System V shared
memory system-wide. Note that <varname>SHMALL</varname> is measured in pages
rather than bytes on many systems.
</para>
<para>
Less likely to cause problems is the minimum size for shared
memory segments (<varname>SHMMIN</varname>), which should be at most
approximately 32 bytes for <productname>PostgreSQL</productname> (it is
usually just 1). The maximum number of segments system-wide
(<varname>SHMMNI</varname>) or per-process (<varname>SHMSEG</varname>) are unlikely
to cause a problem unless your system has them set to zero.
</para>
<para>
When using System V semaphores,
<productname>PostgreSQL</productname> uses one semaphore per allowed connection
(<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
(<xref linkend="guc-autovacuum-max-workers"/>) and allowed background
process (<xref linkend="guc-max-worker-processes"/>), in sets of 16.
Each such set will
also contain a 17th semaphore which contains a <quote>magic
number</quote>, to detect collision with semaphore sets used by
other applications. The maximum number of semaphores in the system
is set by <varname>SEMMNS</varname>, which consequently must be at least
as high as <varname>max_connections</varname> plus
<varname>autovacuum_max_workers</varname> plus <varname>max_worker_processes</varname>,
plus one extra for each 16
allowed connections plus workers (see the formula in <xref
linkend="sysvipc-parameters"/>). The parameter <varname>SEMMNI</varname>
determines the limit on the number of semaphore sets that can
exist on the system at one time. Hence this parameter must be at
least <literal>ceil((max_connections + autovacuum_max_workers + max_worker_processes + 5) / 16)</literal>.
Lowering the number
of allowed connections is a temporary workaround for failures,
which are usually confusingly worded <quote>No space
left on device</quote>, from the function <function>semget</function>.
</para>
<para>
In some cases it might also be necessary to increase
<varname>SEMMAP</varname> to be at least on the order of
<varname>SEMMNS</varname>. This parameter defines the size of the semaphore
resource map, in which each contiguous block of available semaphores
needs an entry. When a semaphore set is freed it is either added to
an existing entry that is adjacent to the freed block or it is
registered under a new map entry. If the map is full, the freed
semaphores get lost (until reboot). Fragmentation of the semaphore
space could over time lead to fewer available semaphores than there
should be.
</para>
<para>
Various other settings related to <quote>semaphore undo</quote>, such as
<varname>SEMMNU</varname> and <varname>SEMUME</varname>, do not affect
<productname>PostgreSQL</productname>.
</para>
<para>
When using POSIX semaphores, the number of semaphores needed is the
same as for System V, that is one semaphore per allowed connection
(<xref linkend="guc-max-connections"/>), allowed autovacuum worker process
(<xref linkend="guc-autovacuum-max-workers"/>) and allowed background
process (<xref linkend="guc-max-worker-processes"/>).
On the platforms where this option is preferred, there is no specific
kernel limit on the number of POSIX semaphores.
</para>
<variablelist>
<varlistentry>
<term><systemitem class="osname">AIX</systemitem>
<indexterm><primary>AIX</primary><secondary>IPC configuration</secondary></indexterm>
</term>
<listitem>
<para>
At least as of version 5.1, it should not be necessary to do
any special configuration for such parameters as
<varname>SHMMAX</varname>, as it appears this is configured to
allow all memory to be used as shared memory. That is the
sort of configuration commonly used for other databases such
as <application>DB/2</application>.</para>
<para> It might, however, be necessary to modify the global
<command>ulimit</command> information in
<filename>/etc/security/limits</filename>, as the default hard
limits for file sizes (<varname>fsize</varname>) and numbers of
files (<varname>nofiles</varname>) might be too low.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><systemitem class="osname">FreeBSD</systemitem>
<indexterm><primary>FreeBSD</primary><secondary>IPC configuration</secondary></indexterm>
</term>
<listitem>
<para>
The default settings can be changed using
the <command>sysctl</command> or
<command>loader</command> interfaces. The following
parameters can be set using <command>sysctl</command>:
<screen>
<prompt>#</prompt> <userinput>sysctl kern.ipc.shmall=32768</userinput>
<prompt>#</prompt> <userinput>sysctl kern.ipc.shmmax=134217728</userinput>
</screen>
To make these settings persist over reboots, modify
<filename>/etc/sysctl.conf</filename>.
</para>
<para>
These semaphore-related settings are read-only as far as
<command>sysctl</command> is concerned, but can be set in
<filename>/boot/loader.conf</filename>:
<programlisting>
kern.ipc.semmni=256
kern.ipc.semmns=512
kern.ipc.semmnu=256
</programlisting>
After modifying these values a reboot is required for the new
settings to take effect.
(Note: FreeBSD does not use <varname>SEMMAP</varname>. Older versions
would accept but ignore a setting for <literal>kern.ipc.semmap</literal>;
newer versions reject it altogether.)
</para>
<para>
You might also want to configure your kernel to lock shared
memory into RAM and prevent it from being paged out to swap.
This can be accomplished using the <command>sysctl</command>
setting <literal>kern.ipc.shm_use_phys</literal>.
</para>
<para>
If running in FreeBSD jails by enabling <application>sysctl</application>'s
<literal>security.jail.sysvipc_allowed</literal>, <application>postmaster</application>s
running in different jails should be run by different operating system
users. This improves security because it prevents non-root users
from interfering with shared memory or semaphores in different jails,
and it allows the PostgreSQL IPC cleanup code to function properly.
(In FreeBSD 6.0 and later the IPC cleanup code does not properly detect
processes in other jails, preventing the running of postmasters on the
same port in different jails.)
</para>
<para>
<systemitem class="osname">FreeBSD</systemitem> versions before 4.0 work like
<systemitem class="osname">OpenBSD</systemitem> (see below).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><systemitem class="osname">NetBSD</systemitem>
<indexterm><primary>NetBSD</primary><secondary>IPC configuration</secondary></indexterm>
</term>
<listitem>
<para>
In <systemitem class="osname">NetBSD</systemitem> 5.0 and later,
IPC parameters can be adjusted using <command>sysctl</command>,
for example:
<screen>
<prompt>$</prompt> <userinput>sysctl -w kern.ipc.shmmax=16777216</userinput>
</screen>
To have these settings persist over reboots, modify
<filename>/etc/sysctl.conf</filename>.
</para>
<para>
You might also want to configure your kernel to lock shared
memory into RAM and prevent it from being paged out to swap.
This can be accomplished using the <command>sysctl</command>
setting <literal>kern.ipc.shm_use_phys</literal>.
</para>
<para>
<systemitem class="osname">NetBSD</systemitem> versions before 5.0 work like
<systemitem class="osname">OpenBSD</systemitem> (see below), except that
parameters should be set with the keyword <literal>options</literal> not
<literal>option</literal>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><systemitem class="osname">OpenBSD</systemitem>
<indexterm><primary>OpenBSD</primary><secondary>IPC configuration</secondary></indexterm>
</term>
<listitem>
<para>
The options <varname>SYSVSHM</varname> and <varname>SYSVSEM</varname> need
to be enabled when the kernel is compiled. (They are by
default.) The maximum size of shared memory is determined by
the option <varname>SHMMAXPGS</varname> (in pages). The following
shows an example of how to set the various parameters:
<programlisting>
option SYSVSHM
option SHMMAXPGS=4096
option SHMSEG=256
option SYSVSEM
option SEMMNI=256
option SEMMNS=512
option SEMMNU=256
</programlisting>
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><systemitem class="osname">HP-UX</systemitem>
<indexterm><primary>HP-UX</primary><secondary>IPC configuration</secondary></indexterm>
</term>
<listitem>
<para>
The default settings tend to suffice for normal installations.
On <productname>HP-UX</productname> 10, the factory default for
<varname>SEMMNS</varname> is 128, which might be too low for larger
database sites.
</para>
<para>
<acronym>IPC</acronym> parameters can be set in the <application>System
Administration Manager</application> (<acronym>SAM</acronym>) under
<menuchoice><guimenu>Kernel
Configuration</guimenu><guimenuitem>Configurable Parameters</guimenuitem></menuchoice>. Choose
<guibutton>Create A New Kernel</guibutton> when you're done.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><systemitem class="osname">Linux</systemitem>
<indexterm><primary>Linux</primary><secondary>IPC configuration</secondary></indexterm>
</term>
<listitem>
<para>
The default maximum segment size is 32 MB, and the
default maximum total size is 2097152
pages. A page is almost always 4096 bytes except in unusual
kernel configurations with <quote>huge pages</quote>
(use <literal>getconf PAGE_SIZE</literal> to verify).
</para>
<para>
The shared memory size settings can be changed via the
<command>sysctl</command> interface. For example, to allow 16 GB:
<screen>
<prompt>$</prompt> <userinput>sysctl -w kernel.shmmax=17179869184</userinput>
<prompt>$</prompt> <userinput>sysctl -w kernel.shmall=4194304</userinput>
</screen>
In addition these settings can be preserved between reboots in
the file <filename>/etc/sysctl.conf</filename>. Doing that is
highly recommended.
</para>
<para>
Ancient distributions might not have the <command>sysctl</command> program,
but equivalent changes can be made by manipulating the
<filename>/proc</filename> file system:
<screen>
<prompt>$</prompt> <userinput>echo 17179869184 &gt;/proc/sys/kernel/shmmax</userinput>
<prompt>$</prompt> <userinput>echo 4194304 &gt;/proc/sys/kernel/shmall</userinput>
</screen>
</para>
<para>
The remaining defaults are quite generously sized, and usually
do not require changes.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><systemitem class="osname">macOS</systemitem>
<indexterm><primary>macOS</primary><secondary>IPC configuration</secondary></indexterm>
</term>
<listitem>
<para>
The recommended method for configuring shared memory in macOS
is to create a file named <filename>/etc/sysctl.conf</filename>,
containing variable assignments such as:
<programlisting>
kern.sysv.shmmax=4194304
kern.sysv.shmmin=1
kern.sysv.shmmni=32
kern.sysv.shmseg=8
kern.sysv.shmall=1024
</programlisting>
Note that in some macOS versions,
<emphasis>all five</emphasis> shared-memory parameters must be set in
<filename>/etc/sysctl.conf</filename>, else the values will be ignored.
</para>
<para>
Beware that recent releases of macOS ignore attempts to set
<varname>SHMMAX</varname> to a value that isn't an exact multiple of 4096.
</para>
<para>
<varname>SHMALL</varname> is measured in 4 kB pages on this platform.
</para>
<para>
In older macOS versions, you will need to reboot to have changes in the
shared memory parameters take effect. As of 10.5 it is possible to
change all but <varname>SHMMNI</varname> on the fly, using
<application>sysctl</application>. But it's still best to set up your preferred
values via <filename>/etc/sysctl.conf</filename>, so that the values will be
kept across reboots.
</para>
<para>
The file <filename>/etc/sysctl.conf</filename> is only honored in macOS
10.3.9 and later. If you are running a previous 10.3.x release,
you must edit the file <filename>/etc/rc</filename>
and change the values in the following commands:
<programlisting>
sysctl -w kern.sysv.shmmax
sysctl -w kern.sysv.shmmin
sysctl -w kern.sysv.shmmni
sysctl -w kern.sysv.shmseg
sysctl -w kern.sysv.shmall
</programlisting>
Note that
<filename>/etc/rc</filename> is usually overwritten by macOS system updates,
so you should expect to have to redo these edits after each update.
</para>
<para>
In macOS 10.2 and earlier, instead edit these commands in the file
<filename>/System/Library/StartupItems/SystemTuning/SystemTuning</filename>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><systemitem class="osname">Solaris</systemitem> 2.6 to 2.9 (Solaris
6 to Solaris 9)
<indexterm><primary>Solaris</primary><secondary>IPC configuration</secondary></indexterm>
</term>
<listitem>
<para>
The relevant settings can be changed in
<filename>/etc/system</filename>, for example:
<programlisting>
set shmsys:shminfo_shmmax=0x2000000
set shmsys:shminfo_shmmin=1
set shmsys:shminfo_shmmni=256
set shmsys:shminfo_shmseg=256
set semsys:seminfo_semmap=256
set semsys:seminfo_semmni=512
set semsys:seminfo_semmns=512
set semsys:seminfo_semmsl=32
</programlisting>
You need to reboot for the changes to take effect. See also
<ulink url="http://sunsite.uakom.sk/sunworldonline/swol-09-1997/swol-09-insidesolaris.html"></ulink>
for information on shared memory under older versions of Solaris.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><systemitem class="osname">Solaris</systemitem> 2.10 (Solaris
10) and later</term>
<term><systemitem class="osname">OpenSolaris</systemitem></term>
<listitem>
<para>
In Solaris 10 and later, and OpenSolaris, the default shared memory and
semaphore settings are good enough for most
<productname>PostgreSQL</productname> applications. Solaris now defaults
to a <varname>SHMMAX</varname> of one-quarter of system <acronym>RAM</acronym>.
To further adjust this setting, use a project setting associated
with the <literal>postgres</literal> user. For example, run the
following as <literal>root</literal>:
<programlisting>
projadd -c "PostgreSQL DB User" -K "project.max-shm-memory=(privileged,8GB,deny)" -U postgres -G postgres user.postgres
</programlisting>
</para>
<para>
This command adds the <literal>user.postgres</literal> project and
sets the shared memory maximum for the <literal>postgres</literal>
user to 8GB, and takes effect the next time that user logs
in, or when you restart <productname>PostgreSQL</productname> (not reload).
The above assumes that <productname>PostgreSQL</productname> is run by
the <literal>postgres</literal> user in the <literal>postgres</literal>
group. No server reboot is required.
</para>
<para>
Other recommended kernel setting changes for database servers which will
have a large number of connections are:
<programlisting>
project.max-shm-ids=(priv,32768,deny)
project.max-sem-ids=(priv,4096,deny)
project.max-msg-ids=(priv,4096,deny)
</programlisting>
</para>
<para>
Additionally, if you are running <productname>PostgreSQL</productname>
inside a zone, you may need to raise the zone resource usage
limits as well. See "Chapter2: Projects and Tasks" in the
<citetitle>System Administrator's Guide</citetitle> for more
information on <literal>projects</literal> and <command>prctl</command>.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="systemd-removeipc">
<title>systemd RemoveIPC</title>
<indexterm>
<primary>systemd</primary>
<secondary>RemoveIPC</secondary>
</indexterm>
<para>
If <productname>systemd</productname> is in use, some care must be taken
that IPC resources (shared memory and semaphores) are not prematurely
removed by the operating system. This is especially of concern when
installing PostgreSQL from source. Users of distribution packages of
PostgreSQL are less likely to be affected, as
the <literal>postgres</literal> user is then normally created as a system
user.
</para>
<para>
The setting <literal>RemoveIPC</literal>
in <filename>logind.conf</filename> controls whether IPC objects are
removed when a user fully logs out. System users are exempt. This
setting defaults to on in stock <productname>systemd</productname>, but
some operating system distributions default it to off.
</para>
<para>
A typical observed effect when this setting is on is that the semaphore
objects used by a PostgreSQL server are removed at apparently random
times, leading to the server crashing with log messages like
<screen>
LOG: semctl(1234567890, 0, IPC_RMID, ...) failed: Invalid argument
</screen>
Different types of IPC objects (shared memory vs. semaphores, System V
vs. POSIX) are treated slightly differently
by <productname>systemd</productname>, so one might observe that some IPC
resources are not removed in the same way as others. But it is not
advisable to rely on these subtle differences.
</para>
<para>
A <quote>user logging out</quote> might happen as part of a maintenance
job or manually when an administrator logs in as
the <literal>postgres</literal> user or something similar, so it is hard
to prevent in general.
</para>
<para>
What is a <quote>system user</quote> is determined
at <productname>systemd</productname> compile time from
the <symbol>SYS_UID_MAX</symbol> setting
in <filename>/etc/login.defs</filename>.
</para>
<para>
Packaging and deployment scripts should be careful to create
the <literal>postgres</literal> user as a system user by
using <literal>useradd -r</literal>, <literal>adduser --system</literal>,
or equivalent.
</para>
<para>
Alternatively, if the user account was created incorrectly or cannot be
changed, it is recommended to set
<programlisting>
RemoveIPC=no
</programlisting>
in <filename>/etc/systemd/logind.conf</filename> or another appropriate
configuration file.
</para>
<caution>
<para>
At least one of these two things has to be ensured, or the PostgreSQL
server will be very unreliable.
</para>
</caution>
</sect2>
<sect2>
<title>Resource Limits</title>
<para>
Unix-like operating systems enforce various kinds of resource limits
that might interfere with the operation of your
<productname>PostgreSQL</productname> server. Of particular
importance are limits on the number of processes per user, the
number of open files per process, and the amount of memory available
to each process. Each of these have a <quote>hard</quote> and a
<quote>soft</quote> limit. The soft limit is what actually counts
but it can be changed by the user up to the hard limit. The hard
limit can only be changed by the root user. The system call
<function>setrlimit</function> is responsible for setting these
parameters. The shell's built-in command <command>ulimit</command>
(Bourne shells) or <command>limit</command> (<application>csh</application>) is
used to control the resource limits from the command line. On
BSD-derived systems the file <filename>/etc/login.conf</filename>
controls the various resource limits set during login. See the
operating system documentation for details. The relevant
parameters are <varname>maxproc</varname>,
<varname>openfiles</varname>, and <varname>datasize</varname>. For
example:
<programlisting>
default:\
...
:datasize-cur=256M:\
:maxproc-cur=256:\
:openfiles-cur=256:\
...
</programlisting>
(<literal>-cur</literal> is the soft limit. Append
<literal>-max</literal> to set the hard limit.)
</para>
<para>
Kernels can also have system-wide limits on some resources.
<itemizedlist>
<listitem>
<para>
On <productname>Linux</productname>
<filename>/proc/sys/fs/file-max</filename> determines the
maximum number of open files that the kernel will support. It can
be changed by writing a different number into the file or by
adding an assignment in <filename>/etc/sysctl.conf</filename>.
The maximum limit of files per process is fixed at the time the
kernel is compiled; see
<filename>/usr/src/linux/Documentation/proc.txt</filename> for
more information.
</para>
</listitem>
</itemizedlist>
</para>
<para>
The <productname>PostgreSQL</productname> server uses one process
per connection so you should provide for at least as many processes
as allowed connections, in addition to what you need for the rest
of your system. This is usually not a problem but if you run
several servers on one machine things might get tight.
</para>
<para>
The factory default limit on open files is often set to
<quote>socially friendly</quote> values that allow many users to
coexist on a machine without using an inappropriate fraction of
the system resources. If you run many servers on a machine this
is perhaps what you want, but on dedicated servers you might want to
raise this limit.
</para>
<para>
On the other side of the coin, some systems allow individual
processes to open large numbers of files; if more than a few
processes do so then the system-wide limit can easily be exceeded.
If you find this happening, and you do not want to alter the
system-wide limit, you can set <productname>PostgreSQL</productname>'s <xref
linkend="guc-max-files-per-process"/> configuration parameter to
limit the consumption of open files.
</para>
</sect2>
<sect2 id="linux-memory-overcommit">
<title>Linux Memory Overcommit</title>
<indexterm>
<primary>memory overcommit</primary>
</indexterm>
<indexterm>
<primary>OOM</primary>
</indexterm>
<indexterm>
<primary>overcommit</primary>
</indexterm>
<para>
In Linux 2.4 and later, the default virtual memory behavior is not
optimal for <productname>PostgreSQL</productname>. Because of the
way that the kernel implements memory overcommit, the kernel might
terminate the <productname>PostgreSQL</productname> postmaster (the
master server process) if the memory demands of either
<productname>PostgreSQL</productname> or another process cause the
system to run out of virtual memory.
</para>
<para>
If this happens, you will see a kernel message that looks like
this (consult your system documentation and configuration on where
to look for such a message):
<programlisting>
Out of Memory: Killed process 12345 (postgres).
</programlisting>
This indicates that the <filename>postgres</filename> process
has been terminated due to memory pressure.
Although existing database connections will continue to function
normally, no new connections will be accepted. To recover,
<productname>PostgreSQL</productname> will need to be restarted.
</para>
<para>
One way to avoid this problem is to run
<productname>PostgreSQL</productname> on a machine where you can
be sure that other processes will not run the machine out of
memory. If memory is tight, increasing the swap space of the
operating system can help avoid the problem, because the
out-of-memory (OOM) killer is invoked only when physical memory and
swap space are exhausted.
</para>
<para>
If <productname>PostgreSQL</productname> itself is the cause of the
system running out of memory, you can avoid the problem by changing
your configuration. In some cases, it may help to lower memory-related
configuration parameters, particularly
<link linkend="guc-shared-buffers"><varname>shared_buffers</varname></link>
and <link linkend="guc-work-mem"><varname>work_mem</varname></link>. In
other cases, the problem may be caused by allowing too many connections
to the database server itself. In many cases, it may be better to reduce
<link linkend="guc-max-connections"><varname>max_connections</varname></link>
and instead make use of external connection-pooling software.
</para>
<para>
On Linux 2.6 and later, it is possible to modify the
kernel's behavior so that it will not <quote>overcommit</quote> memory.
Although this setting will not prevent the <ulink
url="https://lwn.net/Articles/104179/">OOM killer</ulink> from being invoked
altogether, it will lower the chances significantly and will therefore
lead to more robust system behavior. This is done by selecting strict
overcommit mode via <command>sysctl</command>:
<programlisting>
sysctl -w vm.overcommit_memory=2
</programlisting>
or placing an equivalent entry in <filename>/etc/sysctl.conf</filename>.
You might also wish to modify the related setting
<varname>vm.overcommit_ratio</varname>. For details see the kernel documentation
file <ulink url="https://www.kernel.org/doc/Documentation/vm/overcommit-accounting"></ulink>.
</para>
<para>
Another approach, which can be used with or without altering
<varname>vm.overcommit_memory</varname>, is to set the process-specific
<firstterm>OOM score adjustment</firstterm> value for the postmaster process to
<literal>-1000</literal>, thereby guaranteeing it will not be targeted by the OOM
killer. The simplest way to do this is to execute
<programlisting>
echo -1000 > /proc/self/oom_score_adj
</programlisting>
in the postmaster's startup script just before invoking the postmaster.
Note that this action must be done as root, or it will have no effect;
so a root-owned startup script is the easiest place to do it. If you
do this, you should also set these environment variables in the startup
script before invoking the postmaster:
<programlisting>
export PG_OOM_ADJUST_FILE=/proc/self/oom_score_adj
export PG_OOM_ADJUST_VALUE=0
</programlisting>
These settings will cause postmaster child processes to run with the
normal OOM score adjustment of zero, so that the OOM killer can still
target them at need. You could use some other value for
<envar>PG_OOM_ADJUST_VALUE</envar> if you want the child processes to run
with some other OOM score adjustment. (<envar>PG_OOM_ADJUST_VALUE</envar>
can also be omitted, in which case it defaults to zero.) If you do not
set <envar>PG_OOM_ADJUST_FILE</envar>, the child processes will run with the
same OOM score adjustment as the postmaster, which is unwise since the
whole point is to ensure that the postmaster has a preferential setting.
</para>
<para>
Older Linux kernels do not offer <filename>/proc/self/oom_score_adj</filename>,
but may have a previous version of the same functionality called
<filename>/proc/self/oom_adj</filename>. This works the same except the disable
value is <literal>-17</literal> not <literal>-1000</literal>.
</para>
<note>
<para>
Some vendors' Linux 2.4 kernels are reported to have early versions
of the 2.6 overcommit <command>sysctl</command> parameter. However, setting
<literal>vm.overcommit_memory</literal> to 2
on a 2.4 kernel that does not have the relevant code will make
things worse, not better. It is recommended that you inspect
the actual kernel source code (see the function
<function>vm_enough_memory</function> in the file <filename>mm/mmap.c</filename>)
to verify what is supported in your kernel before you try this in a 2.4
installation. The presence of the <filename>overcommit-accounting</filename>
documentation file should <emphasis>not</emphasis> be taken as evidence that the
feature is there. If in any doubt, consult a kernel expert or your
kernel vendor.
</para>
</note>
</sect2>
<sect2 id="linux-huge-pages">
<title>Linux Huge Pages</title>
<para>
Using huge pages reduces overhead when using large contiguous chunks of
memory, as <productname>PostgreSQL</productname> does, particularly when
using large values of <xref linkend="guc-shared-buffers"/>. To use this
feature in <productname>PostgreSQL</productname> you need a kernel
with <varname>CONFIG_HUGETLBFS=y</varname> and
<varname>CONFIG_HUGETLB_PAGE=y</varname>. You will also have to adjust
the kernel setting <varname>vm.nr_hugepages</varname>. To estimate the
number of huge pages needed, start <productname>PostgreSQL</productname>
without huge pages enabled and check the
postmaster's anonymous shared memory segment size, as well as the system's
huge page size, using the <filename>/proc</filename> file system. This might
look like:
<programlisting>
$ <userinput>head -1 $PGDATA/postmaster.pid</userinput>
4170
$ <userinput>pmap 4170 | awk '/rw-s/ &amp;&amp; /zero/ {print $2}'</userinput>
6490428K
$ <userinput>grep ^Hugepagesize /proc/meminfo</userinput>
Hugepagesize: 2048 kB
</programlisting>
<literal>6490428</literal> / <literal>2048</literal> gives approximately
<literal>3169.154</literal>, so in this example we need at
least <literal>3170</literal> huge pages, which we can set with:
<programlisting>
$ <userinput>sysctl -w vm.nr_hugepages=3170</userinput>
</programlisting>
A larger setting would be appropriate if other programs on the machine
also need huge pages. Don't forget to add this setting
to <filename>/etc/sysctl.conf</filename> so that it will be reapplied
after reboots.
</para>
<para>
Sometimes the kernel is not able to allocate the desired number of huge
pages immediately, so it might be necessary to repeat the command or to
reboot. (Immediately after a reboot, most of the machine's memory
should be available to convert into huge pages.) To verify the huge
page allocation situation, use:
<programlisting>
$ <userinput>grep Huge /proc/meminfo</userinput>
</programlisting>
</para>
<para>
It may also be necessary to give the database server's operating system
user permission to use huge pages by setting
<varname>vm.hugetlb_shm_group</varname> via <application>sysctl</application>, and/or
give permission to lock memory with <command>ulimit -l</command>.
</para>
<para>
The default behavior for huge pages in
<productname>PostgreSQL</productname> is to use them when possible and
to fall back to normal pages when failing. To enforce the use of huge
pages, you can set <xref linkend="guc-huge-pages"/>
to <literal>on</literal> in <filename>postgresql.conf</filename>.
Note that with this setting <productname>PostgreSQL</productname> will fail to
start if not enough huge pages are available.
</para>
<para>
For a detailed description of the <productname>Linux</productname> huge
pages feature have a look
at <ulink url="https://www.kernel.org/doc/Documentation/vm/hugetlbpage.txt"></ulink>.
</para>
</sect2>
</sect1>
<sect1 id="server-shutdown">
<title>Shutting Down the Server</title>
<indexterm zone="server-shutdown">
<primary>shutdown</primary>
</indexterm>
<para>
There are several ways to shut down the database server. You control
the type of shutdown by sending different signals to the master
<command>postgres</command> process.
<variablelist>
<varlistentry>
<term><systemitem>SIGTERM</systemitem><indexterm><primary>SIGTERM</primary></indexterm></term>
<listitem>
<para>
This is the <firstterm>Smart Shutdown</firstterm> mode.
After receiving <systemitem>SIGTERM</systemitem>, the server
disallows new connections, but lets existing sessions end their
work normally. It shuts down only after all of the sessions terminate.
If the server is in online backup mode, it additionally waits
until online backup mode is no longer active. While backup mode is
active, new connections will still be allowed, but only to superusers
(this exception allows a superuser to connect to terminate
online backup mode). If the server is in recovery when a smart
shutdown is requested, recovery and streaming replication will be
stopped only after all regular sessions have terminated.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><systemitem>SIGINT</systemitem><indexterm><primary>SIGINT</primary></indexterm></term>
<listitem>
<para>
This is the <firstterm>Fast Shutdown</firstterm> mode.
The server disallows new connections and sends all existing
server processes <systemitem>SIGTERM</systemitem>, which will cause them
to abort their current transactions and exit promptly. It then
waits for all server processes to exit and finally shuts down.
If the server is in online backup mode, backup mode will be
terminated, rendering the backup useless.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><systemitem>SIGQUIT</systemitem><indexterm><primary>SIGQUIT</primary></indexterm></term>
<listitem>
<para>
This is the <firstterm>Immediate Shutdown</firstterm> mode.
The server will send <systemitem>SIGQUIT</systemitem> to all child
processes and wait for them to terminate. If any do not terminate
within 5 seconds, they will be sent <systemitem>SIGKILL</systemitem>.
The master server process exits as soon as all child processes have
exited, without doing normal database shutdown processing.
This will lead to recovery (by
replaying the WAL log) upon next start-up. This is recommended
only in emergencies.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
The <xref linkend="app-pg-ctl"/> program provides a convenient
interface for sending these signals to shut down the server.
Alternatively, you can send the signal directly using <command>kill</command>
on non-Windows systems.
The <acronym>PID</acronym> of the <command>postgres</command> process can be
found using the <command>ps</command> program, or from the file
<filename>postmaster.pid</filename> in the data directory. For
example, to do a fast shutdown:
<screen>
$ <userinput>kill -INT `head -1 /usr/local/pgsql/data/postmaster.pid`</userinput>
</screen>
</para>
<important>
<para>
It is best not to use <systemitem>SIGKILL</systemitem> to shut down
the server. Doing so will prevent the server from releasing
shared memory and semaphores, which might then have to be done
manually before a new server can be started. Furthermore,
<systemitem>SIGKILL</systemitem> kills the <command>postgres</command>
process without letting it relay the signal to its subprocesses,
so it will be necessary to kill the individual subprocesses by hand as
well.
</para>
</important>
<para>
To terminate an individual session while allowing other sessions to
continue, use <function>pg_terminate_backend()</function> (see <xref
linkend="functions-admin-signal-table"/>) or send a
<systemitem>SIGTERM</systemitem> signal to the child process associated with
the session.
</para>
</sect1>
<sect1 id="upgrading">
<title>Upgrading a <productname>PostgreSQL</productname> Cluster</title>
<indexterm zone="upgrading">
<primary>upgrading</primary>
</indexterm>
<indexterm zone="upgrading">
<primary>version</primary>
<secondary>compatibility</secondary>
</indexterm>
<para>
This section discusses how to upgrade your database data from one
<productname>PostgreSQL</productname> release to a newer one.
</para>
<para>
Current <productname>PostgreSQL</productname> version numbers consist of a
major and a minor version number. For example, in the version number 10.1,
the 10 is the major version number and the 1 is the minor version number,
meaning this would be the first minor release of the major release 10. For
releases before <productname>PostgreSQL</productname> version 10.0, version
numbers consist of three numbers, for example, 9.5.3. In those cases, the
major version consists of the first two digit groups of the version number,
e.g., 9.5, and the minor version is the third number, e.g., 3, meaning this
would be the third minor release of the major release 9.5.
</para>
<para>
Minor releases never change the internal storage format and are always
compatible with earlier and later minor releases of the same major version
number. For example, version 10.1 is compatible with version 10.0 and
version 10.6. Similarly, for example, 9.5.3 is compatible with 9.5.0,
9.5.1, and 9.5.6. To update between compatible versions, you simply
replace the executables while the server is down and restart the server.
The data directory remains unchanged &mdash; minor upgrades are that
simple.
</para>
<para>
For <emphasis>major</emphasis> releases of <productname>PostgreSQL</productname>, the
internal data storage format is subject to change, thus complicating
upgrades. The traditional method for moving data to a new major version
is to dump and reload the database, though this can be slow. A
faster method is <xref linkend="pgupgrade"/>. Replication methods are
also available, as discussed below.
</para>
<para>
New major versions also typically introduce some user-visible
incompatibilities, so application programming changes might be required.
All user-visible changes are listed in the release notes (<xref
linkend="release"/>); pay particular attention to the section
labeled "Migration". If you are upgrading across several major
versions, be sure to read the release notes for each intervening
version.
</para>
<para>
Cautious users will want to test their client applications on the new
version before switching over fully; therefore, it's often a good idea to
set up concurrent installations of old and new versions. When
testing a <productname>PostgreSQL</productname> major upgrade, consider the
following categories of possible changes:
</para>
<variablelist>
<varlistentry>
<term>Administration</term>
<listitem>
<para>
The capabilities available for administrators to monitor and control
the server often change and improve in each major release.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>SQL</term>
<listitem>
<para>
Typically this includes new SQL command capabilities and not changes
in behavior, unless specifically mentioned in the release notes.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Library API</term>
<listitem>
<para>
Typically libraries like <application>libpq</application> only add new
functionality, again unless mentioned in the release notes.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>System Catalogs</term>
<listitem>
<para>
System catalog changes usually only affect database management tools.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Server C-language API</term>
<listitem>
<para>
This involves changes in the backend function API, which is written
in the C programming language. Such changes affect code that
references backend functions deep inside the server.
</para>
</listitem>
</varlistentry>
</variablelist>
<sect2 id="upgrading-via-pgdumpall">
<title>Upgrading Data via <application>pg_dumpall</application></title>
<para>
One upgrade method is to dump data from one major version of
<productname>PostgreSQL</productname> and reload it in another &mdash; to do
this, you must use a <emphasis>logical</emphasis> backup tool like
<application>pg_dumpall</application>; file system
level backup methods will not work. (There are checks in place that prevent
you from using a data directory with an incompatible version of
<productname>PostgreSQL</productname>, so no great harm can be done by
trying to start the wrong server version on a data directory.)
</para>
<para>
It is recommended that you use the <application>pg_dump</application> and
<application>pg_dumpall</application> programs from the <emphasis>newer</emphasis>
version of
<productname>PostgreSQL</productname>, to take advantage of enhancements
that might have been made in these programs. Current releases of the
dump programs can read data from any server version back to 7.0.
</para>
<para>
These instructions assume that your existing installation is under the
<filename>/usr/local/pgsql</filename> directory, and that the data area is in
<filename>/usr/local/pgsql/data</filename>. Substitute your paths
appropriately.
</para>
<procedure>
<step>
<para>
If making a backup, make sure that your database is not being updated.
This does not affect the integrity of the backup, but the changed
data would of course not be included. If necessary, edit the
permissions in the file <filename>/usr/local/pgsql/data/pg_hba.conf</filename>
(or equivalent) to disallow access from everyone except you.
See <xref linkend="client-authentication"/> for additional information on
access control.
</para>
<para>
<indexterm>
<primary>pg_dumpall</primary>
<secondary>use during upgrade</secondary>
</indexterm>
To back up your database installation, type:
<screen>
<userinput>pg_dumpall &gt; <replaceable>outputfile</replaceable></userinput>
</screen>
</para>
<para>
To make the backup, you can use the <application>pg_dumpall</application>
command from the version you are currently running; see <xref
linkend="backup-dump-all"/> for more details. For best
results, however, try to use the <application>pg_dumpall</application>
command from <productname>PostgreSQL</productname> &version;,
since this version contains bug fixes and improvements over older
versions. While this advice might seem idiosyncratic since you
haven't installed the new version yet, it is advisable to follow
it if you plan to install the new version in parallel with the
old version. In that case you can complete the installation
normally and transfer the data later. This will also decrease
the downtime.
</para>
</step>
<step>
<para>
Shut down the old server:
<screen>
<userinput>pg_ctl stop</userinput>
</screen>
On systems that have <productname>PostgreSQL</productname> started at boot time,
there is probably a start-up file that will accomplish the same thing. For
example, on a <systemitem class="osname">Red Hat Linux</systemitem> system one
might find that this works:
<screen>
<userinput>/etc/rc.d/init.d/postgresql stop</userinput>
</screen>
See <xref linkend="runtime"/> for details about starting and
stopping the server.
</para>
</step>
<step>
<para>
If restoring from backup, rename or delete the old installation
directory if it is not version-specific. It is a good idea to
rename the directory, rather than
delete it, in case you have trouble and need to revert to it. Keep
in mind the directory might consume significant disk space. To rename
the directory, use a command like this:
<screen>
<userinput>mv /usr/local/pgsql /usr/local/pgsql.old</userinput>
</screen>
(Be sure to move the directory as a single unit so relative paths
remain unchanged.)
</para>
</step>
<step>
<para>
Install the new version of <productname>PostgreSQL</productname> as
outlined in <xref linkend="install-procedure"/>.
</para>
</step>
<step>
<para>
Create a new database cluster if needed. Remember that you must
execute these commands while logged in to the special database user
account (which you already have if you are upgrading).
<programlisting>
<userinput>/usr/local/pgsql/bin/initdb -D /usr/local/pgsql/data</userinput>
</programlisting>
</para>
</step>
<step>
<para>
Restore your previous <filename>pg_hba.conf</filename> and any
<filename>postgresql.conf</filename> modifications.
</para>
</step>
<step>
<para>
Start the database server, again using the special database user
account:
<programlisting>
<userinput>/usr/local/pgsql/bin/postgres -D /usr/local/pgsql/data</userinput>
</programlisting>
</para>
</step>
<step>
<para>
Finally, restore your data from backup with:
<screen>
<userinput>/usr/local/pgsql/bin/psql -d postgres -f <replaceable>outputfile</replaceable></userinput>
</screen>
using the <emphasis>new</emphasis> <application>psql</application>.
</para>
</step>
</procedure>
<para>
The least downtime can be achieved by installing the new server in
a different directory and running both the old and the new servers
in parallel, on different ports. Then you can use something like:
<programlisting>
pg_dumpall -p 5432 | psql -d postgres -p 5433
</programlisting>
to transfer your data.
</para>
</sect2>
<sect2 id="upgrading-via-pg-upgrade">
<title>Upgrading Data via <application>pg_upgrade</application></title>
<para>
The <xref linkend="pgupgrade"/> module allows an installation to
be migrated in-place from one major <productname>PostgreSQL</productname>
version to another. Upgrades can be performed in minutes,
particularly with <option>--link</option> mode. It requires steps similar to
<application>pg_dumpall</application> above, e.g. starting/stopping the server,
running <application>initdb</application>. The <application>pg_upgrade</application> <link
linkend="pgupgrade">documentation</link> outlines the necessary steps.
</para>
</sect2>
<sect2 id="upgrading-via-replication">
<title>Upgrading Data via Replication</title>
<para>
It is also possible to use logical replication methods to create a standby
server with the updated version of <productname>PostgreSQL</productname>.
This is possible because logical replication supports
replication between different major versions of
<productname>PostgreSQL</productname>. The standby can be on the same computer or
a different computer. Once it has synced up with the master server
(running the older version of <productname>PostgreSQL</productname>), you can
switch masters and make the standby the master and shut down the older
database instance. Such a switch-over results in only several seconds
of downtime for an upgrade.
</para>
<para>
This method of upgrading can be performed using the built-in logical
replication facilities as well as using external logical replication
systems such as <productname>pglogical</productname>,
<productname>Slony</productname>, <productname>Londiste</productname>, and
<productname>Bucardo</productname>.
</para>
</sect2>
</sect1>
<sect1 id="preventing-server-spoofing">
<title>Preventing Server Spoofing</title>
<indexterm zone="preventing-server-spoofing">
<primary>server spoofing</primary>
</indexterm>
<para>
While the server is running, it is not possible for a malicious user
to take the place of the normal database server. However, when the
server is down, it is possible for a local user to spoof the normal
server by starting their own server. The spoof server could read
passwords and queries sent by clients, but could not return any data
because the <varname>PGDATA</varname> directory would still be secure because
of directory permissions. Spoofing is possible because any user can
start a database server; a client cannot identify an invalid server
unless it is specially configured.
</para>
<para>
One way to prevent spoofing of <literal>local</literal>
connections is to use a Unix domain socket directory (<xref
linkend="guc-unix-socket-directories"/>) that has write permission only
for a trusted local user. This prevents a malicious user from creating
their own socket file in that directory. If you are concerned that
some applications might still reference <filename>/tmp</filename> for the
socket file and hence be vulnerable to spoofing, during operating system
startup create a symbolic link <filename>/tmp/.s.PGSQL.5432</filename> that points
to the relocated socket file. You also might need to modify your
<filename>/tmp</filename> cleanup script to prevent removal of the symbolic link.
</para>
<para>
Another option for <literal>local</literal> connections is for clients to use
<link linkend="libpq-connect-requirepeer"><literal>requirepeer</literal></link>
to specify the required owner of the server process connected to
the socket.
</para>
<para>
To prevent spoofing on TCP connections, the best solution is to use
SSL certificates and make sure that clients check the server's certificate.
To do that, the server
must be configured to accept only <literal>hostssl</literal> connections (<xref
linkend="auth-pg-hba-conf"/>) and have SSL key and certificate files
(<xref linkend="ssl-tcp"/>). The TCP client must connect using
<literal>sslmode=verify-ca</literal> or
<literal>verify-full</literal> and have the appropriate root certificate
file installed (<xref linkend="libq-ssl-certificates"/>).
</para>
</sect1>
<sect1 id="encryption-options">
<title>Encryption Options</title>
<indexterm zone="encryption-options">
<primary>encryption</primary>
</indexterm>
<para>
<productname>PostgreSQL</productname> offers encryption at several
levels, and provides flexibility in protecting data from disclosure
due to database server theft, unscrupulous administrators, and
insecure networks. Encryption might also be required to secure
sensitive data such as medical records or financial transactions.
</para>
<variablelist>
<varlistentry>
<term>Password Encryption</term>
<listitem>
<para>
Database user passwords are stored as hashes (determined by the setting
<xref linkend="guc-password-encryption"/>), so the administrator cannot
determine the actual password assigned to the user. If SCRAM or MD5
encryption is used for client authentication, the unencrypted password is
never even temporarily present on the server because the client encrypts
it before being sent across the network. SCRAM is preferred, because it
is an Internet standard and is more secure than the PostgreSQL-specific
MD5 authentication protocol.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Encryption For Specific Columns</term>
<listitem>
<para>
The <xref linkend="pgcrypto"/> module allows certain fields to be
stored encrypted.
This is useful if only some of the data is sensitive.
The client supplies the decryption key and the data is decrypted
on the server and then sent to the client.
</para>
<para>
The decrypted data and the decryption key are present on the
server for a brief time while it is being decrypted and
communicated between the client and server. This presents a brief
moment where the data and keys can be intercepted by someone with
complete access to the database server, such as the system
administrator.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Data Partition Encryption</term>
<listitem>
<para>
Storage encryption can be performed at the file system level or the
block level. Linux file system encryption options include eCryptfs
and EncFS, while FreeBSD uses PEFS. Block level or full disk
encryption options include dm-crypt + LUKS on Linux and GEOM
modules geli and gbde on FreeBSD. Many other operating systems
support this functionality, including Windows.
</para>
<para>
This mechanism prevents unencrypted data from being read from the
drives if the drives or the entire computer is stolen. This does
not protect against attacks while the file system is mounted,
because when mounted, the operating system provides an unencrypted
view of the data. However, to mount the file system, you need some
way for the encryption key to be passed to the operating system,
and sometimes the key is stored somewhere on the host that mounts
the disk.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Encrypting Data Across A Network</term>
<listitem>
<para>
SSL connections encrypt all data sent across the network: the
password, the queries, and the data returned. The
<filename>pg_hba.conf</filename> file allows administrators to specify
which hosts can use non-encrypted connections (<literal>host</literal>)
and which require SSL-encrypted connections
(<literal>hostssl</literal>). Also, clients can specify that they
connect to servers only via SSL. <application>Stunnel</application> or
<application>SSH</application> can also be used to encrypt transmissions.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>SSL Host Authentication</term>
<listitem>
<para>
It is possible for both the client and server to provide SSL
certificates to each other. It takes some extra configuration
on each side, but this provides stronger verification of identity
than the mere use of passwords. It prevents a computer from
pretending to be the server just long enough to read the password
sent by the client. It also helps prevent <quote>man in the middle</quote>
attacks where a computer between the client and server pretends to
be the server and reads and passes all data between the client and
server.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>Client-Side Encryption</term>
<listitem>
<para>
If the system administrator for the server's machine cannot be trusted,
it is necessary
for the client to encrypt the data; this way, unencrypted data
never appears on the database server. Data is encrypted on the
client before being sent to the server, and database results have
to be decrypted on the client before being used.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect1>
<sect1 id="ssl-tcp">
<title>Secure TCP/IP Connections with SSL</title>
<indexterm zone="ssl-tcp">
<primary>SSL</primary>
</indexterm>
<para>
<productname>PostgreSQL</productname> has native support for using
<acronym>SSL</acronym> connections to encrypt client/server communications
for increased security. This requires that
<productname>OpenSSL</productname> is installed on both client and
server systems and that support in <productname>PostgreSQL</productname> is
enabled at build time (see <xref linkend="installation"/>).
</para>
<sect2 id="ssl-setup">
<title>Basic Setup</title>
<para>
With <acronym>SSL</acronym> support compiled in, the
<productname>PostgreSQL</productname> server can be started with
<acronym>SSL</acronym> enabled by setting the parameter
<xref linkend="guc-ssl"/> to <literal>on</literal> in
<filename>postgresql.conf</filename>. The server will listen for both normal
and <acronym>SSL</acronym> connections on the same TCP port, and will negotiate
with any connecting client on whether to use <acronym>SSL</acronym>. By
default, this is at the client's option; see <xref
linkend="auth-pg-hba-conf"/> about how to set up the server to require
use of <acronym>SSL</acronym> for some or all connections.
</para>
<para>
To start in <acronym>SSL</acronym> mode, files containing the server certificate
and private key must exist. By default, these files are expected to be
named <filename>server.crt</filename> and <filename>server.key</filename>, respectively, in
the server's data directory, but other names and locations can be specified
using the configuration parameters <xref linkend="guc-ssl-cert-file"/>
and <xref linkend="guc-ssl-key-file"/>.
</para>
<para>
On Unix systems, the permissions on <filename>server.key</filename> must
disallow any access to world or group; achieve this by the command
<command>chmod 0600 server.key</command>. Alternatively, the file can be
owned by root and have group read access (that is, <literal>0640</literal>
permissions). That setup is intended for installations where certificate
and key files are managed by the operating system. The user under which
the <productname>PostgreSQL</productname> server runs should then be made a
member of the group that has access to those certificate and key files.
</para>
<para>
If the data directory allows group read access then certificate files may
need to be located outside of the data directory in order to conform to the
security requirements outlined above. Generally, group access is enabled
to allow an unprivileged user to backup the database, and in that case the
backup software will not be able to read the certificate files and will
likely error.
</para>
<para>
If the private key is protected with a passphrase, the
server will prompt for the passphrase and will not start until it has
been entered.
Using a passphrase also disables the ability to change the server's SSL
configuration without a server restart.
Furthermore, passphrase-protected private keys cannot be used at all
on Windows.
</para>
<para>
The first certificate in <filename>server.crt</filename> must be the
server's certificate because it must match the server's private key.
The certificates of <quote>intermediate</quote> certificate authorities
can also be appended to the file. Doing this avoids the necessity of
storing intermediate certificates on clients, assuming the root and
intermediate certificates were created with <literal>v3_ca</literal>
extensions. This allows easier expiration of intermediate certificates.
</para>
<para>
It is not necessary to add the root certificate to
<filename>server.crt</filename>. Instead, clients must have the root
certificate of the server's certificate chain.
</para>
</sect2>
<sect2 id="ssl-openssl-config">
<title>OpenSSL Configuration</title>
<para>
<productname>PostgreSQL</productname> reads the system-wide
<productname>OpenSSL</productname> configuration file. By default, this
file is named <filename>openssl.cnf</filename> and is located in the
directory reported by <literal>openssl version -d</literal>.
This default can be overridden by setting environment variable
<envar>OPENSSL_CONF</envar> to the name of the desired configuration file.
</para>
<para>
<productname>OpenSSL</productname> supports a wide range of ciphers
and authentication algorithms, of varying strength. While a list of
ciphers can be specified in the <productname>OpenSSL</productname>
configuration file, you can specify ciphers specifically for use by
the database server by modifying <xref linkend="guc-ssl-ciphers"/> in
<filename>postgresql.conf</filename>.
</para>
<note>
<para>
It is possible to have authentication without encryption overhead by
using <literal>NULL-SHA</literal> or <literal>NULL-MD5</literal> ciphers. However,
a man-in-the-middle could read and pass communications between client
and server. Also, encryption overhead is minimal compared to the
overhead of authentication. For these reasons NULL ciphers are not
recommended.
</para>
</note>
</sect2>
<sect2 id="ssl-client-certificates">
<title>Using Client Certificates</title>
<para>
To require the client to supply a trusted certificate,
place certificates of the root certificate authorities
(<acronym>CA</acronym>s) you trust in a file in the data
directory, set the parameter <xref linkend="guc-ssl-ca-file"/> in
<filename>postgresql.conf</filename> to the new file name, and add the
authentication option <literal>clientcert=1</literal> to the appropriate
<literal>hostssl</literal> line(s) in <filename>pg_hba.conf</filename>.
A certificate will then be requested from the client during SSL
connection startup. (See <xref linkend="libpq-ssl"/> for a description
of how to set up certificates on the client.) The server will
verify that the client's certificate is signed by one of the trusted
certificate authorities.
</para>
<para>
Intermediate certificates that chain up to existing root certificates
can also appear in the <xref linkend="guc-ssl-ca-file"/> file if
you wish to avoid storing them on clients (assuming the root and
intermediate certificates were created with <literal>v3_ca</literal>
extensions). Certificate Revocation List (CRL) entries are also
checked if the parameter <xref linkend="guc-ssl-crl-file"/> is set.
(See <ulink
url="http://h41379.www4.hpe.com/doc/83final/ba554_90007/ch04s02.html"></ulink>
for diagrams showing SSL certificate usage.)
</para>
<para>
The <literal>clientcert</literal> authentication option is available for
all authentication methods, but only in <filename>pg_hba.conf</filename> lines
specified as <literal>hostssl</literal>. When <literal>clientcert</literal> is
not specified or is set to 0, the server will still verify any presented
client certificates against its CA file, if one is configured &mdash; but
it will not insist that a client certificate be presented.
</para>
<para>
If you are setting up client certificates, you may wish to use
the <literal>cert</literal> authentication method, so that the certificates
control user authentication as well as providing connection security.
See <xref linkend="auth-cert"/> for details. (It is not necessary to
specify <literal>clientcert=1</literal> explicitly when using
the <literal>cert</literal> authentication method.)
</para>
</sect2>
<sect2 id="ssl-server-files">
<title>SSL Server File Usage</title>
<para>
<xref linkend="ssl-file-usage"/> summarizes the files that are
relevant to the SSL setup on the server. (The shown file names are default
names. The locally configured names could be different.)
</para>
<table id="ssl-file-usage">
<title>SSL Server File Usage</title>
<tgroup cols="3">
<thead>
<row>
<entry>File</entry>
<entry>Contents</entry>
<entry>Effect</entry>
</row>
</thead>
<tbody>
<row>
<entry><xref linkend="guc-ssl-cert-file"/> (<filename>$PGDATA/server.crt</filename>)</entry>
<entry>server certificate</entry>
<entry>sent to client to indicate server's identity</entry>
</row>
<row>
<entry><xref linkend="guc-ssl-key-file"/> (<filename>$PGDATA/server.key</filename>)</entry>
<entry>server private key</entry>
<entry>proves server certificate was sent by the owner; does not indicate
certificate owner is trustworthy</entry>
</row>
<row>
<entry><xref linkend="guc-ssl-ca-file"/></entry>
<entry>trusted certificate authorities</entry>
<entry>checks that client certificate is
signed by a trusted certificate authority</entry>
</row>
<row>
<entry><xref linkend="guc-ssl-crl-file"/></entry>
<entry>certificates revoked by certificate authorities</entry>
<entry>client certificate must not be on this list</entry>
</row>
</tbody>
</tgroup>
</table>
<para>
The server reads these files at server start and whenever the server
configuration is reloaded. On <systemitem class="osname">Windows</systemitem>
systems, they are also re-read whenever a new backend process is spawned
for a new client connection.
</para>
<para>
If an error in these files is detected at server start, the server will
refuse to start. But if an error is detected during a configuration
reload, the files are ignored and the old SSL configuration continues to
be used. On <systemitem class="osname">Windows</systemitem> systems, if an error in
these files is detected at backend start, that backend will be unable to
establish an SSL connection. In all these cases, the error condition is
reported in the server log.
</para>
</sect2>
<sect2 id="ssl-certificate-creation">
<title>Creating Certificates</title>
<para>
To create a simple self-signed certificate for the server, valid for 365
days, use the following <productname>OpenSSL</productname> command,
replacing <replaceable>dbhost.yourdomain.com</replaceable> with the
server's host name:
<programlisting>
openssl req -new -x509 -days 365 -nodes -text -out server.crt \
-keyout server.key -subj "/CN=<replaceable>dbhost.yourdomain.com</replaceable>"
</programlisting>
Then do:
<programlisting>
chmod og-rwx server.key
</programlisting>
because the server will reject the file if its permissions are more
liberal than this.
For more details on how to create your server private key and
certificate, refer to the <productname>OpenSSL</productname> documentation.
</para>
<para>
While a self-signed certificate can be used for testing, a certificate
signed by a certificate authority (<acronym>CA</acronym>) (usually an
enterprise-wide root <acronym>CA</acronym>) should be used in production.
</para>
<para>
To create a server certificate whose identity can be validated
by clients, first create a certificate signing request
(<acronym>CSR</acronym>) and a public/private key file:
<programlisting>
openssl req -new -nodes -text -out root.csr \
-keyout root.key -subj "/CN=<replaceable>root.yourdomain.com</replaceable>"
chmod og-rwx root.key
</programlisting>
Then, sign the request with the key to create a root certificate
authority (using the default <productname>OpenSSL</productname>
configuration file location on <productname>Linux</productname>):
<programlisting>
openssl x509 -req -in root.csr -text -days 3650 \
-extfile /etc/ssl/openssl.cnf -extensions v3_ca \
-signkey root.key -out root.crt
</programlisting>
Finally, create a server certificate signed by the new root certificate
authority:
<programlisting>
openssl req -new -nodes -text -out server.csr \
-keyout server.key -subj "/CN=<replaceable>dbhost.yourdomain.com</replaceable>"
chmod og-rwx server.key
openssl x509 -req -in server.csr -text -days 365 \
-CA root.crt -CAkey root.key -CAcreateserial \
-out server.crt
</programlisting>
<filename>server.crt</filename> and <filename>server.key</filename>
should be stored on the server, and <filename>root.crt</filename> should
be stored on the client so the client can verify that the server's leaf
certificate was signed by its trusted root certificate.
<filename>root.key</filename> should be stored offline for use in
creating future certificates.
</para>
<para>
It is also possible to create a chain of trust that includes
intermediate certificates:
<programlisting>
# root
openssl req -new -nodes -text -out root.csr \
-keyout root.key -subj "/CN=<replaceable>root.yourdomain.com</replaceable>"
chmod og-rwx root.key
openssl x509 -req -in root.csr -text -days 3650 \
-extfile /etc/ssl/openssl.cnf -extensions v3_ca \
-signkey root.key -out root.crt
# intermediate
openssl req -new -nodes -text -out intermediate.csr \
-keyout intermediate.key -subj "/CN=<replaceable>intermediate.yourdomain.com</replaceable>"
chmod og-rwx intermediate.key
openssl x509 -req -in intermediate.csr -text -days 1825 \
-extfile /etc/ssl/openssl.cnf -extensions v3_ca \
-CA root.crt -CAkey root.key -CAcreateserial \
-out intermediate.crt
# leaf
openssl req -new -nodes -text -out server.csr \
-keyout server.key -subj "/CN=<replaceable>dbhost.yourdomain.com</replaceable>"
chmod og-rwx server.key
openssl x509 -req -in server.csr -text -days 365 \
-CA intermediate.crt -CAkey intermediate.key -CAcreateserial \
-out server.crt
</programlisting>
<filename>server.crt</filename> and
<filename>intermediate.crt</filename> should be concatenated
into a certificate file bundle and stored on the server.
<filename>server.key</filename> should also be stored on the server.
<filename>root.crt</filename> should be stored on the client so
the client can verify that the server's leaf certificate was signed
by a chain of certificates linked to its trusted root certificate.
<filename>root.key</filename> and <filename>intermediate.key</filename>
should be stored offline for use in creating future certificates.
</para>
</sect2>
</sect1>
<sect1 id="ssh-tunnels">
<title>Secure TCP/IP Connections with <application>SSH</application> Tunnels</title>
<indexterm zone="ssh-tunnels">
<primary>ssh</primary>
</indexterm>
<para>
It is possible to use <application>SSH</application> to encrypt the network
connection between clients and a
<productname>PostgreSQL</productname> server. Done properly, this
provides an adequately secure network connection, even for non-SSL-capable
clients.
</para>
<para>
First make sure that an <application>SSH</application> server is
running properly on the same machine as the
<productname>PostgreSQL</productname> server and that you can log in using
<command>ssh</command> as some user. Then you can establish a secure
tunnel with a command like this from the client machine:
<programlisting>
ssh -L 63333:localhost:5432 joe@foo.com
</programlisting>
The first number in the <option>-L</option> argument, 63333, is the
port number of your end of the tunnel; it can be any unused port.
(IANA reserves ports 49152 through 65535 for private use.) The
second number, 5432, is the remote end of the tunnel: the port
number your server is using. The name or IP address between the
port numbers is the host with the database server you are going to
connect to, as seen from the host you are logging in to, which
is <literal>foo.com</literal> in this example. In order to connect
to the database server using this tunnel, you connect to port 63333
on the local machine:
<programlisting>
psql -h localhost -p 63333 postgres
</programlisting>
To the database server it will then look as though you are really
user <literal>joe</literal> on host <literal>foo.com</literal>
connecting to <literal>localhost</literal> in that context, and it
will use whatever authentication procedure was configured for
connections from this user and host. Note that the server will not
think the connection is SSL-encrypted, since in fact it is not
encrypted between the
<application>SSH</application> server and the
<productname>PostgreSQL</productname> server. This should not pose any
extra security risk as long as they are on the same machine.
</para>
<para>
In order for the
tunnel setup to succeed you must be allowed to connect via
<command>ssh</command> as <literal>joe@foo.com</literal>, just
as if you had attempted to use <command>ssh</command> to create a
terminal session.
</para>
<para>
You could also have set up the port forwarding as
<programlisting>
ssh -L 63333:foo.com:5432 joe@foo.com
</programlisting>
but then the database server will see the connection as coming in
on its <literal>foo.com</literal> interface, which is not opened by
the default setting <literal>listen_addresses =
'localhost'</literal>. This is usually not what you want.
</para>
<para>
If you have to <quote>hop</quote> to the database server via some
login host, one possible setup could look like this:
<programlisting>
ssh -L 63333:db.foo.com:5432 joe@shell.foo.com
</programlisting>
Note that this way the connection
from <literal>shell.foo.com</literal>
to <literal>db.foo.com</literal> will not be encrypted by the SSH
tunnel.
SSH offers quite a few configuration possibilities when the network
is restricted in various ways. Please refer to the SSH
documentation for details.
</para>
<tip>
<para>
Several other applications exist that can provide secure tunnels using
a procedure similar in concept to the one just described.
</para>
</tip>
</sect1>
<sect1 id="event-log-registration">
<title>Registering <application>Event Log</application> on <systemitem
class="osname">Windows</systemitem></title>
<indexterm zone="event-log-registration">
<primary>event log</primary>
<secondary>event log</secondary>
</indexterm>
<para>
To register a <systemitem class="osname">Windows</systemitem>
<application>event log</application> library with the operating system,
issue this command:
<screen>
<userinput>regsvr32 <replaceable>pgsql_library_directory</replaceable>/pgevent.dll</userinput>
</screen>
This creates registry entries used by the event viewer, under the default
event source named <literal>PostgreSQL</literal>.
</para>
<para>
To specify a different event source name (see
<xref linkend="guc-event-source"/>), use the <literal>/n</literal>
and <literal>/i</literal> options:
<screen>
<userinput>regsvr32 /n /i:<replaceable>event_source_name</replaceable> <replaceable>pgsql_library_directory</replaceable>/pgevent.dll</userinput>
</screen>
</para>
<para>
To unregister the <application>event log</application> library from
the operating system, issue this command:
<screen>
<userinput>regsvr32 /u [/i:<replaceable>event_source_name</replaceable>] <replaceable>pgsql_library_directory</replaceable>/pgevent.dll</userinput>
</screen>
</para>
<note>
<para>
To enable event logging in the database server, modify
<xref linkend="guc-log-destination"/> to include
<literal>eventlog</literal> in <filename>postgresql.conf</filename>.
</para>
</note>
</sect1>
</chapter>
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