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doc: Add advice about systemd RemoveIPC

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Reviewed-by: Magnus Hagander <magnus@hagander.net>
This commit is contained in:
Peter Eisentraut
2017-02-15 10:44:07 -05:00
parent e6c7ff9e38
commit 6a1459f67c
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doc/src/sgml/runtime.sgml
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@@ -1162,6 +1162,85 @@ project.max-msg-ids=(priv,4096,deny)
</sect2> </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> <sect2>
<title>Resource Limits</title> <title>Resource Limits</title>