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Add note about risks involved in replaying CREATE TABLESPACE commands

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from WAL.  A couple other grammatical improvements too.
This commit is contained in:
Tom Lane
2005-03-23 19:39:06 +00:00
parent a4500f3262
commit 63b0d7b35f
1 changed files with 36 additions and 7 deletions
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43
doc/src/sgml/backup.sgml
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@@ -1,5 +1,5 @@
<!--
$PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.54.4.1 2005/01/22 23:05:47 momjian Exp $
$PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.54.4.2 2005/03/23 19:39:06 tgl Exp $
-->
<chapter id="backup">
<title>Backup and Restore</title>
@@ -177,7 +177,7 @@ pg_dumpall &gt; <replaceable>outfile</>
</synopsis>
The resulting dump can be restored with <application>psql</>:
<synopsis>
psql template1 &lt; <replaceable class="parameter">infile</replaceable>
psql -f <replaceable class="parameter">infile</replaceable> template1
</synopsis>
(Actually, you can specify any existing database name to start from,
but if you are reloading in an empty cluster then <literal>template1</>
@@ -364,15 +364,29 @@ tar -cf backup.tar /usr/local/pgsql/data
</para>
<para>
If your database is spread across multiple volumes (for example,
data files and WAL log on different disks) there may not be any way
to obtain exactly-simultaneous frozen snapshots of all the volumes.
If your database is spread across multiple file systems, there may not
be any way to obtain exactly-simultaneous frozen snapshots of all
the volumes. For example, if your data files and WAL log are on different
disks, or if tablespaces are on different file systems, it might
not be possible to use snapshot backup because the snapshots must be
simultaneous.
Read your file system documentation very carefully before trusting
to the consistent-snapshot technique in such situations. The safest
approach is to shut down the database server for long enough to
establish all the frozen snapshots.
</para>
<para>
Another option is to use <application>rsync</> to perform a file
system backup. This is done by first running <application>rsync</>
while the database server is running, then shutting down the database
server just long enough to do a second <application>rsync</>. The
second <application>rsync</> will be much quicker than the first,
because it has relatively little data to transfer, and the end result
will be consistent because the server was down. This method
allows a file system backup to be performed with minimal downtime.
</para>
<para>
Note that a file system backup will not necessarily be
smaller than an SQL dump. On the contrary, it will most likely be
@@ -1111,6 +1125,19 @@ restore_command = 'copy /mnt/server/archivedir/%f "%p"' # Windows
such index after completing a recovery operation.
</para>
</listitem>
<listitem>
<para>
<command>CREATE TABLESPACE</> commands are WAL-logged with the literal
absolute path, and will therefore be replayed as tablespace creations
with the same absolute path. This might be undesirable if the log is
being replayed on a different machine. It can be dangerous even if
the log is being replayed on the same machine, but into a new data
directory: the replay will still overwrite the contents of the original
tablespace. To avoid potential gotchas of this sort, the best practice
is to take a new base backup after creating or dropping tablespaces.
</para>
</listitem>
</itemizedlist>
</para>
@@ -1121,7 +1148,9 @@ restore_command = 'copy /mnt/server/archivedir/%f "%p"' # Windows
since we may need to fix partially-written disk pages. It is not
necessary to store so many page copies for PITR operations, however.
An area for future development is to compress archived WAL data by
removing unnecessary page copies.
removing unnecessary page copies. In the meantime, administrators
may wish to reduce the number of page snapshots included in WAL by
increasing the checkpoint interval parameters as much as feasible.
</para>
</sect2>
</sect1>
@@ -1210,7 +1239,7 @@ cd ~/postgresql-&version;
gmake install
initdb -D /usr/local/pgsql/data
postmaster -D /usr/local/pgsql/data
psql template1 &lt; backup
psql -f backup template1
</programlisting>
See <xref linkend="runtime"> about ways to start and stop the