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Improvements to the PITR docs. Initial patch from Gavin Sherry, additional

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improvements by Neil Conway.
REL8_0_STABLE
Neil Conway 21 years ago
parent a236dd9536
commit 1c7a47cea4
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  1. 265
      doc/src/sgml/backup.sgml

265
doc/src/sgml/backup.sgml
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@ -1,5 +1,5 @@
<!--
$PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.49 2004/11/08 18:01:28 tgl Exp $
$PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.50 2004/11/14 06:10:12 neilc Exp $
-->
<chapter id="backup">
<title>Backup and Restore</title>
@ -358,7 +358,7 @@ tar -cf backup.tar /usr/local/pgsql/data
properly shut down; therefore, when you start the database server
on the backed-up data, it will think the server had crashed
and replay the WAL log. This is not a problem, just be aware of
it (and be sure to include the WAL files in your dump).
it (and be sure to include the WAL files in your backup).
</para>
<para>
@ -379,7 +379,7 @@ tar -cf backup.tar /usr/local/pgsql/data
</sect1>
<sect1 id="backup-online">
<title>On-line backup and point-in-time recovery</title>
<title>On-line backup and point-in-time recovery (PITR)</title>
<indexterm zone="backup">
<primary>on-line backup</primary>
@ -389,18 +389,25 @@ tar -cf backup.tar /usr/local/pgsql/data
<primary>point-in-time recovery</primary>
</indexterm>
<indexterm zone="backup">
<primary>PITR</primary>
</indexterm>
<para>
At all times, <productname>PostgreSQL</> maintains a <firstterm>write ahead
log</> (WAL) that shows details of every change made to the database's data
files. This log exists primarily for crash-safety purposes: if the system
crashes, the database can be restored to consistency by <quote>replaying</>
the log entries made since the last checkpoint. However, the existence
At all times, <productname>PostgreSQL</> maintains a
<firstterm>write ahead log</> (WAL) in the <filename>pg_xlog/</>
subdirectory of the cluster's data directory. The log describes
every change made to the database's data files. This log exists
primarily for crash-safety purposes: if the system crashes, the
database can be restored to consistency by <quote>replaying</> the
log entries made since the last checkpoint. However, the existence
of the log makes it possible to use a third strategy for backing up
databases: we can combine a filesystem-level backup with backup of the WAL
files. If recovery is needed, we restore the backup and then replay from
the backed-up WAL files to bring the backup up to current time. This
approach is notably more complex to administer than either of the previous
approaches, but it has some significant benefits to offer:
databases: we can combine a filesystem-level backup with backup of
the WAL files. If recovery is needed, we restore the backup and
then replay from the backed-up WAL files to bring the backup up to
current time. This approach is more complex to administer than
either of the previous approaches, but it has some significant
benefits:
<itemizedlist>
<listitem>
<para>
@ -414,7 +421,7 @@ tar -cf backup.tar /usr/local/pgsql/data
<listitem>
<para>
Since we can string together an indefinitely long sequence of WAL files
for replay, continuous backup can be had simply by continuing to archive
for replay, continuous backup can be achieved simply by continuing to archive
the WAL files. This is particularly valuable for large databases, where
it may not be convenient to take a full backup frequently.
</para>
@ -431,10 +438,11 @@ tar -cf backup.tar /usr/local/pgsql/data
</listitem>
<listitem>
<para>
If we continuously feed the series of WAL files to another machine
that's been loaded with the same base backup file, we have a <quote>hot
standby</> system: at any point we can bring up the second machine
and it will have a nearly-current copy of the database.
If we continuously feed the series of WAL files to another
machine that has been loaded with the same base backup file, we
have a <quote>hot standby</> system: at any point we can bring up
the second machine and it will have a nearly-current copy of the
database.
</para>
</listitem>
</itemizedlist>
@ -464,15 +472,16 @@ tar -cf backup.tar /usr/local/pgsql/data
<para>
In an abstract sense, a running <productname>PostgreSQL</> system
produces an indefinitely long sequence of WAL records. The system
physically divides this sequence into WAL <firstterm>segment files</>,
which are normally 16Mb apiece (although the size can be altered when
building the server). The segment files are given numeric names that
reflect their position in the abstract WAL sequence. When not using WAL
archiving, the system normally creates just a few segment files and then
<quote>recycles</> them by renaming no-longer-needed segment files to
higher segment numbers. It's assumed that a segment file whose contents
precede the checkpoint-before-last is no longer of interest and can be
recycled.
physically divides this sequence into WAL <firstterm>segment
files</>, which are normally 16MB apiece (although the size can be
altered when building <productname>PostgreSQL</>). The segment
files are given numeric names that reflect their position in the
abstract WAL sequence. When not using WAL archiving, the system
normally creates just a few segment files and then
<quote>recycles</> them by renaming no-longer-needed segment files
to higher segment numbers. It's assumed that a segment file whose
contents precede the checkpoint-before-last is no longer of
interest and can be recycled.
</para>
<para>
@ -481,7 +490,8 @@ tar -cf backup.tar /usr/local/pgsql/data
file is recycled for reuse. Depending on the application and the
available hardware, there could be many different ways of <quote>saving
the data somewhere</>: we could copy the segment files to an NFS-mounted
directory on another machine, or write them onto a tape drive, or batch
directory on another machine, write them onto a tape drive (ensuring that
you have a way of restoring the file with its original file name), or batch
them together and burn them onto CDs, or something else entirely. To
provide the database administrator with as much flexibility as possible,
<productname>PostgreSQL</> tries not to make any assumptions about how
@ -561,7 +571,7 @@ archive_command = 'test ! -f .../%f &amp;&amp; cp %p .../%f'
</para>
<para>
Speed of the archiving command is not important, so long as it can keep up
The speed of the archiving command is not important, so long as it can keep up
with the average rate at which your server generates WAL data. Normal
operation continues even if the archiving process falls a little behind.
If archiving falls significantly behind, this will increase the amount of
@ -573,24 +583,24 @@ archive_command = 'test ! -f .../%f &amp;&amp; cp %p .../%f'
</para>
<para>
If you are concerned about being able to recover right up to the current
instant, you may want to take additional steps to ensure that the current,
partially-filled WAL segment is also copied someplace. This is
particularly important if your server generates only little WAL traffic
(or has slack periods where it does so), since it could take a long time
before a WAL segment file is completely filled and ready to archive.
One possible way to handle this is to set up a <application>cron</> job
that periodically (once a minute, perhaps) identifies the current WAL
segment file and saves it someplace safe. Then the combination of the
archived WAL segments and the saved current segment will be enough to
ensure you can always restore to within a minute of current time. This
behavior is not presently built into <productname>PostgreSQL</> because
we did not want to complicate the definition of the <xref
linkend="guc-archive-command"> by requiring it to keep track of
successively archived, but different, copies of the same WAL file.
The <xref linkend="guc-archive-command"> is only invoked on finished
WAL segments that will not change anymore; and except in the case of
retrying a failure, it will be called only once for any given file name.
If you are concerned about being able to recover right up to the
current instant, you may want to take additional steps to ensure that
the current, partially-filled WAL segment is also copied someplace.
This is particularly important if your server generates only little WAL
traffic (or has slack periods where it does so), since it could take a
long time before a WAL segment file is completely filled and ready to
archive. One possible way to handle this is to set up a
<application>cron</> job that periodically (once a minute, perhaps)
identifies the current WAL segment file and saves it someplace safe.
Then the combination of the archived WAL segments and the saved current
segment will be enough to ensure you can always restore to within a
minute of current time. This behavior is not presently built into
<productname>PostgreSQL</> because we did not want to complicate the
definition of the <xref linkend="guc-archive-command"> by requiring it
to keep track of successively archived, but different, copies of the
same WAL file. The <xref linkend="guc-archive-command"> is only
invoked on completed WAL segments. Except in the case of retrying a
failure, it will be called only once for any given file name.
</para>
<para>
@ -600,6 +610,14 @@ archive_command = 'test ! -f .../%f &amp;&amp; cp %p .../%f'
remember the original full path (<literal>%p</>) but it is necessary to
remember the file name (<literal>%f</>).
</para>
<para>
Note that although WAL archiving will allow you to restore any
modifications made to the data in your <productname>PostgreSQL</> database
it will not restore changes made to configuration files (that is,
<filename>postgresql.conf</>, <filename>pg_hba.conf</> and
<filename>pg_ident.conf</>) after the initial base backup.
</para>
</sect2>
<sect2 id="backup-base-backup">
@ -620,10 +638,16 @@ archive_command = 'test ! -f .../%f &amp;&amp; cp %p .../%f'
SELECT pg_start_backup('label');
</programlisting>
where <literal>label</> is any string you want to use to uniquely
identify this backup operation. (One good practice is to use the
full path where you intend to put the backup dump file.) It does
not matter which database within the cluster you connect to to issue
this command. You can ignore the result returned by the function;
identify this backup operation. <function>pg_start_backup</> creates
a <firstterm>backup label</> file, called <filename>backup_label</>,
in the cluster directory with information about your backup.
One good practice is to use the full path where you intend to put the
backup dump file as.
</para>
<para>
It does not matter which database within the cluster you connect to to
issue this command. You can ignore the result returned by the function;
but if it reports an error, deal with that before proceeding.
</para>
</listitem>
@ -653,7 +677,7 @@ SELECT pg_stop_backup();
nor between the end of the backup and <function>pg_stop_backup</>; a
few minutes' delay won't hurt anything. You
must however be quite sure that these operations are carried out in
sequence and don't overlap.
sequence and do not overlap.
</para>
<para>
@ -698,17 +722,17 @@ SELECT pg_stop_backup();
<para>
Since you have to keep around all the archived WAL files back to your
last full dump, your interval between full dumps would usually be chosen
based on how much storage you want to expend on archived WAL files.
You should also consider how long you are prepared to spend recovering,
if recovery should be necessary --- the system will have to replay all
those segments, and that could take awhile if it's been a long time
since the full dump.
last base backup, the interval between base backups should usually be
chosen based on how much storage you want to expend on archived WAL
files. You should also consider how long you are prepared to spend
recovering, if recovery should be necessary --- the system will have to
replay all those WAL segments, and that could take awhile if it has
been a long time since the last base backup.
</para>
<para>
It's also worth noting that the <function>pg_start_backup</> function
makes a file named <literal>backup_label</> in the database cluster
makes a file named <filename>backup_label</> in the database cluster
directory, which is then removed again by <function>pg_stop_backup</>.
This file will of course be archived as a part of your backup dump file.
The backup label file includes the label string you gave to
@ -721,11 +745,11 @@ SELECT pg_stop_backup();
<para>
It is also possible to make a backup dump while the postmaster is
stopped. In this case, obviously you can't use
stopped. In this case, you obviously cannot use
<function>pg_start_backup</> or <function>pg_stop_backup</>, and
you will therefore be left to your own devices to keep track of which
backup dump is which and how far back the associated WAL files go.
It's generally better to follow the on-line backup procedure above.
It is generally better to follow the on-line backup procedure above.
</para>
</sect2>
@ -738,12 +762,19 @@ SELECT pg_stop_backup();
<orderedlist>
<listitem>
<para>
Stop the postmaster, if it's running, and clean out all existing files
under the cluster data directory and under the root directories of any
tablespaces you are using.
(If there are recent, unarchived WAL segment files in
<filename>pg_xlog/</> that you want to use during restore, move these aside
instead of removing them.)
Stop the postmaster, if it's running. If you have the space to do so,
copy the cluster data directory and any tablespaces to a temporary
location so that you can reference them later. Note that this will
require that you have enough free space on your system to hold two
copies of your existing database. If you do not have enough space,
you need at the least to backup the <filename>pg_xlog</> directory in
the cluster data directory as it may contain logs which were not archived
before the system went down.
</para>
<para>
Next, clean out all existing files under the cluster data directory and
under the root directories of any tablespaces you are using.
</para>
</listitem>
<listitem>
@ -766,18 +797,18 @@ SELECT pg_stop_backup();
</listitem>
<listitem>
<para>
If you had unarchived WAL segment files that you saved aside in step 1,
copy them into <filename>pg_xlog/</>. (It's best to copy them, not move
them back in, so that you still have the unmodified files if the worst
happens and you have to start over.)
If you had unarchived WAL segment files that you saved in step 1,
copy them into <filename>pg_xlog/</>. (It is best to copy them,
not move them, so that you still have the unmodified files if a
problem occurs and you have to start over.)
</para>
</listitem>
<listitem>
<para>
Create a recovery command file <filename>recovery.conf</> in the cluster
data directory, as discussed below. You may also want to temporarily
modify <filename>pg_hba.conf</> to prevent ordinary users from connecting
until you are sure the recovery has worked.
data directory (see <xref linkend="recovery-config-settings">). You may
also want to temporarily modify <filename>pg_hba.conf</> to prevent
ordinary users from connecting until you are sure the recovery has worked.
</para>
</listitem>
<listitem>
@ -801,20 +832,20 @@ SELECT pg_stop_backup();
</para>
<para>
The key part of all this is to set up a recovery command file
that describes how you want to recover and how far the recovery
should run. You can use <filename>recovery.conf.sample</> (normally
The key part of all this is to set up a recovery command file that
describes how you want to recover and how far the recovery should
run. You can use <filename>recovery.conf.sample</> (normally
installed in the installation <filename>share/</> directory) as a
prototype. The one thing that you absolutely must specify in
<filename>recovery.conf</> is the <literal>restore_command</>,
which tells how to get back archived WAL file segments. Like
the <literal>archive_command</>, this is a shell command string.
It may contain <literal>%f</>,
which is replaced by the name of the desired log file, and <literal>%p</>,
<filename>recovery.conf</> is the <varname>restore_command</>,
which tells <productname>PostgreSQL</> how to get back archived
WAL file segments. Like the <varname>archive_command</>, this is
a shell command string. It may contain <literal>%f</>, which is
replaced by the name of the desired log file, and <literal>%p</>,
which is replaced by the absolute path to copy the log file to.
Write <literal>%%</> if you need to embed an actual <literal>%</>
character in the command. The simplest useful command is something
like
character in the command. The simplest useful command is
something like
<programlisting>
restore_command = 'cp /mnt/server/archivedir/%f %p'
</programlisting>
@ -844,34 +875,36 @@ restore_command = 'cp /mnt/server/archivedir/%f %p'
<para>
Normally, recovery will proceed through all available WAL segments,
thereby restoring the database to current time (or as close as we can
get given the available WAL segments). But if you want to recover to
some previous point in time (say, right before the junior DBA dropped your
main transaction table), just specify the required stopping point in
<filename>recovery.conf</>. You can specify the stop point, known as the
<quote>recovery target</>, either by date/time or by completion of a
specific transaction ID. As of this writing
only the date/time option is very usable, since there are no tools
to help you identify with any accuracy which transaction ID to use.
</para>
<para>
Note that the stop point must be after the ending time of the backup
(ie, the time of <function>pg_stop_backup</>). You cannot use a base
backup to recover to a time when that backup was still going on. (To
recover to such a time, you must go back to your previous base backup
and roll forward from there.)
thereby restoring the database to the current point in time (or as
close as we can get given the available WAL segments). But if you want
to recover to some previous point in time (say, right before the junior
DBA dropped your main transaction table), just specify the required
stopping point in <filename>recovery.conf</>. You can specify the stop
point, known as the <quote>recovery target</>, either by date/time or
by completion of a specific transaction ID. As of this writing only
the date/time option is very usable, since there are no tools to help
you identify with any accuracy which transaction ID to use.
</para>
<sect3 id="recovery-config-settings">
<note>
<para>
The stop point must be after the ending time of the base backup (the
time of <function>pg_stop_backup</>). You cannot use a base backup
to recover to a time when that backup was still going on. (To
recover to such a time, you must go back to your previous base backup
and roll forward from there.)
</para>
</note>
<sect3 id="recovery-config-settings" xreflabel="Recovery Settings">
<title>Recovery Settings</title>
<para>
These settings can only be made in the
<filename>recovery.conf</filename> file, and apply only for the
duration of the recovery. They must be reset for any subsequent
recovery you wish to perform. They cannot be changed once recovery
has begun.
</para>
<para>
These settings can only be made in the <filename>recovery.conf</>
file, and apply only for the duration of the recovery. They must be
reset for any subsequent recovery you wish to perform. They cannot be
changed once recovery has begun.
</para>
<variablelist>
@ -889,11 +922,10 @@ restore_command = 'cp /mnt/server/archivedir/%f %p'
in the command.
</para>
<para>
It is important for the command to return a zero exit status only if
it succeeds. The command <emphasis>will</> be asked for file names
that are not present in the archive;
it must return nonzero when so asked.
Examples:
It is important for the command to return a zero exit status only
if it succeeds. The command <emphasis>will</> be asked for file
names that are not present in the archive; it must return nonzero
when so asked. Examples:
<programlisting>
restore_command = 'cp /mnt/server/archivedir/%f "%p"'
restore_command = 'copy /mnt/server/archivedir/%f "%p"' # Windows
@ -996,7 +1028,7 @@ restore_command = 'copy /mnt/server/archivedir/%f "%p"' # Windows
Tuesday evening, and are up and running. In <emphasis>this</> history of
the database universe, you never dropped the table at all. But suppose
you later realize this wasn't such a great idea after all, and would like
to return to some later point in the original history? You won't be able
to return to some later point in the original history. You won't be able
to if, while your database was up-and-running, it overwrote some of the
sequence of WAL segment files that led up to the time you now wish you
could get back to. So you really want to distinguish the series of
@ -1060,9 +1092,8 @@ restore_command = 'copy /mnt/server/archivedir/%f "%p"' # Windows
<para>
Operations on non-btree indexes (hash, R-tree, and GiST indexes) are
not presently WAL-logged, so replay will not update these index types.
The recommended workaround, if you use any non-btree indexes, is to
manually <command>REINDEX</> each such index after completing a
recovery operation.
The recommended workaround is to manually <command>REINDEX</> each
such index after completing a recovery operation.
</para>
</listitem>
</itemizedlist>

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