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Preliminary documentation for PITR.

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doc/src/sgml/backup.sgml
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<!--
$PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.40 2004/08/03 20:32:30 tgl Exp $
$PostgreSQL: pgsql/doc/src/sgml/backup.sgml,v 2.41 2004/08/03 23:42:59 tgl Exp $
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<chapter id="backup">
<title>Backup and Restore</title>
@ -381,6 +381,14 @@ tar -cf backup.tar /usr/local/pgsql/data
<sect1 id="backup-online">
<title>On-line backup and point-in-time recovery</title>
<indexterm zone="backup">
<primary>on-line backup</primary>
</indexterm>
<indexterm zone="backup">
<primary>point-in-time recovery</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
@ -424,7 +432,7 @@ tar -cf backup.tar /usr/local/pgsql/data
<listitem>
<para>
If we continuously feed the series of WAL files to another machine
that's been loaded with the same base backup, we have a <quote>hot
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.
</para>
@ -441,6 +449,448 @@ tar -cf backup.tar /usr/local/pgsql/data
many situations where high reliability is needed.
</para>
<para>
To recover successfully using an on-line backup, you need a continuous
sequence of archived WAL files that extends back at least as far as the
start time of your backup. So to get started, you should set up and test
your procedure for archiving WAL files <emphasis>before</> you take your
first base backup. Accordingly, we first discuss the mechanics of
archiving WAL files.
</para>
<sect2 id="backup-archiving-wal">
<title>Setting up WAL archiving</title>
<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.
</para>
<para>
When archiving WAL data, we want to capture the contents of each segment
file once it is filled, and save that data somewhere before the segment
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
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
the archiving will be done. Instead, <productname>PostgreSQL</> lets
the administrator specify a shell command to be executed to copy a
completed segment file to wherever it needs to go. The command could be
as simple as a <application>cp</>, or it could invoke a complex shell
script --- it's all up to you.
</para>
<para>
The shell command to use is specified by the <xref
linkend="guc-archive-command"> configuration parameter, which in practice
will always be placed in the <filename>postgresql.conf</filename> file.
In this string,
any <literal>%p</> is replaced by the absolute path of the file to
archive, while any <literal>%f</> is replaced by the file name only.
Write <literal>%%</> if you need to embed an actual <literal>%</>
character in the command. The simplest useful command is something
like
<programlisting>
archive_command = 'cp %p /mnt/server/archivedir/%f'
</programlisting>
which will copy archivable WAL segments to the directory
<literal>/mnt/server/archivedir</>.
</para>
<para>
The archive command will be executed under the ownership of the same
user that the <productname>PostgreSQL</> server is running as. Since
the series of WAL files being archived contains effectively everything
in your database, you will want to be sure that the archived data is
protected from prying eyes; for example, archive into a directory that
does not have group or world read access.
</para>
<para>
It is important that the archive command return zero exit status if and
only if it succeeded. Upon getting a zero result,
<productname>PostgreSQL</> will assume that the WAL segment file has been
successfully archived, and it may be overwritten with new data very
soon thereafter. However, a nonzero status tells
<productname>PostgreSQL</> that the file was not archived; it will try
again periodically until it succeeds.
</para>
<para>
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. It is okay
if the archiving process falls a little behind (or even a lot behind, if
you don't mind the <literal>pg_xlog/</> directory filling up with
not-yet-archived segment files).
</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. The combination of the archived
WAL segments and the saved current segment will then 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.
</para>
<para>
In writing your archive command, you should assume that the filenames to
be archived may be up to 64 characters long and may contain any
combination of ASCII letters, digits, and dots. It is not necessary to
remember the original full path (<literal>%p</>) but it is necessary to
remember the file name (<literal>%f</>).
</para>
</sect2>
<sect2 id="backup-base-backup">
<title>Making a Base Backup</title>
<para>
The procedure for making a base backup is relatively simple:
<orderedlist>
<listitem>
<para>
Ensure that WAL archiving is enabled and working.
</para>
</listitem>
<listitem>
<para>
Connect to the database as a superuser, and issue the command
<programlisting>
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;
but if it reports an error, deal with that before proceeding.
</para>
</listitem>
<listitem>
<para>
Perform the backup, using any convenient filesystem-backup tool
such as <application>tar</> or <application>cpio</>. It is neither
necessary nor desirable to stop normal operation of the database
while you do this.
</para>
</listitem>
<listitem>
<para>
Again connect to the database as a superuser, and issue the command
<programlisting>
SELECT pg_stop_backup();
</programlisting>
If this returns successfully, you're done.
</para>
</listitem>
</orderedlist>
</para>
<para>
It is not necessary to be very concerned about the amount of time elapsed
between <function>pg_start_backup</> and the start of the actual 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.
</para>
<para>
Be certain that your backup dump includes all of the files underneath
the database cluster directory (e.g., <literal>/usr/local/pgsql/data</>).
If you are using tablespaces that do not reside underneath this directory,
be careful to include them as well (and be sure that your backup dump
archives symbolic links as links, otherwise the restore will mess up
your tablespaces).
</para>
<para>
You may, however, omit from the backup dump the files within the
<literal>pg_xlog/</> subdirectory of the cluster directory. This
slight complication is worthwhile because it reduces the risk
of mistakes when restoring. This is easy to arrange if
<literal>pg_xlog/</> is a symbolic link pointing to someplace outside
the cluster directory, which is a common setup anyway for performance
reasons.
</para>
<para>
To make use of this backup, you will need to keep around all the WAL
segment files generated at or after the starting time of the backup.
To aid you in doing this, the <function>pg_stop_backup</> function
creates a <firstterm>backup history file</> that is immediately stored
into the WAL archive area. This file is named after the first WAL
segment file that you need to have to make use of the backup. For
example, if the starting WAL file is <literal>0000000100001234000055CD</>
the backup history file will be named something like
<literal>0000000100001234000055CD.007C9330.backup</>. (The second part of
this file name stands for an exact position within the WAL file, and can
ordinarily be ignored.) Once you have safely archived the backup dump
file, you can delete all archived WAL segments with names numerically
preceding this one. The backup history file is just a small text file.
It contains the label string you gave to <function>pg_start_backup</>, as
well as the starting and ending times of the backup. If you used the
label to identify where the associated dump file is kept, then the
archived history file is enough to tell you which dump file to restore,
should you need to do so.
</para>
<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.
</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
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
<function>pg_start_backup</>, as well as the time at which
<function>pg_start_backup</> was run, and the name of the starting WAL
file. In case of confusion it will
therefore be possible to look inside a backup dump file and determine
exactly which backup session the dump file came from.
</para>
<para>
It is also possible to make a backup dump while the postmaster is
stopped. In this case, obviously you can't 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.
</para>
</sect2>
<sect2 id="backup-pitr-recovery">
<title>Recovering with an On-line Backup</title>
<para>
Okay, the worst has happened and you need to recover from your backup.
Here is the procedure:
<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
<literal>pg_xlog/</> that you want to use during restore, move these aside
instead of removing them.)
</para>
</listitem>
<listitem>
<para>
Restore the database files from your backup dump. Be careful that they
are restored with the right ownership (the database system user, not
root!) and with the right permissions. If you are using tablespaces,
you may want to verify that the symbolic links in <literal>pg_tblspc/</>
were correctly restored.
</para>
</listitem>
<listitem>
<para>
Remove any files present in <literal>pg_xlog/</>; these came from the
backup dump and are therefore probably obsolete rather than current.
If you didn't archive <literal>pg_xlog/</> at all, then re-create it,
and be sure to re-create the subdirectory
<literal>pg_xlog/archive_status/</> as well.
</para>
</listitem>
<listitem>
<para>
If you had unarchived WAL segment files that you saved aside in step 1,
copy them into <literal>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.)
</para>
</listitem>
<listitem>
<para>
Create a recovery command file <literal>recovery.conf</> in the cluster
data directory, as discussed below. You may also want to temporarily
modify <literal>pg_hba.conf</> to prevent ordinary users from connecting
until you are sure the recovery has worked.
</para>
</listitem>
<listitem>
<para>
Start the postmaster. The postmaster will go into recovery mode and
proceed to read through the archived WAL files it needs. Upon completion
of the recovery process, the postmaster will rename
<literal>recovery.conf</> to <literal>recovery.done</> (to prevent
accidentally re-entering recovery mode in case of a crash later) and then
commence normal database operations.
</para>
</listitem>
<listitem>
<para>
Inspect the contents of the database to ensure you have recovered to
where you want to be. If not, return to step 1. If all is well,
let in your users by restoring <literal>pg_hba.conf</> to normal.
</para>
</listitem>
</orderedlist>
</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 <literal>recovery.conf.sample</> (normally
installed in the installation <literal>share/</> directory) as a
prototype. The one thing that you absolutely must specify in
<literal>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</>,
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
<programlisting>
restore_command = 'cp /mnt/server/archivedir/%f %p'
</programlisting>
which will copy previously archived WAL segments from the directory
<literal>/mnt/server/archivedir</>. You could of course use something
much more complicated, perhaps even a shell script that requests the
operator to mount an appropriate tape.
</para>
<para>
It is important that the command return nonzero exit status on failure.
The command <emphasis>will</> be asked for log files that are not present
in the archive; it must return nonzero when so asked. This is not an
error condition. Be aware also that the basename of the <literal>%p</>
path will be different from <literal>%f</>; do not expect them to be
interchangeable.
</para>
<para>
WAL segments that cannot be found in the archive will be sought in
<literal>pg_xlog/</>; this allows use of recent un-archived segments.
However segments that are available from the archive will be used in
preference to files in <literal>pg_xlog/</>. The system will not
overwrite the existing contents of <literal>pg_xlog/</> when retrieving
archived files.
</para>
<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
<literal>recovery.conf</>. You can specify the stop point either by
date/time or by transaction ID. As of this writing only the date/time
option is very usable, since there are no tools to help you identify
which transaction ID to use. 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.)
</para>
</sect2>
<sect2 id="backup-timelines">
<title>Timelines</title>
<indexterm zone="backup">
<primary>timelines</primary>
</indexterm>
<para>
The ability to restore the database to a previous point in time creates
some complexities that are akin to science-fiction stories about time
travel and parallel universes. In the original history of the database,
perhaps you dropped a critical table at 5:15PM on Tuesday evening.
Unfazed, you get out your backup, restore to the point-in-time 5:14PM
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 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
WAL records generated after you've done a point-in-time recovery from
those that were generated in the original database history.
</para>
<para>
To deal with these problems, <productname>PostgreSQL</> has a notion
of <firstterm>timelines</>. Each time you recover to a point-in-time
earlier than the end of the WAL sequence, a new timeline is created
to identify the series of WAL records generated after that recovery.
(If recovery proceeds all the way to the end of WAL, however, we do not
start a new timeline: we just extend the existing one.) The timeline
ID number is part of WAL segment file names, and so a new timeline does
not overwrite the WAL data generated by previous timelines. It is
in fact possible to archive many different timelines. While that might
seem like a useless feature, it's often a lifesaver. Consider the
situation where you aren't quite sure what point-in-time to recover to,
and so have to do several point-in-time recoveries by trial and error
until you find the best place to branch off from the old history. Without
timelines this process would soon generate an unmanageable mess. With
timelines, you can recover to <emphasis>any</> prior state, including
states in timeline branches that you later abandoned.
</para>
<para>
Each time a new timeline is created, <productname>PostgreSQL</> creates
a <quote>timeline history</> file that shows which timeline it branched
off from and when. These history files are necessary to allow the system
to pick the right WAL segment files when recovering from an archive that
contains multiple timelines. Therefore, they are archived into the WAL
archive area just like WAL segment files. The history files are just
small text files, so it's cheap and appropriate to keep them around
indefinitely (unlike the segment files which are large). You can, if
you like, add comments to a history file to make your own notes about
how and why this particular timeline came to be. Such comments will be
especially valuable when you have a thicket of different timelines as
a result of experimentation.
</para>
<para>
The default behavior of recovery is to recover along the same timeline
that was current when the base backup was taken. If you want to recover
into some child timeline (that is, you want to return to some state that
was itself generated after a recovery attempt), you need to specify the
target timeline in <literal>recovery.conf</>. You cannot recover into
timelines that branched off earlier than the base backup.
</para>
</sect2>
</sect1>
<sect1 id="migration">

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$PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.270 2004/07/24 19:51:22 tgl Exp $
$PostgreSQL: pgsql/doc/src/sgml/runtime.sgml,v 1.271 2004/08/03 23:42:59 tgl Exp $
-->
<Chapter Id="runtime">
@ -1342,6 +1342,30 @@ SET ENABLE_SEQSCAN TO OFF;
</para>
</listitem>
</varlistentry>
</variablelist>
</sect3>
<sect3 id="runtime-config-wal-archiving">
<title>Archiving</title>
<variablelist>
<varlistentry id="guc-archive-command" xreflabel="archive_command">
<term><varname>archive_command</varname> (<type>string</type>)</term>
<listitem>
<para>
The shell command to execute to archive a completed segment of the
WAL file series. If this is an empty string (which is the default),
WAL archiving is disabled. Any <literal>%p</> in the string is
replaced
by the absolute path of the file to archive, while any <literal>%f</>
is replaced by the file name only. Write <literal>%%</> if you need
to embed an actual <literal>%</> character in the command. For more
information see <xref linkend="backup-archiving-wal">. This option
can only be set at server start or in the
<filename>postgresql.conf</filename> file.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect3>