backup.sgml

PostgreSQL 8.1.4的源码 适用于Linux下的开源数据库系统

   to make a <quote>frozen snapshot</> of the volume containing the
   database, then copy the whole data directory (not just parts, see
   above) from the snapshot to a backup device, then release the frozen
   snapshot.  This will work even while the database server is running.
   However, a backup created in this way saves
   the database files in a state where the database server was not
   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 backup).
  </para>

  <para>
   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
   larger. (<application>pg_dump</application> does not need to dump
   the contents of indexes for example, just the commands to recreate
   them.)
  </para>
 </sect1>

 <sect1 id="backup-online">
  <title>On-line backup and point-in-time recovery (PITR)</title>

  <indexterm zone="backup">
   <primary>on-line backup</primary>
  </indexterm>

  <indexterm zone="backup">
   <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) 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 file-system-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>
     We do not need a perfectly consistent backup as the starting point.
     Any internal inconsistency in the backup will be corrected by log
     replay (this is not significantly different from what happens during
     crash recovery).  So we don't need file system snapshot capability,
     just <application>tar</> or a similar archiving tool.
    </para>
   </listitem>
   <listitem>
    <para>
     Since we can string together an indefinitely long sequence of WAL files
     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>
   </listitem>
   <listitem>
    <para>
     There is nothing that says we have to replay the WAL entries all the
     way to the end.  We could stop the replay at any point and have a
     consistent snapshot of the database as it was at that time.  Thus,
     this technique supports <firstterm>point-in-time recovery</>: it is
     possible to restore the database to its state at any time since your base
     backup was taken.
    </para>
   </listitem>
   <listitem>
    <para>
     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>
  </para>

  <para>
   As with the plain file-system-backup technique, this method can only
   support restoration of an entire database cluster, not a subset.
   Also, it requires a lot of archival storage: the base backup may be bulky,
   and a busy system will generate many megabytes of WAL traffic that
   have to be archived.  Still, it is the preferred backup technique in
   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 <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>
    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, 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 
    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 &mdash; 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 -i %p /mnt/server/archivedir/%f &lt;/dev/null'
</programlisting>
    which will copy archivable WAL segments to the directory
    <filename>/mnt/server/archivedir</>.  (This is an example, not a 
    recommendation, and may not work on all platforms.)
   </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 will remove or recycle it.
    However, a nonzero status tells
    <productname>PostgreSQL</> that the file was not archived; it will try
    again periodically until it succeeds.
   </para>

   <para>
    The archive command should generally be designed to refuse to overwrite
    any pre-existing archive file.  This is an important safety feature to
    preserve the integrity of your archive in case of administrator error
    (such as sending the output of two different servers to the same archive
    directory).
    It is advisable to test your proposed archive command to ensure that it
    indeed does not overwrite an existing file, <emphasis>and that it returns
    nonzero status in this case</>.  We have found that <literal>cp -i</> does
    this correctly on some platforms but not others.  If the chosen command
    does not itself handle this case correctly, you should add a command
    to test for pre-existence of the archive file.  For example, something
    like
<programlisting>
archive_command = 'test ! -f .../%f &amp;&amp; cp %p .../%f'
</programlisting>
    works correctly on most Unix variants.
   </para>

   <para>
    While designing your archiving setup, consider what will happen if
    the archive command fails repeatedly because some aspect requires 
    operator intervention or the archive runs out of space. For example, this
    could occur if you write to tape without an autochanger; when the tape 
    fills, nothing further can be archived until the tape is swapped.
    You should ensure that any error condition or request to a human operator
    is reported appropriately so that the situation can be 
    resolved relatively quickly. The <filename>pg_xlog/</> directory will
    continue to fill with WAL segment files until the situation is resolved.
   </para>

   <para>
    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
    data that would be lost in the event of a disaster. It will also mean that
    the <filename>pg_xlog/</> directory will contain large numbers of
    not-yet-archived segment files, which could eventually exceed available
    disk space. You are advised to monitor the archiving process to ensure that
    it is working as you intend.
   </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 completed WAL segments. 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 file names 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>

   <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</>), since those are edited manually rather
    than through SQL operations.
    You may wish to keep the configuration files in a location that will
    be backed up by your regular file system backup procedures.  See
    <xref linkend="runtime-config-file-locations"> for how to relocate the
    configuration files.
   </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.)
     <function>pg_start_backup</> creates a <firstterm>backup label</> file,
     called <filename>backup_label</>, in the cluster directory with
     information about your backup.
    </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>
   <listitem>
    <para>
     Perform the backup, using any convenient file-system-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.