backup.sgml

PostgreSQL 8.1.4的源码 适用于Linux下的开源数据库系统

        The precise stopping point is also influenced by 
        <xref linkend="recovery-target-inclusive">.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="recovery-target-xid" xreflabel="recovery_target_xid">
      <term><varname>recovery_target_xid</varname> (<type>string</type>)</term>
      <listitem>
       <para>
        This parameter specifies the transaction ID up to which recovery
        will proceed. Keep in mind 
        that while transaction IDs are assigned sequentially at transaction 
        start, transactions can complete in a different numeric order.
        The transactions that will be recovered are those that committed
        before (and optionally including) the specified one.
        At most one of <varname>recovery_target_xid</> and
        <xref linkend="recovery-target-time"> can be specified.
        The default is to recover to the end of the WAL log.
        The precise stopping point is also influenced by 
        <xref linkend="recovery-target-inclusive">.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="recovery-target-inclusive" 
                   xreflabel="recovery_target_inclusive">
      <term><varname>recovery_target_inclusive</varname> 
        (<type>boolean</type>)
      </term>
      <listitem>
       <para>
        Specifies whether we stop just after the specified recovery target
        (<literal>true</literal>), or just before the recovery target 
        (<literal>false</literal>).
        Applies to both <xref linkend="recovery-target-time">
        and <xref linkend="recovery-target-xid">, whichever one is
        specified for this recovery.  This indicates whether transactions
        having exactly the target commit time or ID, respectively, will
        be included in the recovery.  Default is <literal>true</>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="recovery-target-timeline" 
                   xreflabel="recovery_target_timeline">
      <term><varname>recovery_target_timeline</varname> 
        (<type>string</type>)
      </term>
      <listitem>
       <para>
        Specifies recovering into a particular timeline.  The default is
        to recover along the same timeline that was current when the
        base backup was taken.  You would only need to set this parameter
        in complex re-recovery situations, where you need to return to
        a state that itself was reached after a point-in-time recovery.
        See <xref linkend="backup-timelines"> for discussion.
       </para>
      </listitem>
     </varlistentry>

   </variablelist>

   </sect3>

  </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 ID in <filename>recovery.conf</>.  You cannot recover into
    timelines that branched off earlier than the base backup.
   </para>
  </sect2>

  <sect2 id="backup-online-caveats">
   <title>Caveats</title>

   <para>
    At this writing, there are several limitations of the on-line backup
    technique.  These will probably be fixed in future releases:

  <itemizedlist>
   <listitem>
    <para>
     Operations on hash and R-tree indexes are
     not presently WAL-logged, so replay will not update these index types.
     The recommended workaround is to manually <command>REINDEX</> each
     such index after completing a recovery operation.
    </para>
   </listitem>

   <listitem>
    <para>
     If a <command>CREATE DATABASE</> command is executed while a base
     backup is being taken, and then the template database that the
     <command>CREATE DATABASE</> copied is modified while the base backup
     is still in progress, it is possible that recovery will cause those
     modifications to be propagated into the created database as well.
     This is of course undesirable.  To avoid this risk, it is best not to
     modify any template databases while taking a base backup.
    </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>

   <para>
    It should also be noted that the default <acronym>WAL</acronym>
    format is fairly bulky since it includes many disk page snapshots.
    These page snapshots are designed to support crash recovery,
    since we may need to fix partially-written disk pages.  Depending
    on your system hardware and software, the risk of partial writes may
    be small enough to ignore, in which case you can significantly reduce
    the total volume of archived logs by turning off page snapshots 
    using the <xref linkend="guc-full-page-writes"> parameter.
    (Read the notes and warnings in 
    <xref linkend="wal"> before you do so.)
    Turning off page snapshots does not prevent use of the logs for PITR
    operations.
    An area for future development is to compress archived WAL data by
    removing unnecessary page copies even when <varname>full_page_writes</>
    is on.  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>

 <sect1 id="migration">
  <title>Migration Between Releases</title>

  <indexterm zone="migration">
   <primary>upgrading</primary>
  </indexterm>

  <indexterm zone="migration">
   <primary>version</primary>
   <secondary>compatibility</secondary>
  </indexterm>

  <para>
   This section discusses how to migrate your database data from one
   <productname>PostgreSQL</> release to a newer one.
   The software installation procedure <foreignphrase>per se</> is not the
   subject of this section; those details are in <xref linkend="installation">.
  </para>

  <para>
   As a general rule, the internal data storage format is subject to
   change between major releases of <productname>PostgreSQL</> (where
   the number after the first dot changes). This does not apply to
   different minor releases under the same major release (where the
   number after the second dot changes); these always have compatible
   storage formats. For example, releases 7.0.1, 7.1.2, and 7.2 are
   not compatible, whereas 7.1.1 and 7.1.2 are. When you update
   between compatible versions, you can simply replace the executables
   and reuse the data directory on disk. Otherwise you need to back
   up your data and restore it on the new server.  This has to be done
   using <application>pg_dump</>; file system level backup methods
   obviously won't work. There are checks in place that prevent you
   from using a data directory with an incompatible version of
   <productname>PostgreSQL</productname>, so no great harm can be done by
   trying to start the wrong server version on a data directory.
  </para>

  <para>
   It is recommended that you use the <application>pg_dump</> and
   <application>pg_dumpall</> programs from the newer version of
   <productname>PostgreSQL</>, to take advantage of any enhancements
   that may have been made in these programs.  Current releases of the
   dump programs can read data from any server version back to 7.0.
  </para>

  <para>
   The least downtime can be achieved by installing the new server in
   a different directory and running both the old and the new servers
   in parallel, on different ports. Then you can use something like

<programlisting>
pg_dumpall -p 5432 | psql -d postgres -p 6543
</programlisting>

   to transfer your data.  Or use an intermediate file if you want.
   Then you can shut down the old server and start the new server at
   the port the old one was running at. You should make sure that the
   old database is not updated after you run <application>pg_dumpall</>,
   otherwise you will obviously lose that data. See <xref
   linkend="client-authentication"> for information on how to prohibit
   access.
  </para>

  <para>
   In practice you probably want to test your client
   applications on the new setup before switching over completely.
   This is another reason for setting up concurrent installations
   of old and new versions.
  </para>

  <para>
   If you cannot or do not want to run two servers in parallel you can
   do the backup step before installing the new version, bring down
   the server, move the old version out of the way, install the new
   version, start the new server, restore the data. For example:

<programlisting>
pg_dumpall &gt; backup
pg_ctl stop
mv /usr/local/pgsql /usr/local/pgsql.old
cd ~/postgresql-&version;
gmake install
initdb -D /usr/local/pgsql/data
postmaster -D /usr/local/pgsql/data
psql -f backup postgres
</programlisting>

   See <xref linkend="runtime"> about ways to start and stop the
   server and other details. The installation instructions will advise
   you of strategic places to perform these steps.
  </para>

  <note>
   <para>
    When you <quote>move the old installation out of the way</quote>
    it may no longer be perfectly usable. Some of the executable programs
    contain absolute paths to various installed programs and data files.
    This is usually not a big problem but if you plan on using two
    installations in parallel for a while you should assign them
    different installation directories at build time.  (This problem
    is rectified in <productname>PostgreSQL</> 8.0 and later, but you
    need to be wary of moving older installations.)
   </para>
  </note>
 </sect1>
</chapter>

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