postgresql/doc/src/sgml/maintenance.sgml

<!-- $PostgreSQL: pgsql/doc/src/sgml/maintenance.sgml,v 1.57 2006/08/01 00:09:06 momjian Exp $ -->

<chapter id="maintenance">
 <title>Routine Database Maintenance Tasks</title>

 <indexterm zone="maintenance">
  <primary>maintenance</primary>
 </indexterm>

 <indexterm zone="maintenance">
  <primary>Routine maintenance</primary>
 </indexterm>

  <para>
   PostgreSQL, like any database software, requires that certain tasks
   be performed regularly to achieve optimum performance. The tasks 
   discussed here are <emphasis>required</emphasis>, but they
   are repetitive in nature and can easily be automated using standard 
   Unix tools such as <application>cron</application> scripts.  
  </para>
  
  <para>
   <productname>PostgreSQL</productname> is low-maintenance compared
   to other database management systems. It is also fairly easy, provided
   you read the documentation.
  </para>

  <para>
   There are three tasks that <emphasis>must</emphasis> be performed on 
   a periodic basis. The first is backups. If you do not have a current backup
   and your system experiences a catastrophic failure, you will lose your data.
   You can read further about backups procedures in <xref linkend="backup">. 
   The second is vacuum which is discussed in <xref linkend="routine-vacuuming">. 
   The third is to update the planner statistics using the analyze command as 
   discussed in <xref linkend="vacuum-for-statistics">.
  </para>

  <para>
   Another task that may need periodic attention is log file management.
   This is discussed in <xref linkend="logfile-maintenance">.
  </para>

  

 <sect1 id="routine-vacuuming">
  <title>Routine Vacuuming</title>

  <indexterm zone="routine-vacuuming">
   <primary>vacuum</primary>
  </indexterm>

  <para>
   <productname>PostgreSQL</productname>'s <command>VACUUM</> command
   <emphasis>must</emphasis> be run on a regular basis for several reasons:

    <orderedlist>
     <listitem>
      <simpara>To recover or reuse disk space occupied by updated or deleted
      rows.</simpara>
     </listitem>

     <listitem>
      <simpara>To update data statistics used by the
      <productname>PostgreSQL</productname> query planner.</simpara>
     </listitem>

     <listitem>
      <simpara>To protect against loss of very old data due to
      <firstterm>transaction ID wraparound</>.</simpara>
     </listitem>
    </orderedlist>

   The frequency and scope of the <command>VACUUM</> operations
   performed for each of these reasons will vary depending on the
   needs of each site.  Therefore, database administrators must
   understand these issues and develop an appropriate maintenance
   strategy.  This section concentrates on explaining the high-level
   issues; for details about command syntax and so on, see the <xref
   linkend="sql-vacuum" endterm="sql-vacuum-title"> reference page.
  </para>

  <para>
   The standard form of <command>VACUUM</> does not intefere with production
   database operations. Items such as SELECTS, INSERTS, UPDATES and DELETES
   will continue to function as normal, though you will not be able to modify the
   definition (Such as ALTER TABLE ADD COLUMN) of a table while it is being vacuumed.
   The release of <productname>PostgreSQL</productname> 8.0, introduced new
   configuration parameters to further reduce the potentially negative 
   performance impact of background vacuuming.  See
   <xref linkend="runtime-config-resource-vacuum-cost">.
  </para>

  <para>
   An automated mechanism for performing the necessary <command>VACUUM</>
   operations has been added in <productname>PostgreSQL</productname> 8.1.
   See <xref linkend="autovacuum">.
  </para>

  <sect2 id="vacuum-for-space-recovery">
   <title>Recovering disk space</title>

   <indexterm zone="vacuum-for-space-recovery">
    <primary>disk space</primary>
   </indexterm>

   <para>
    In normal <productname>PostgreSQL</productname> operation, an
    <command>UPDATE</> or <command>DELETE</> of a row does not
    immediately remove the old version of the row.
    This approach is necessary to gain the benefits of multiversion
    concurrency control (see <xref linkend="mvcc">): the row version
    must not be deleted while it is still potentially visible to other
    transactions. But eventually, an outdated or deleted row version is no
    longer of interest to any transaction. The space it occupies must be
    reclaimed for reuse by new rows, to avoid infinite growth of disk
    space requirements. This is done by running <command>VACUUM</>.
   </para>

   <para>
    Clearly, a table that receives frequent updates or deletes will need
    to be vacuumed more often than tables that are seldom updated. It
    may be useful to set up periodic <application>cron</> tasks that
    <command>VACUUM</command> only selected tables, skipping tables that are known not to
    change often. This is only likely to be helpful if you have both
    large heavily-updated tables and large seldom-updated tables &mdash; the
    extra cost of vacuuming a small table isn't enough to be worth
    worrying about.
   </para>

   <para>
    There are two variants of the <command>VACUUM</command>
    command. The first form, known as <quote>lazy vacuum</quote> or
    just <command>VACUUM</command>, marks expired data in tables and
    indexes for future reuse; it does <emphasis>not</emphasis> attempt
    to reclaim the space used by this expired data unless the space is
    at the end of the table and an exclusive table lock can be easily 
    obtained. Unused space at the start or middle of the file does
    not result in the file being shortened and space returned to the
    operating system. This variant of <command>VACUUM</command> can be
    run concurrently with normal database operations.
   </para>

   <para>
    The second form is the <command>VACUUM FULL</command>
    command. This uses a more aggressive algorithm for reclaiming the
    space consumed by expired row versions. Any space that is freed by
    <command>VACUUM FULL</command> is immediately returned to the
    operating system. Unfortunately, this variant of the
    <command>VACUUM</command> command acquires an exclusive lock on
    each table while <command>VACUUM FULL</command> is processing
    it. Therefore, frequently using <command>VACUUM FULL</command> can
    have an extremely negative effect on the performance of concurrent
    database queries.
   </para>

   <para>
    The standard form of <command>VACUUM</> is best used with the goal
    of maintaining a fairly level steady-state usage of disk space. If
    you need to return disk space to the operating system you can use
    <command>VACUUM FULL</> &mdash; but what's the point of releasing disk
    space that will only have to be allocated again soon?  Moderately
    frequent standard <command>VACUUM</> runs are a better approach
    than infrequent <command>VACUUM FULL</> runs for maintaining
    heavily-updated tables.
   </para>

   <para>
    Recommended practice for most sites is to schedule a database-wide
    <command>VACUUM</> once a day at a low-usage time of day,
    supplemented by more frequent vacuuming of heavily-updated tables
    if necessary. (Some installations with an extremely high
    rate of data modification <command>VACUUM</command> busy tables as
    often as once every few minutes.)  If you have multiple databases
    in a cluster, don't forget to <command>VACUUM</command> each one;
    the program <xref linkend="app-vacuumdb" endterm="app-vacuumdb-title">
    may be helpful.
   </para>

   <para>
    <command>VACUUM FULL</> is recommended for cases where you know
    you have deleted the majority of rows in a table, so that the
    steady-state size of the table can be shrunk substantially with
    <command>VACUUM FULL</>'s more aggressive approach.  Use plain
    <command>VACUUM</>, not <command>VACUUM FULL</>, for routine
    vacuuming for space recovery.
   </para>

   <para>
    If you have a table whose entire contents are deleted on a periodic
    basis, consider doing it with the <command>TRUNCATE</command> rather
    than using the <command>DELETE</command> followed by
    <command>VACUUM</command>. <command>TRUNCATE</command> removes the
    entire content of the table immediately, without requiring a
    subsequent <command>VACUUM</command> or <command>VACUUM
    FULL</command> to reclaim the now-unused disk space.
   </para>
  </sect2>

  <sect2 id="vacuum-for-statistics">
   <title>Updating planner statistics</title>

   <indexterm zone="vacuum-for-statistics">
    <primary>statistics</primary>
    <secondary>of the planner</secondary>
   </indexterm>

   <indexterm zone="vacuum-for-statistics">
    <primary>ANALYZE</primary>
   </indexterm>

   <para>
    The <productname>PostgreSQL</productname> query planner relies on
    statistical information about the contents of tables in order to
    generate good plans for queries.  These statistics are gathered by
    the <command>ANALYZE</> command, which can be invoked by itself or
    as an optional step in <command>VACUUM</>.  It is important to have
    reasonably accurate statistics, otherwise poor choices of plans may
    degrade database performance.
   </para>

   <para>
    As with vacuuming for space recovery, frequent updates of statistics
    are more useful for heavily-updated tables than for seldom-updated
    ones. But even for a heavily-updated table, there may be no need for
    statistics updates if the statistical distribution of the data is
    not changing much. A simple rule of thumb is to think about how much
    the minimum and maximum values of the columns in the table change.
    For example, a <type>timestamp</type> column that contains the time
    of row update will have a constantly-increasing maximum value as
    rows are added and updated; such a column will probably need more
    frequent statistics updates than, say, a column containing URLs for
    pages accessed on a website. The URL column may receive changes just
    as often, but the statistical distribution of its values probably
    changes relatively slowly.
   </para>

   <para>
    It is possible to run <command>ANALYZE</> on specific tables and even
    just specific columns of a table, so the flexibility exists to update some
    statistics more frequently than others if your application requires it.
    In practice, however, it is usually best to just analyze the entire database
    because it is a fast operation.  It uses a statistical random sampling of 
    the rows of a table rather than reading every single row.
   </para>

   <tip>
    <para>
     Although per-column tweaking of <command>ANALYZE</> frequency may not be
     very productive, you may well find it worthwhile to do per-column
     adjustment of the level of detail of the statistics collected by
     <command>ANALYZE</>.  Columns that are heavily used in <literal>WHERE</> clauses
     and have highly irregular data distributions may require a finer-grain
     data histogram than other columns.  See <command>ALTER TABLE SET
     STATISTICS</>.
    </para>
   </tip>

   <para>
    Recommended practice for most sites is to schedule a database-wide
    <command>ANALYZE</> once a day at a low-usage time of day; this can
    usefully be combined with a nightly <command>VACUUM</>.  However,
    sites with relatively slowly changing table statistics may find that
    this is overkill, and that less-frequent <command>ANALYZE</> runs
    are sufficient.
   </para>
  </sect2>

  <sect2 id="vacuum-for-wraparound">
   <title>Preventing transaction ID wraparound failures</title>

   <indexterm zone="vacuum-for-wraparound">
    <primary>transaction ID</primary>
    <secondary>wraparound</secondary>
   </indexterm>

   <para>
    <productname>PostgreSQL</productname>'s MVCC transaction semantics
    depend on being able to compare transaction ID (<acronym>XID</>)
    numbers: a row version with an insertion XID greater than the current
    transaction's XID is <quote>in the future</> and should not be visible
    to the current transaction.  But since transaction IDs have limited size
    (32 bits at this writing) a cluster that runs for a long time (more
    than 4 billion transactions) would suffer <firstterm>transaction ID
    wraparound</>: the XID counter wraps around to zero, and all of a sudden
    transactions that were in the past appear to be in the future &mdash; which
    means their outputs become invisible.  In short, catastrophic data loss.
    (Actually the data is still there, but that's cold comfort if you can't
    get at it.)  To avoid this, it is <emphasis>necessary to vacuum every table
    in every database at least once every billion transactions</emphasis>.
   </para>

   <para>
    In practice this isn't an onerous requirement, but since the
    consequences of failing to meet it can be complete data loss (not
    just wasted disk space or slow performance), some special provisions
    have been made to help database administrators avoid disaster.
    For each database in the cluster, <productname>PostgreSQL</productname>
    keeps track of the time of the last database-wide <command>VACUUM</>.
    When any database approaches the billion-transaction danger level,
    the system begins to emit warning messages.  If nothing is done, it
    will eventually shut down normal operations until appropriate
    manual maintenance is done.  The remainder of this
    section gives the details.
   </para>

   <para>
    The new approach to XID comparison distinguishes two special XIDs,
    numbers 1 and 2 (<literal>BootstrapXID</> and
    <literal>FrozenXID</>). These two XIDs are always considered older
    than every normal XID. Normal XIDs (those greater than 2) are
    compared using modulo-2<superscript>31</> arithmetic. This means
    that for every normal XID, there are two billion XIDs that are
    <quote>older</> and two billion that are <quote>newer</>; another
    way to say it is that the normal XID space is circular with no
    endpoint. Therefore, once a row version has been created with a particular
    normal XID, the row version will appear to be <quote>in the past</> for
    the next two billion transactions, no matter which normal XID we are
    talking about. If the row version still exists after more than two billion
    transactions, it will suddenly appear to be in the future. To
    prevent data loss, old row versions must be reassigned the XID
    <literal>FrozenXID</> sometime before they reach the
    two-billion-transactions-old mark. Once they are assigned this
    special XID, they will appear to be <quote>in the past</> to all
    normal transactions regardless of wraparound issues, and so such
    row versions will be good until deleted, no matter how long that is. This
    reassignment of XID is handled by <command>VACUUM</>.
   </para>

   <para>
    <command>VACUUM</>'s normal policy is to reassign <literal>FrozenXID</>
    to any row version with a normal XID more than one billion transactions in the
    past.  This policy preserves the original insertion XID until it is not
    likely to be of interest anymore.  (In fact, most row versions will probably
    live and die without ever being <quote>frozen</>.)  With this policy,
    the maximum safe interval between <command>VACUUM</> runs on any table
    is exactly one billion transactions: if you wait longer, it's possible
    that a row version that was not quite old enough to be reassigned last time
    is now more than two billion transactions old and has wrapped around
    into the future &mdash; i.e., is lost to you.  (Of course, it'll reappear
    after another two billion transactions, but that's no help.)
   </para>

   <para>
    Since periodic <command>VACUUM</> runs are needed anyway for the reasons
    described earlier, it's unlikely that any table would not be vacuumed
    for as long as a billion transactions.  But to help administrators ensure
    this constraint is met, <command>VACUUM</> stores transaction ID
    statistics in the system table <literal>pg_database</>.  In particular,
    the <literal>datfrozenxid</> column of a database's
    <literal>pg_database</> row is updated at the completion of any
    database-wide <command>VACUUM</command> operation (i.e.,
    <command>VACUUM</> that does not
    name a specific table).  The value stored in this field is the freeze
    cutoff XID that was used by that <command>VACUUM</> command.  All normal
    XIDs older than this cutoff XID are guaranteed to have been replaced by
    <literal>FrozenXID</> within that database.  A convenient way to
    examine this information is to execute the query

<programlisting>
SELECT datname, age(datfrozenxid) FROM pg_database;
</programlisting>

    The <literal>age</> column measures the number of transactions from the
    cutoff XID to the current transaction's XID.
   </para>

   <para>
    With the standard freezing policy, the <literal>age</> column will start
    at one billion for a freshly-vacuumed database.  When the <literal>age</>
    approaches two billion, the database must be vacuumed again to avoid
    risk of wraparound failures.  Recommended practice is to <command>VACUUM</command> each
    database at least once every half-a-billion (500 million) transactions,
    so as to provide plenty of safety margin.  To help meet this rule,
    each database-wide <command>VACUUM</> automatically delivers a warning
    if there are any <literal>pg_database</> entries showing an
    <literal>age</> of more than 1.5 billion transactions, for example:

<programlisting>
play=# VACUUM;
WARNING:  database "mydb" must be vacuumed within 177009986 transactions
HINT:  To avoid a database shutdown, execute a full-database VACUUM in "mydb".
VACUUM
</programlisting>
   </para>

   <para>
    If the warnings emitted by <command>VACUUM</> go ignored, then
    <productname>PostgreSQL</productname> will begin to emit a warning
    like the above on every transaction start once there are fewer than 10
    million transactions left until wraparound.  If those warnings also are
    ignored, the system will shut down and refuse to execute any new
    transactions once there are fewer than 1 million transactions left
    until wraparound:

<programlisting>
play=# select 2+2;
ERROR:  database is shut down to avoid wraparound data loss in database "mydb"
HINT:  Stop the postmaster and use a standalone backend to VACUUM in "mydb".
</programlisting>

    The 1-million-transaction safety margin exists to let the
    administrator recover without data loss, by manually executing the
    required <command>VACUUM</> commands.  However, since the system will not
    execute commands once it has gone into the safety shutdown mode,
    the only way to do this is to stop the server and use a single-user
    backend to execute <command>VACUUM</>.  The shutdown mode is not enforced
    by a single-user backend.  See the <xref linkend="app-postgres"> reference
    page for details about using a single-user backend.
   </para>

   <para>
    <command>VACUUM</> with the <command>FREEZE</> option uses a more
    aggressive freezing policy: row versions are frozen if they are old enough
    to be considered good by all open transactions. In particular, if a
    <command>VACUUM FREEZE</> is performed in an otherwise-idle
    database, it is guaranteed that <emphasis>all</> row versions in that
    database will be frozen. Hence, as long as the database is not
    modified in any way, it will not need subsequent vacuuming to avoid
    transaction ID wraparound problems. This technique is used by
    <command>initdb</> to prepare the <literal>template0</> database.
    It should also be used to prepare any user-created databases that
    are to be marked <literal>datallowconn</> = <literal>false</> in
    <literal>pg_database</>, since there isn't any convenient way to
    <command>VACUUM</command> a database that you can't connect to.
   </para>

   <warning>
    <para>
     A database that is marked <literal>datallowconn</> = <literal>false</>
     in <literal>pg_database</> is assumed to be properly frozen; the
     automatic warnings and wraparound protection shutdown do not take
     such databases into account.  Therefore it's up to you to ensure
     you've correctly frozen a database before you mark it with
     <literal>datallowconn</> = <literal>false</>.
    </para>
   </warning>
  </sect2>

  <sect2 id="autovacuum">
   <title id="autovacuum-title">The auto-vacuum daemon</title>

   <indexterm>
    <primary>autovacuum</primary>
    <secondary>general information</secondary>
   </indexterm>
   <para>
    Beginning in <productname>PostgreSQL </productname> 8.1, there is a
    separate optional server process called the <firstterm>autovacuum
    daemon</firstterm>, whose purpose is to automate the execution of
    <command>VACUUM</command> and <command>ANALYZE </command> commands.
    When enabled, the autovacuum daemon runs periodically and checks for
    tables that have had a large number of inserted, updated or deleted
    tuples.  These checks use the row-level statistics collection facility;
    therefore, the autovacuum daemon cannot be used unless <xref
    linkend="guc-stats-start-collector"> and <xref
    linkend="guc-stats-row-level"> are set to <literal>true</literal>.  Also,
    it's important to allow a slot for the autovacuum process when choosing
    the value of <xref linkend="guc-superuser-reserved-connections">.
   </para>

   <para>
    The autovacuum daemon, when enabled, runs every <xref
    linkend="guc-autovacuum-naptime"> seconds and determines which database
    to process.  Any database which is close to transaction ID wraparound
    is immediately processed.  In this case, autovacuum issues a
    database-wide <command>VACUUM</command> call, or <command>VACUUM
    FREEZE</command> if it's a template database, and then terminates.  If
    no database fulfills this criterion, the one that was least recently
    processed by autovacuum is chosen.  In this case each table in
    the selected database is checked, and individual <command>VACUUM</command>
    or <command>ANALYZE</command> commands are issued as needed.
   </para>

   <para>
    For each table, two conditions are used to determine which operation(s)
    to apply.  If the number of obsolete tuples since the last
    <command>VACUUM</command> exceeds the <quote>vacuum threshold</quote>, the
    table is vacuumed.  The vacuum threshold is defined as:
<programlisting>
vacuum threshold = vacuum base threshold + vacuum scale factor * number of tuples
</programlisting>
    where the vacuum base threshold is
    <xref linkend="guc-autovacuum-vacuum-threshold">,
    the vacuum scale factor is
    <xref linkend="guc-autovacuum-vacuum-scale-factor">,
    and the number of tuples is
    <structname>pg_class</structname>.<structfield>reltuples</structfield>.
    The number of obsolete tuples is obtained from the statistics
    collector; it is a semi-accurate count updated by each
    <command>UPDATE</command> and <command>DELETE</command> operation.  (It
    is only semi-accurate because some information may be lost under heavy
    load.)  For analyze, a similar condition is used: the threshold, defined as
<programlisting>
analyze threshold = analyze base threshold + analyze scale factor * number of tuples
</programlisting>
    is compared to the total number of tuples inserted, updated, or deleted
    since the last <command>ANALYZE</command>.
   </para>

   <para>
    The default thresholds and scale factors are taken from
    <filename>postgresql.conf</filename>, but it is possible to override them
    on a table-by-table basis by making entries in the system catalog
    <link
    linkend="catalog-pg-autovacuum"><structname>pg_autovacuum</></link>.
    If a <structname>pg_autovacuum</structname> row exists for a particular
    table, the settings it specifies are applied; otherwise the global
    settings are used.  See <xref linkend="runtime-config-autovacuum"> for
    more details on the global settings.
   </para>

   <para>
    Besides the base threshold values and scale factors, there are three
    more parameters that can be set for each table in
    <structname>pg_autovacuum</structname>.
    The first, <structname>pg_autovacuum</>.<structfield>enabled</>,
    can be set to <literal>false</literal> to instruct the autovacuum daemon
    to skip that particular table entirely.  In this case
    autovacuum will only touch the table when it vacuums the entire database
    to prevent transaction ID wraparound.
    The other two parameters, the vacuum cost delay
    (<structname>pg_autovacuum</structname>.<structfield>vac_cost_delay</structfield>)
    and the vacuum cost limit
    (<structname>pg_autovacuum</structname>.<structfield>vac_cost_limit</structfield>), 
    are used to set table-specific values for the
    <xref linkend="runtime-config-resource-vacuum-cost" endterm="runtime-config-resource-vacuum-cost-title">
    feature.
   </para>

   <para>
    If any of the values in <structname>pg_autovacuum</structname>
    are set to a negative number, or if a row is not present at all in
    <structname>pg_autovacuum</structname> for any particular table, the
    corresponding values from <filename>postgresql.conf</filename> are used.
   </para>

   <para>
    There is not currently any support for making
    <structname>pg_autovacuum</structname> entries, except by doing
    manual <command>INSERT</>s into the catalog.  This feature will be
    improved in future releases, and it is likely that the catalog
    definition will change.
   </para>

   <caution>
    <para>
     The contents of the <structname>pg_autovacuum</structname> system
     catalog are currently not saved in database dumps created by
     the tools <command>pg_dump</command> and <command>pg_dumpall</command>.
     If you want to preserve them across a dump/reload cycle, make sure you
     dump the catalog manually.
    </para>
   </caution>

  </sect2>
 </sect1>


 <sect1 id="routine-reindex">
  <title>Routine Reindexing</title>

  <indexterm zone="routine-reindex">
   <primary>reindex</primary>
  </indexterm>

  <para>
   In some situations it is worthwhile to rebuild indexes periodically
   with the <xref linkend="sql-reindex" endterm="sql-reindex-title">
   command.
  </para>

  <para>
   In <productname>PostgreSQL</> releases before 7.4, periodic reindexing
   was frequently necessary to avoid <quote>index bloat</>, due to lack of
   internal space reclamation in B-tree indexes.  Any situation in which the
   range of index keys changed over time &mdash; for example, an index on
   timestamps in a table where old entries are eventually deleted &mdash;
   would result in bloat, because index pages for no-longer-needed portions
   of the key range were not reclaimed for re-use.  Over time, the index size
   could become indefinitely much larger than the amount of useful data in it.
  </para>

  <para>
   In <productname>PostgreSQL</> 7.4 and later, index pages that have become
   completely empty are reclaimed for re-use.  There is still a possibility
   for inefficient use of space: if all but a few index keys on a page have
   been deleted, the page remains allocated.  So a usage pattern in which all
   but a few keys in each range are eventually deleted will see poor use of
   space.  The potential for bloat is not indefinite &mdash; at worst there
   will be one key per page &mdash; but it may still be worthwhile to schedule
   periodic reindexing for indexes that have such usage patterns.
  </para>

  <para>
   The potential for bloat in non-B-tree indexes has not been well
   characterized.  It is a good idea to keep an eye on the index's physical
   size when using any non-B-tree index type.
  </para>

  <para>
   Also, for B-tree indexes a freshly-constructed index is somewhat faster to
   access than one that has been updated many times, because logically
   adjacent pages are usually also physically adjacent in a newly built index.
   (This consideration does not currently apply to non-B-tree indexes.)  It
   might be worthwhile to reindex periodically just to improve access speed.
  </para>
 </sect1>


 <sect1 id="logfile-maintenance">
  <title>Log File Maintenance</title>

  <indexterm zone="logfile-maintenance">
   <primary>server log</primary>
   <secondary>log file maintenance</secondary>
  </indexterm>

  <para>
   It is a good idea to save the database server's log output
   somewhere, rather than just routing it to <filename>/dev/null</>.
   The log output is invaluable when it comes time to diagnose
   problems.  However, the log output tends to be voluminous
   (especially at higher debug levels) and you won't want to save it
   indefinitely.  You need to <quote>rotate</> the log files so that
   new log files are started and old ones removed after a reasonable
   period of time.
  </para>

  <para>
   If you simply direct the <systemitem>stderr</> of
   <command>postgres</command> into a
   file, you will have log output, but
   the only way to truncate the log file is to stop and restart
   the server. This may be OK if you are using
   <productname>PostgreSQL</productname> in a development environment,
   but few production servers would find this behavior acceptable.
  </para>

  <para>
   A better approach is to send the server's
   <systemitem>stderr</> output to some type of log rotation program.
   There is a built-in log rotation program, which you can use by
   setting the configuration parameter <literal>redirect_stderr</> to
   <literal>true</> in <filename>postgresql.conf</>.  The control
   parameters for this program are described in <xref
   linkend="runtime-config-logging-where">.
  </para>

  <para>
   Alternatively, you might prefer to use an external log rotation
   program, if you have one that you are already using with other
   server software. For example, the <application>rotatelogs</application>
   tool included in the <productname>Apache</productname> distribution
   can be used with <productname>PostgreSQL</productname>.  To do this,
   just pipe the server's
   <systemitem>stderr</> output to the desired program.
   If you start the server with
   <command>pg_ctl</>, then <systemitem>stderr</>
   is already redirected to <systemitem>stdout</>, so you just need a
   pipe command, for example:

<programlisting>
pg_ctl start | rotatelogs /var/log/pgsql_log 86400
</programlisting>
  </para>

  <para>
   Another production-grade approach to managing log output is to
   send it all to <application>syslog</> and let
   <application>syslog</> deal with file rotation. To do this, set the
   configuration parameter <literal>log_destination</> to <literal>syslog</>
   (to log to <application>syslog</> only) in
   <filename>postgresql.conf</>. Then you can send a <literal>SIGHUP</literal>
   signal to the <application>syslog</> daemon whenever you want to force it
   to start writing a new log file.  If you want to automate log
   rotation, the <application>logrotate</application> program can be
   configured to work with log files from
   <application>syslog</application>.
  </para>

  <para>
   On many systems, however, <application>syslog</> is not very reliable,
   particularly with large log messages; it may truncate or drop messages
   just when you need them the most.  Also, on <productname>Linux</>,
   <application>syslog</> will sync each message to disk, yielding poor
   performance.  (You can use a <literal>-</> at the start of the file name
   in the <application>syslog</> configuration file to disable this behavior.)
  </para>

  <para>
   Note that all the solutions described above take care of starting new
   log files at configurable intervals, but they do not handle deletion
   of old, no-longer-interesting log files.  You will probably want to set
   up a batch job to periodically delete old log files.  Another possibility
   is to configure the rotation program so that old log files are overwritten
   cyclically.
  </para>
 </sect1>
</chapter>

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