maintenance.sgml

PostgreSQL7.4.6 for Linux

    <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) will 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 --- 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.)
   </para>

   <para>
    Prior to <productname>PostgreSQL</productname> 7.2, the only defense
    against XID wraparound was to re-<command>initdb</> at least every 4
    billion transactions. This of course was not very satisfactory for
    high-traffic sites, so a better solution has been devised. The new
    approach allows a server to remain up indefinitely, without
    <command>initdb</> or any sort of restart. The price is this
    maintenance requirement: <emphasis>every table in the database must
    be vacuumed 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 keep track of the
    time since the last <command>VACUUM</>. 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 --- 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 vacuum 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 vacuum 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:  some databases have not been vacuumed in 1613770184 transactions
HINT:  Better vacuum them within 533713463 transactions, or you may have a wraparound failure.
VACUUM
</programlisting>
   </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
    vacuum a database that you can't connect to. Note that
    <command>VACUUM</command>'s automatic warning message about
    unvacuumed databases will ignore <literal>pg_database</> entries
    with <literal>datallowconn</> = <literal>false</>, so as to avoid
    giving false warnings about these databases; therefore it's up to
    you to ensure that such databases are frozen correctly.
   </para>

  </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 <command>REINDEX</> command. (There is also
   <filename>contrib/reindexdb</> which can reindex an entire database.)
   However, <productname>PostgreSQL</> 7.4 has substantially reduced the need
   for this activity compared to earlier releases.
  </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's 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 thrown
   away every so often.
  </para>

  <para>
   If you simply direct the <systemitem>stderr</> of the <command>postmaster</command> into a
   file, the only way to truncate the log file is to stop and restart
   the <command>postmaster</command>. This may be OK for development setups but you won't
   want to run a production server that way.
  </para>

  <para>
   The simplest 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
   configurations parameter <literal>syslog</> to 2 (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. You may find it more useful to pipe the
   <systemitem>stderr</> of the <command>postmaster</> to some type of
   log rotation program. If you start the server with
   <command>pg_ctl</>, then the <systemitem>stderr</> of the <command>postmaster</command>
   is already redirected to <systemitem>stdout</>, so you just need a
   pipe command:
   
<programlisting>
pg_ctl start | logrotate
</programlisting>

   The <productname>PostgreSQL</> distribution doesn't include a suitable
   log rotation program, but there are many available on the Internet;
   one is included in the Apache distribution, for example.
  </para>
 </sect1>
</chapter>

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