maintenance.sgml

PostgreSQL 8.1.4的源码 适用于Linux下的开源数据库系统

    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, the usefulness of this feature is doubtful.
    Beginning in <productname>PostgreSQL</productname> 7.2,
    <command>ANALYZE</> is a fairly fast operation even on large tables,
    because it uses a statistical random sampling of the rows of a table
    rather than reading every single row.  So it's probably much simpler
    to just run it over the whole database every so often.
   </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.)
   </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 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 postmaster and use a standalone
    backend to execute <command>VACUUM</>.  The shutdown mode is not enforced
    by a standalone backend.  See the <xref linkend="app-postgres"> reference
    page for details about using a standalone 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>