wal.sgml

PostgreSQL7.4.6 for Linux

   interval, and in any case causing more disk I/O.
  </para>

  <para>
   There will be at least one 16 MB segment file, and will normally
   not be more than 2 * <varname>checkpoint_segments</varname> + 1
   files.  You can use this to estimate space requirements for WAL.
   Ordinarily, when old log segment files are no longer needed, they
   are recycled (renamed to become the next segments in the numbered
   sequence). If, due to a short-term peak of log output rate, there
   are more than 2 * <varname>checkpoint_segments</varname> + 1
   segment files, the unneeded segment files will be deleted instead
   of recycled until the system gets back under this limit.
  </para>

  <para>
   There are two commonly used <acronym>WAL</acronym> functions:
   <function>LogInsert</function> and <function>LogFlush</function>.
   <function>LogInsert</function> is used to place a new record into
   the <acronym>WAL</acronym> buffers in shared memory. If there is no
   space for the new record, <function>LogInsert</function> will have
   to write (move to kernel cache) a few filled <acronym>WAL</acronym>
   buffers. This is undesirable because <function>LogInsert</function>
   is used on every database low level modification (for example,
   row insertion) at a time when an exclusive lock is held on
   affected data pages, so the operation needs to be as fast as
   possible.  What is worse, writing <acronym>WAL</acronym> buffers may
   also force the creation of a new log segment, which takes even more
   time. Normally, <acronym>WAL</acronym> buffers should be written
   and flushed by a <function>LogFlush</function> request, which is
   made, for the most part, at transaction commit time to ensure that
   transaction records are flushed to permanent storage. On systems
   with high log output, <function>LogFlush</function> requests may
   not occur often enough to prevent <acronym>WAL</acronym> buffers
   being written by <function>LogInsert</function>. On such systems
   one should increase the number of <acronym>WAL</acronym> buffers by
   modifying the configuration parameter <varname>wal_buffers</varname>.
   The default number of <acronym>
   WAL</acronym> buffers is 8.  Increasing this value will 
   correspondingly increase shared memory usage.
  </para>

  <para>
   Checkpoints are fairly expensive because they force all dirty kernel
   buffers to disk using the operating system <literal>sync()</> call.
   Busy servers may fill checkpoint segment files too quickly,
   causing excessive checkpointing. If such forced checkpoints happen
   more frequently than <varname>checkpoint_warning</varname> seconds, 
   a message, will be output to the server logs recommending increasing 
   <varname>checkpoint_segments</varname>.
  </para>

  <para>
   The <varname>commit_delay</varname> parameter defines for how many
   microseconds the server process will sleep after writing a commit
   record to the log with <function>LogInsert</function> but before
   performing a <function>LogFlush</function>. This delay allows other
   server processes to add their commit records to the log so as to have all
   of them flushed with a single log sync. No sleep will occur if
   <varname>fsync</varname>
   is not enabled, nor if fewer than <varname>commit_siblings</varname>
   other sessions are currently in active transactions; this avoids
   sleeping when it's unlikely that any other session will commit soon.
   Note that on most platforms, the resolution of a sleep request is
   ten milliseconds, so that any nonzero <varname>commit_delay</varname>
   setting between 1 and 10000 microseconds would have the same effect.
   Good values for these parameters are not yet clear; experimentation
   is encouraged.
  </para>

  <para>
   The <varname>wal_sync_method</varname> parameter determines how
   <productname>PostgreSQL</productname> will ask the kernel to force
    WAL updates out to disk. 
   All the options should be the same as far as reliability goes,
   but it's quite platform-specific which one will be the fastest.
   Note that this parameter is irrelevant if <varname>fsync</varname>
   has been turned off.
  </para>

  <para>
   Setting the <varname>wal_debug</varname> parameter to any nonzero
   value will result in each <function>LogInsert</function> and
   <function>LogFlush</function> <acronym>WAL</acronym> call being
   logged to the server log.  At present, it makes no difference what
   the nonzero value is.  This option may be replaced by a more
   general mechanism in the future.
  </para>
 </sect1>

 <sect1 id="wal-internals">
  <title>Internals</title>

  <para>
   <acronym>WAL</acronym> is automatically enabled; no action is
   required from the administrator except ensuring that the additional
   disk-space requirements of the <acronym>WAL</acronym> logs are met,
   and that any necessary tuning is done (see <xref
   linkend="wal-configuration">).
  </para>

  <para>
   <acronym>WAL</acronym> logs are stored in the directory
   <filename>pg_xlog</filename> under the data directory, as a set of
   segment files, each 16 MB in size.  Each segment is divided into 8
   kB pages. The log record headers are described in
   <filename>access/xlog.h</filename>; the record content is dependent
   on the type of event that is being logged.  Segment files are given
   ever-increasing numbers as names, starting at
   <filename>0000000000000000</filename>.  The numbers do not wrap, at
   present, but it should take a very long time to exhaust the
   available stock of numbers.
  </para>

  <para>
   The <acronym>WAL</acronym> buffers and control structure are in
   shared memory and are handled by the server child processes; they
   are protected by lightweight locks.  The demand on shared memory is
   dependent on the number of buffers.  The default size of the
   <acronym>WAL</acronym> buffers is 8 buffers of 8 kB each, or 64 kB
   total.
  </para>

  <para>
   It is of advantage if the log is located on another disk than the
   main database files.  This may be achieved by moving the directory
   <filename>pg_xlog</filename> to another location (while the server
   is shut down, of course) and creating a symbolic link from the
   original location in the main data directory to the new location.
  </para>

  <para>
   The aim of <acronym>WAL</acronym>, to ensure that the log is
   written before database records are altered, may be subverted by
   disk drives<indexterm><primary>disk drive</></> that falsely report a successful write to the kernel,
   when, in fact, they have only cached the data and not yet stored it
   on the disk.  A power failure in such a situation may still lead to
   irrecoverable data corruption.  Administrators should try to ensure
   that disks holding <productname>PostgreSQL</productname>'s
   <acronym>WAL</acronym> log files do not make such false reports.
  </para>

  <para>
   After a checkpoint has been made and the log flushed, the
   checkpoint's position is saved in the file
   <filename>pg_control</filename>. Therefore, when recovery is to be
   done, the server first reads <filename>pg_control</filename> and
   then the checkpoint record; then it performs the REDO operation by
   scanning forward from the log position indicated in the checkpoint
   record.  Because the entire content of data pages is saved in the
   log on the first page modification after a checkpoint, all pages
   changed since the checkpoint will be restored to a consistent
   state.
  </para>

  <para>
   Using <filename>pg_control</filename> to get the checkpoint
   position speeds up the recovery process, but to handle possible
   corruption of <filename>pg_control</filename>, we should actually
   implement the reading of existing log segments in reverse order --
   newest to oldest -- in order to find the last checkpoint.  This has
   not been implemented, yet.
  </para>
 </sect1>
</chapter>

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