postgresql/doc/src/sgml/wal.sgml

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<chapter id="wal">
 <title>Write-Ahead Logging (<acronym>WAL</acronym>)</title>

 <note>
  <title>Author</title>
  <para>
   Vadim Mikheev and Oliver Elphick
  </para>
 </note>

 <sect1 id="wal-general">
  <title>General Description</Title>

  <para>
   <firstterm>Write Ahead Logging</firstterm> (<acronym>WAL</acronym>)
   is a standard approach to transaction logging.  Its detailed
   description may be found in most (if not all) books about
   transaction processing. Briefly, <acronym>WAL</acronym>'s central
   concept is that changes to data files (where tables and indexes
   reside) must be written only after those changes have been logged -
   that is, when log records have been flushed to permanent
   storage. When we follow this procedure, we do not need to flush
   data pages to disk on every transaction commit, because we know
   that in the event of a crash we will be able to recover the
   database using the log: any changes that have not been applied to
   the data pages will first be redone from the log records (this is
   roll-forward recovery, also known as REDO) and then changes made by
   uncommitted transactions will be removed from the data pages
   (roll-backward recovery - UNDO).
  </para>

  <sect2 id="wal-benefits-now">
   <title>Immediate Benefits of <acronym>WAL</acronym></title>

   <para>
    The first obvious benefit of using <acronym>WAL</acronym> is a
    significantly reduced number of disk writes, since only the log
    file needs to be flushed to disk at the time of transaction
    commit; in multiuser environments, commits of many transactions
    may be accomplished with a single <function>fsync()</function> of
    the log file. Furthermore, the log file is written sequentially,
    and so the cost of syncing the log is much less than the cost of
    flushing the data pages.
   </para>

   <para>
    The next benefit is consistency of the data pages. The truth is
    that, before <acronym>WAL</acronym>,
    <productname>PostgreSQL</productname> was never able to guarantee
    consistency in the case of a crash.  Before
    <acronym>WAL</acronym>, any crash during writing could result in:

    <orderedlist>
     <listitem>
      <simpara>index tuples pointing to non-existent table rows</simpara>
     </listitem>

     <listitem>
      <simpara>index tuples lost in split operations</simpara>
     </listitem>

     <listitem>
      <simpara>totally corrupted table or index page content, because
      of partially written data pages</simpara>
     </listitem>
    </orderedlist>

    Problems with indexes (problems 1 and 2) could possibly have been
    fixed by additional <function>fsync()</function> calls, but it is
    not obvious how to handle the last case without
    <acronym>WAL</acronym>; <acronym>WAL</acronym> saves the entire
    data page content in the log if that is required to ensure page
    consistency for after-crash recovery.
   </para>
  </sect2>
  
  <sect2 id="wal-benefits-later">
   <title>Future Benefits</title>

   <para>
    In this first release of <acronym>WAL</acronym>, UNDO operation is
    not implemented, because of lack of time. This means that changes
    made by aborted transactions will still occupy disk space and that
    we still need a permanent <filename>pg_clog</filename> file to hold
    the status of transactions, since we are not able to re-use
    transaction identifiers.  Once UNDO is implemented,
    <filename>pg_clog</filename> will no longer be required to be
    permanent; it will be possible to remove
    <filename>pg_clog</filename> at shutdown.  (However, the urgency
    of this concern has decreased greatly with the adoption of a segmented
    storage method for <filename>pg_clog</filename> --- it is no longer
    necessary to keep old <filename>pg_clog</filename> entries around
    forever.)
   </para>

   <para>
    With UNDO, it will also be possible to implement
    <firstterm>savepoints</firstterm> to allow partial rollback of
    invalid transaction operations (parser errors caused by mistyping
    commands, insertion of duplicate primary/unique keys and so on)
    with the ability to continue or commit valid operations made by
    the transaction before the error.  At present, any error will
    invalidate the whole transaction and require a transaction abort.
   </para>

   <para>
    <acronym>WAL</acronym> offers the opportunity for a new method for
    database on-line backup and restore (<acronym>BAR</acronym>).  To
    use this method, one would have to make periodic saves of data
    files to another disk, a tape or another host and also archive the
    <acronym>WAL</acronym> log files.  The database file copy and the
    archived log files could be used to restore just as if one were
    restoring after a crash. Each time a new database file copy was
    made the old log files could be removed.  Implementing this
    facility will require the logging of data file and index creation
    and deletion; it will also require development of a method for
    copying the data files (operating system copy commands are not
    suitable).
   </para>

   <para>
    A difficulty standing in the way of realizing these benefits is that they
    require saving <acronym>WAL</acronym> entries for considerable periods
    of time (eg, as long as the longest possible transaction if transaction
    UNDO is wanted).  The present <acronym>WAL</acronym> format is
    extremely bulky since it includes many disk page snapshots.
    This is not a serious concern at present, since the entries only need
    to be kept for one or two checkpoint intervals; but to achieve
    these future benefits some sort of compressed <acronym>WAL</acronym>
    format will be needed.
   </para>
  </sect2>
 </sect1>

 <sect1 id="wal-implementation">
  <title>Implementation</title>

  <para>
   <acronym>WAL</acronym> is automatically enabled from release 7.1
   onwards. No action is required from the administrator with the
   exception of 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><replaceable>$PGDATA</replaceable>/pg_xlog</Filename>, as
   a set of segment files, each 16MB in size.  Each segment is
   divided into 8KB pages. The log record headers are described in
   <filename>access/xlog.h</filename>; 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 backends; 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 8KB buffers, or 64KB.
  </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
   postmaster is shut down, of course) and creating a symbolic link
   from the original location in <replaceable>$PGDATA</replaceable> 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 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
   log files do not make such false reports.
  </para>

  <sect2 id="wal-recovery">
   <title>Database Recovery with <acronym>WAL</acronym></title>

   <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 backend 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 yet been done in release 7.1.
   </para>
  </sect2>
 </sect1>

 <sect1 id="wal-configuration">
  <title><acronym>WAL</acronym> Configuration</title>

  <para>
   There are several <acronym>WAL</acronym>-related parameters that
   affect database performance. This section explains their use.
   Consult <xref linkend="runtime-config"> for details about setting
   configuration parameters.
  </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,
   tuple 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 <varname>WAL_BUFFERS</varname> parameter. The default
   number of <acronym>WAL</acronym> buffers is 8.  Increasing this
   value will correspondingly increase shared memory usage.
  </para>

  <para>
   <firstterm>Checkpoints</firstterm> are points in the sequence of
   transactions at which it is guaranteed that the data files have
   been updated with all information logged before the checkpoint.  At
   checkpoint time, all dirty data pages are flushed to disk and a
   special checkpoint record is written to the log file. As result, in
   the event of a crash, the recoverer knows from what record in the
   log (known as the redo record) it should start the REDO operation,
   since any changes made to data files before that record are already
   on disk. After a checkpoint has been made, any log segments written
   before the undo records are no longer needed and can be recycled or
   removed. (When <acronym>WAL</acronym>-based <acronym>BAR</acronym> is
   implemented, the log segments would be archived before being recycled
   or removed.)
  </para>

  <para>
   The checkpoint maker is also able to create a few log segments for
   future use, so as to avoid the need for
   <function>LogInsert</function> or <function>LogFlush</function> to
   spend time in creating them.  (If that happens, the entire database
   system will be delayed by the creation operation, so it's better if
   the files can be created in the checkpoint maker, which is not on
   anyone's critical path.)
   By default a new 16MB segment file is created only if more than 75% of
   the current segment has been used.  This is inadequate if the system
   generates more than 4MB of log output between checkpoints.
   One can instruct the server to pre-create up to 64 log segments
   at checkpoint time by modifying the <varname>WAL_FILES</varname>
   configuration parameter.
  </para>

  <para>
   The postmaster spawns a special backend process every so often
   to create the next checkpoint.  A checkpoint is created every
   <varname>CHECKPOINT_SEGMENTS</varname> log segments, or every
   <varname>CHECKPOINT_TIMEOUT</varname> seconds, whichever comes first.
   The default settings are 3 segments and 300 seconds respectively.
   It is also possible to force a checkpoint by using the SQL command
   <command>CHECKPOINT</command>.
  </para>

  <para>
   Reducing <varname>CHECKPOINT_SEGMENTS</varname> and/or
   <varname>CHECKPOINT_TIMEOUT</varname> causes checkpoints to be
   done more often.  This allows faster after-crash recovery (since
   less work will need to be redone).  However, one must balance this against
   the increased cost of flushing dirty data pages more often.  In addition,
   to ensure data page consistency, the first modification of a data page
   after each checkpoint results in logging the entire page content.
   Thus a smaller checkpoint interval increases the volume of output to
   the log, partially negating the goal of using a smaller interval, and
   in any case causing more disk I/O.
  </para>

  <para>
   The number of 16MB segment files will always be at least
   <varname>WAL_FILES</varname> + 1, and will normally not exceed
   <varname>WAL_FILES</varname> + 2 * <varname>CHECKPOINT_SEGMENTS</varname>
   + 1.  This may be used to estimate space requirements for WAL.  Ordinarily,
   when an old log segment file is no longer needed, it is recycled (renamed
   to become the next sequential future segment).  If, due to a short-term
   peak of log output rate, there are more than <varname>WAL_FILES</varname> +
   2 * <varname>CHECKPOINT_SEGMENTS</varname> + 1 segment files, then unneeded
   segment files will be deleted instead of recycled until the system gets
   back under this limit.  (If this happens on a regular basis,
   <varname>WAL_FILES</varname> should be increased to avoid it.  Deleting log
   segments that will only have to be created again later is expensive and
   pointless.)
  </para>

  <para>
   The <varname>COMMIT_DELAY</varname> parameter defines for how many
   microseconds the backend 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
   backends 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 or if fewer than <varname>COMMIT_SIBLINGS</varname>
   other backends are not currently in active transactions; this avoids
   sleeping when it's unlikely that any other backend 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 will 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 non-zero
   value will result in each <function>LogInsert</function> and
   <function>LogFlush</function> <acronym>WAL</acronym> call being
   logged to standard error.  At present, it makes no difference what
   the non-zero value is.  This option may be replaced by a more
   general mechanism in the future.
  </para>
 </sect1>
</chapter>

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