wal.sgml

PostgreSQL 8.1.4的源码 适用于Linux下的开源数据库系统

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<chapter id="wal">
 <title>Reliability and the Write-Ahead Log</title>

 <para>
  This chapter explain how the Write-Ahead Log is used to obtain
  efficient, reliable operation.
 </para>

 <sect1 id="wal-reliability">
  <title>Reliability</title>

  <para>
   Reliability is an important property of any serious database
   system, and <productname>PostgreSQL</> does everything possible to
   guarantee reliable operation. One aspect of reliable operation is
   that all data recorded by a committed transaction should be stored
   in a nonvolatile area that is safe from power loss, operating
   system failure, and hardware failure (except failure of the
   nonvolatile area itself, of course).  Successfully writing the data
   to the computer's permanent storage (disk drive or equivalent)
   ordinarily meets this requirement.  In fact, even if a computer is
   fatally damaged, if the disk drives survive they can be moved to
   another computer with similar hardware and all committed
   transactions will remain intact.
  </para>

  <para>
   While forcing data periodically to the disk platters might seem like
   a simple operation, it is not. Because disk drives are dramatically
   slower than main memory and CPUs, several layers of caching exist
   between the computer's main memory and the disk platters.
   First, there is the operating system's buffer cache, which caches
   frequently requested disk blocks and combines disk writes. Fortunately,
   all operating systems give applications a way to force writes from
   the buffer cache to disk, and <productname>PostgreSQL</> uses those
   features.  (See the <xref linkend="guc-wal-sync-method"> parameter
   to adjust how this is done.)
  </para>

  <para>
   Next, there may be a cache in the disk drive controller; this is
   particularly common on <acronym>RAID</> controller cards. Some of
   these caches are <firstterm>write-through</>, meaning writes are passed
   along to the drive as soon as they arrive. Others are
   <firstterm>write-back</>, meaning data is passed on to the drive at
   some later time. Such caches can be a reliability hazard because the
   memory in the disk controller cache is volatile, and will lose its
   contents in a power failure.  Better controller cards have
   <firstterm>battery-backed</> caches, meaning the card has a battery that
   maintains power to the cache in case of system power loss.  After power
   is restored the data will be written to the disk drives.
  </para>

  <para>
   And finally, most disk drives have caches. Some are write-through
   while some are write-back, and the
   same concerns about data loss exist for write-back drive caches as
   exist for disk controller caches.  Consumer-grade IDE drives are
   particularly likely to contain write-back caches that will not
   survive a power failure.
  </para>

  <para>
   When the operating system sends a write request to the disk hardware,
   there is little it can do to make sure the data has arrived at a truly
   non-volatile storage area. Rather, it is the
   administrator's responsibility to be sure that all storage components
   ensure data integrity.  Avoid disk controllers that have non-battery-backed
   write caches.  At the drive level, disable write-back caching if the
   drive cannot guarantee the data will be written before shutdown.
  </para>
  
  <para>
   Another risk of data loss is posed by the disk platter write
   operations themselves. Disk platters are divided into sectors,
   commonly 512 bytes each.  Every physical read or write operation
   processes a whole sector.
   When a write request arrives at the drive, it might be for 512 bytes,
   1024 bytes, or 8192 bytes, and the process of writing could fail due
   to power loss at any time, meaning some of the 512-byte sectors were
   written, and others were not.  To guard against such failures,
   <productname>PostgreSQL</> periodically writes full page images to
   permanent storage <emphasis>before</> modifying the actual page on
   disk. By doing this, during crash recovery <productname>PostgreSQL</> can
   restore partially-written pages.  If you have a battery-backed disk
   controller or file-system software (e.g., Reiser4) that prevents partial
   page writes,  you can turn off this page imaging by using the 
   <xref linkend="guc-full-page-writes"> parameter.
  </para>
 </sect1>
 
  <sect1 id="wal-intro">
   <title>Write-Ahead Logging (<acronym>WAL</acronym>)</title>

   <indexterm zone="wal">
    <primary>WAL</primary>
   </indexterm>

   <indexterm>
    <primary>transaction log</primary>
    <see>WAL</see>
   </indexterm>

   <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 describing the changes have been flushed
    to permanent storage. If 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 can be redone from the log records.  (This is
    roll-forward recovery, also known as REDO.)
   </para>

   <para>
    A major benefit of using <acronym>WAL</acronym> is a
    significantly reduced number of disk writes, because only the log
    file needs to be flushed to disk at the time of transaction
    commit, rather than every data file changed by the transaction.
    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.   This is especially true for servers
    handling many small transactions touching different parts of the data
    store.
   </para>

   <para>
    <acronym>WAL</acronym> also makes it possible to support on-line
    backup and point-in-time recovery, as described in <xref
    linkend="backup-online">.  By archiving the WAL data we can support
    reverting to any time instant covered by the available WAL data:
    we simply install a prior physical backup of the database, and
    replay the WAL log just as far as the desired time.  What's more,
    the physical backup doesn't have to be an instantaneous snapshot
    of the database state &mdash; if it is made over some period of time,
    then replaying the WAL log for that period will fix any internal
    inconsistencies.
   </para>
  </sect1>

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

  <para>
   There are several <acronym>WAL</>-related configuration parameters that
   affect database performance. This section explains their use.
   Consult <xref linkend="runtime-config"> for general information about
   setting server configuration parameters.
  </para>

  <para>
   <firstterm>Checkpoints</firstterm><indexterm><primary>checkpoint</></>
   are points in the sequence of transactions at which it is guaranteed
   that the data files have been updated with all information written 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.
   In the event of a crash, the crash recovery procedure looks at the latest
   checkpoint record to determine the point in the log (known as the redo
   record) from which it should start the REDO operation.  Any changes made to
   data files before that point are known to be already on disk.  Hence, after
   a checkpoint has been made, any log segments preceding the one containing
   the redo record are no longer needed and can be recycled or removed. (When
   <acronym>WAL</acronym> archiving is being done, the log segments must be
   archived before being recycled or removed.)
  </para>

  <para>
   The server's background writer process will automatically perform
   a checkpoint every so often.  A checkpoint is created every <xref
   linkend="guc-checkpoint-segments"> log segments, or every <xref
   linkend="guc-checkpoint-timeout"> 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. If 
   <xref linkend="guc-full-page-writes"> is set (as is the default), there is 
   another factor to consider. To ensure data page consistency, 
   the first modification of a data page after each checkpoint results in 
   logging the entire page content. In that case,