2010-02-27 02:39:46 +01:00
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<!-- $PostgreSQL: pgsql/doc/src/sgml/wal.sgml,v 1.63 2010/02/27 01:39:46 momjian Exp $ -->
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<chapter id="wal">
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<title>Reliability and the Write-Ahead Log</title>
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<para>
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2007-08-02 00:45:09 +02:00
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This chapter explains how the Write-Ahead Log is used to obtain
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2005-11-05 00:14:02 +01:00
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efficient, reliable operation.
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</para>
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<sect1 id="wal-reliability">
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<title>Reliability</title>
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2005-09-28 20:18:02 +02:00
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<para>
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2005-11-05 00:14:02 +01:00
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Reliability is an important property of any serious database
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system, and <productname>PostgreSQL</> does everything possible to
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guarantee reliable operation. One aspect of reliable operation is
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that all data recorded by a committed transaction should be stored
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in a nonvolatile area that is safe from power loss, operating
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system failure, and hardware failure (except failure of the
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nonvolatile area itself, of course). Successfully writing the data
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to the computer's permanent storage (disk drive or equivalent)
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ordinarily meets this requirement. In fact, even if a computer is
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fatally damaged, if the disk drives survive they can be moved to
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another computer with similar hardware and all committed
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transactions will remain intact.
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2005-09-28 20:18:02 +02:00
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</para>
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2003-08-31 19:32:24 +02:00
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2005-09-28 20:18:02 +02:00
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<para>
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While forcing data periodically to the disk platters might seem like
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a simple operation, it is not. Because disk drives are dramatically
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slower than main memory and CPUs, several layers of caching exist
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2005-10-22 23:56:07 +02:00
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between the computer's main memory and the disk platters.
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First, there is the operating system's buffer cache, which caches
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frequently requested disk blocks and combines disk writes. Fortunately,
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2005-09-28 20:18:02 +02:00
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all operating systems give applications a way to force writes from
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2005-10-22 23:56:07 +02:00
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the buffer cache to disk, and <productname>PostgreSQL</> uses those
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features. (See the <xref linkend="guc-wal-sync-method"> parameter
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to adjust how this is done.)
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2005-09-28 20:18:02 +02:00
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</para>
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2005-10-22 23:56:07 +02:00
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2005-09-28 20:18:02 +02:00
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<para>
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Update documentation on may/can/might:
Standard English uses "may", "can", and "might" in different ways:
may - permission, "You may borrow my rake."
can - ability, "I can lift that log."
might - possibility, "It might rain today."
Unfortunately, in conversational English, their use is often mixed, as
in, "You may use this variable to do X", when in fact, "can" is a better
choice. Similarly, "It may crash" is better stated, "It might crash".
Also update two error messages mentioned in the documenation to match.
2007-01-31 21:56:20 +01:00
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Next, there might be a cache in the disk drive controller; this is
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2005-10-22 23:56:07 +02:00
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particularly common on <acronym>RAID</> controller cards. Some of
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2010-02-03 18:25:06 +01:00
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these caches are <firstterm>write-through</>, meaning writes are sent
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to the drive as soon as they arrive. Others are
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<firstterm>write-back</>, meaning data is sent to the drive at
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2005-10-22 23:56:07 +02:00
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some later time. Such caches can be a reliability hazard because the
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memory in the disk controller cache is volatile, and will lose its
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contents in a power failure. Better controller cards have
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<firstterm>battery-backed</> caches, meaning the card has a battery that
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maintains power to the cache in case of system power loss. After power
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is restored the data will be written to the disk drives.
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2005-09-28 20:18:02 +02:00
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</para>
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2003-08-31 19:32:24 +02:00
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2001-01-25 00:15:19 +01:00
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<para>
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2005-09-28 20:18:02 +02:00
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And finally, most disk drives have caches. Some are write-through
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2005-10-22 23:56:07 +02:00
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while some are write-back, and the
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2005-09-28 20:18:02 +02:00
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same concerns about data loss exist for write-back drive caches as
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2007-12-10 15:51:10 +01:00
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exist for disk controller caches. Consumer-grade IDE and SATA drives are
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2007-12-10 15:05:05 +01:00
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particularly likely to have write-back caches that will not survive a
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2010-02-27 02:39:46 +01:00
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power failure, though <acronym>ATAPI-6</> introduced a drive cache
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flush command that some file systems use, e.g. <acronym>ZFS</>.
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Many solid-state drives also have volatile write-back
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caches, and many do not honor cache flush commands by default.
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To check write caching on <productname>Linux</> use
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2007-12-10 15:05:05 +01:00
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<command>hdparm -I</>; it is enabled if there is a <literal>*</> next
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2010-02-03 18:25:06 +01:00
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to <literal>Write cache</>; <command>hdparm -W</> to turn off
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2007-12-10 15:05:05 +01:00
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write caching. On <productname>FreeBSD</> use
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<application>atacontrol</>. (For SCSI disks use <ulink
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url="http://sg.torque.net/sg/sdparm.html"><application>sdparm</></ulink>
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2007-12-29 18:55:07 +01:00
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to turn off <literal>WCE</>.) On <productname>Solaris</> the disk
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write cache is controlled by <ulink
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url="http://www.sun.com/bigadmin/content/submitted/format_utility.jsp"><literal>format
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-e</></ulink>. (The Solaris <acronym>ZFS</> file system is safe with
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disk write-cache enabled because it issues its own disk cache flush
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commands.) On <productname>Windows</> if <varname>wal_sync_method</>
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is <literal>open_datasync</> (the default), write caching is disabled
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by unchecking <literal>My Computer\Open\{select disk
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2007-12-10 15:05:05 +01:00
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drive}\Properties\Hardware\Properties\Policies\Enable write caching on
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2007-12-16 15:03:32 +01:00
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the disk</>. Also on Windows, <literal>fsync</> and
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2007-12-16 15:05:12 +01:00
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<literal>fsync_writethrough</> never do write caching.
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2005-10-22 23:56:07 +02:00
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</para>
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<para>
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2010-02-03 18:25:06 +01:00
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When the operating system sends a write request to the storage hardware,
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2005-10-22 23:56:07 +02:00
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there is little it can do to make sure the data has arrived at a truly
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non-volatile storage area. Rather, it is the
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2010-02-03 18:25:06 +01:00
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administrator's responsibility to make certain that all storage components
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2005-10-22 23:56:07 +02:00
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ensure data integrity. Avoid disk controllers that have non-battery-backed
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write caches. At the drive level, disable write-back caching if the
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drive cannot guarantee the data will be written before shutdown.
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2009-11-28 17:21:31 +01:00
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You can test for reliable I/O subsystem behavior using <ulink
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url="http://brad.livejournal.com/2116715.html">diskchecker.pl</ulink>.
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2005-09-28 20:18:02 +02:00
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</para>
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2007-08-02 00:45:09 +02:00
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2005-09-28 20:18:02 +02:00
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<para>
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2005-10-22 23:56:07 +02:00
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Another risk of data loss is posed by the disk platter write
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operations themselves. Disk platters are divided into sectors,
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commonly 512 bytes each. Every physical read or write operation
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processes a whole sector.
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2005-09-28 20:18:02 +02:00
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When a write request arrives at the drive, it might be for 512 bytes,
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1024 bytes, or 8192 bytes, and the process of writing could fail due
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to power loss at any time, meaning some of the 512-byte sectors were
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2005-10-22 23:56:07 +02:00
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written, and others were not. To guard against such failures,
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2005-09-28 20:18:02 +02:00
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<productname>PostgreSQL</> periodically writes full page images to
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2010-02-03 18:25:06 +01:00
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permanent WAL storage <emphasis>before</> modifying the actual page on
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2005-10-13 19:32:42 +02:00
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disk. By doing this, during crash recovery <productname>PostgreSQL</> can
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2005-10-22 23:56:07 +02:00
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restore partially-written pages. If you have a battery-backed disk
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2006-10-23 20:10:32 +02:00
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controller or file-system software that prevents partial page writes
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2010-02-03 18:25:06 +01:00
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(e.g., ZFS), you can turn off this page imaging by turning off the
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2005-10-22 23:56:07 +02:00
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<xref linkend="guc-full-page-writes"> parameter.
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2005-09-28 20:18:02 +02:00
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</para>
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2005-11-05 00:14:02 +01:00
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</sect1>
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2007-08-02 00:45:09 +02:00
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2005-11-05 00:14:02 +01:00
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<sect1 id="wal-intro">
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2005-09-28 20:18:02 +02:00
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<title>Write-Ahead Logging (<acronym>WAL</acronym>)</title>
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<indexterm zone="wal">
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<primary>WAL</primary>
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</indexterm>
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<indexterm>
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<primary>transaction log</primary>
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<see>WAL</see>
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</indexterm>
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<para>
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<firstterm>Write-Ahead Logging</firstterm> (<acronym>WAL</acronym>)
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2007-08-02 00:45:09 +02:00
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is a standard method for ensuring data integrity. A detailed
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Update documentation on may/can/might:
Standard English uses "may", "can", and "might" in different ways:
may - permission, "You may borrow my rake."
can - ability, "I can lift that log."
might - possibility, "It might rain today."
Unfortunately, in conversational English, their use is often mixed, as
in, "You may use this variable to do X", when in fact, "can" is a better
choice. Similarly, "It may crash" is better stated, "It might crash".
Also update two error messages mentioned in the documenation to match.
2007-01-31 21:56:20 +01:00
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description can be found in most (if not all) books about
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2005-09-28 20:18:02 +02:00
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transaction processing. Briefly, <acronym>WAL</acronym>'s central
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concept is that changes to data files (where tables and indexes
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reside) must be written only after those changes have been logged,
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2007-08-02 00:45:09 +02:00
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that is, after log records describing the changes have been flushed
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to permanent storage. If we follow this procedure, we do not need
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to flush data pages to disk on every transaction commit, because we
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know that in the event of a crash we will be able to recover the
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database using the log: any changes that have not been applied to
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the data pages can be redone from the log records. (This is
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roll-forward recovery, also known as REDO.)
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</para>
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2001-01-25 00:15:19 +01:00
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2008-12-06 22:34:27 +01:00
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<tip>
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<para>
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Because <acronym>WAL</acronym> restores database file
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2010-02-03 18:25:06 +01:00
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contents after a crash, journaled file systems are not necessary for
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2008-12-18 23:21:16 +01:00
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reliable storage of the data files or WAL files. In fact, journaling
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2008-12-10 12:05:49 +01:00
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overhead can reduce performance, especially if journaling
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causes file system <emphasis>data</emphasis> to be flushed
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to disk. Fortunately, data flushing during journaling can
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2010-02-03 18:25:06 +01:00
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often be disabled with a file system mount option, e.g.
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2008-12-10 12:05:49 +01:00
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<literal>data=writeback</> on a Linux ext3 file system.
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Journaled file systems do improve boot speed after a crash.
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2008-12-06 22:34:27 +01:00
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</para>
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</tip>
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2005-10-13 19:32:42 +02:00
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<para>
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2007-08-02 00:45:09 +02:00
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Using <acronym>WAL</acronym> results in a
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2004-08-08 06:34:43 +02:00
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significantly reduced number of disk writes, because only the log
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file needs to be flushed to disk to guarantee that a transaction is
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committed, rather than every data file changed by the transaction.
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The log file is written sequentially,
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2001-01-25 00:15:19 +01:00
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and so the cost of syncing the log is much less than the cost of
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flushing the data pages. This is especially true for servers
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handling many small transactions touching different parts of the data
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store. Furthermore, when the server is processing many small concurrent
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transactions, one <function>fsync</function> of the log file may
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suffice to commit many transactions.
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2001-01-25 00:15:19 +01:00
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</para>
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<para>
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2005-10-22 23:56:07 +02:00
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<acronym>WAL</acronym> also makes it possible to support on-line
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backup and point-in-time recovery, as described in <xref
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2006-03-03 23:02:08 +01:00
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linkend="continuous-archiving">. By archiving the WAL data we can support
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reverting to any time instant covered by the available WAL data:
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we simply install a prior physical backup of the database, and
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replay the WAL log just as far as the desired time. What's more,
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the physical backup doesn't have to be an instantaneous snapshot
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of the database state — if it is made over some period of time,
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then replaying the WAL log for that period will fix any internal
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inconsistencies.
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2001-10-27 01:10:21 +02:00
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</para>
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2003-03-24 15:32:51 +01:00
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</sect1>
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2001-01-25 00:15:19 +01:00
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2007-08-02 00:45:09 +02:00
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<sect1 id="wal-async-commit">
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<title>Asynchronous Commit</title>
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<indexterm>
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<primary>synchronous commit</primary>
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</indexterm>
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<indexterm>
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<primary>asynchronous commit</primary>
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</indexterm>
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<para>
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<firstterm>Asynchronous commit</> is an option that allows transactions
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to complete more quickly, at the cost that the most recent transactions may
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be lost if the database should crash. In many applications this is an
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acceptable trade-off.
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</para>
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<para>
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As described in the previous section, transaction commit is normally
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<firstterm>synchronous</>: the server waits for the transaction's
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<acronym>WAL</acronym> records to be flushed to permanent storage
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before returning a success indication to the client. The client is
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therefore guaranteed that a transaction reported to be committed will
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be preserved, even in the event of a server crash immediately after.
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However, for short transactions this delay is a major component of the
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total transaction time. Selecting asynchronous commit mode means that
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the server returns success as soon as the transaction is logically
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completed, before the <acronym>WAL</acronym> records it generated have
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actually made their way to disk. This can provide a significant boost
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in throughput for small transactions.
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</para>
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<para>
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Asynchronous commit introduces the risk of data loss. There is a short
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time window between the report of transaction completion to the client
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and the time that the transaction is truly committed (that is, it is
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guaranteed not to be lost if the server crashes). Thus asynchronous
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commit should not be used if the client will take external actions
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relying on the assumption that the transaction will be remembered.
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As an example, a bank would certainly not use asynchronous commit for
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a transaction recording an ATM's dispensing of cash. But in many
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scenarios, such as event logging, there is no need for a strong
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guarantee of this kind.
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</para>
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<para>
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The risk that is taken by using asynchronous commit is of data loss,
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not data corruption. If the database should crash, it will recover
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by replaying <acronym>WAL</acronym> up to the last record that was
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flushed. The database will therefore be restored to a self-consistent
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state, but any transactions that were not yet flushed to disk will
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not be reflected in that state. The net effect is therefore loss of
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the last few transactions. Because the transactions are replayed in
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commit order, no inconsistency can be introduced — for example,
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if transaction B made changes relying on the effects of a previous
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transaction A, it is not possible for A's effects to be lost while B's
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effects are preserved.
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</para>
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<para>
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The user can select the commit mode of each transaction, so that
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it is possible to have both synchronous and asynchronous commit
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transactions running concurrently. This allows flexible trade-offs
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between performance and certainty of transaction durability.
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The commit mode is controlled by the user-settable parameter
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<xref linkend="guc-synchronous-commit">, which can be changed in any of
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the ways that a configuration parameter can be set. The mode used for
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any one transaction depends on the value of
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<varname>synchronous_commit</varname> when transaction commit begins.
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</para>
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<para>
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|
|
|
Certain utility commands, for instance <command>DROP TABLE</>, are
|
|
|
|
forced to commit synchronously regardless of the setting of
|
|
|
|
<varname>synchronous_commit</varname>. This is to ensure consistency
|
2007-11-28 16:42:31 +01:00
|
|
|
between the server's file system and the logical state of the database.
|
2007-08-02 00:45:09 +02:00
|
|
|
The commands supporting two-phase commit, such as <command>PREPARE
|
|
|
|
TRANSACTION</>, are also always synchronous.
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
If the database crashes during the risk window between an
|
|
|
|
asynchronous commit and the writing of the transaction's
|
|
|
|
<acronym>WAL</acronym> records,
|
|
|
|
then changes made during that transaction <emphasis>will</> be lost.
|
|
|
|
The duration of the
|
2007-11-28 16:42:31 +01:00
|
|
|
risk window is limited because a background process (the <quote>WAL
|
2007-08-02 00:45:09 +02:00
|
|
|
writer</>) flushes unwritten <acronym>WAL</acronym> records to disk
|
|
|
|
every <xref linkend="guc-wal-writer-delay"> milliseconds.
|
|
|
|
The actual maximum duration of the risk window is three times
|
2007-11-28 16:42:31 +01:00
|
|
|
<varname>wal_writer_delay</varname> because the WAL writer is
|
2007-08-02 00:45:09 +02:00
|
|
|
designed to favor writing whole pages at a time during busy periods.
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<caution>
|
|
|
|
<para>
|
|
|
|
An immediate-mode shutdown is equivalent to a server crash, and will
|
|
|
|
therefore cause loss of any unflushed asynchronous commits.
|
|
|
|
</para>
|
|
|
|
</caution>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
Asynchronous commit provides behavior different from setting
|
|
|
|
<xref linkend="guc-fsync"> = off.
|
|
|
|
<varname>fsync</varname> is a server-wide
|
|
|
|
setting that will alter the behavior of all transactions. It disables
|
|
|
|
all logic within <productname>PostgreSQL</> that attempts to synchronize
|
|
|
|
writes to different portions of the database, and therefore a system
|
|
|
|
crash (that is, a hardware or operating system crash, not a failure of
|
|
|
|
<productname>PostgreSQL</> itself) could result in arbitrarily bad
|
|
|
|
corruption of the database state. In many scenarios, asynchronous
|
|
|
|
commit provides most of the performance improvement that could be
|
|
|
|
obtained by turning off <varname>fsync</varname>, but without the risk
|
|
|
|
of data corruption.
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
<xref linkend="guc-commit-delay"> also sounds very similar to
|
|
|
|
asynchronous commit, but it is actually a synchronous commit method
|
|
|
|
(in fact, <varname>commit_delay</varname> is ignored during an
|
|
|
|
asynchronous commit). <varname>commit_delay</varname> causes a delay
|
|
|
|
just before a synchronous commit attempts to flush
|
|
|
|
<acronym>WAL</acronym> to disk, in the hope that a single flush
|
|
|
|
executed by one such transaction can also serve other transactions
|
|
|
|
committing at about the same time. Setting <varname>commit_delay</varname>
|
|
|
|
can only help when there are many concurrently committing transactions,
|
|
|
|
and it is difficult to tune it to a value that actually helps rather
|
2010-02-03 18:25:06 +01:00
|
|
|
than hurt throughput.
|
2007-08-02 00:45:09 +02:00
|
|
|
</para>
|
|
|
|
|
|
|
|
</sect1>
|
|
|
|
|
2001-01-25 00:15:19 +01:00
|
|
|
<sect1 id="wal-configuration">
|
|
|
|
<title><acronym>WAL</acronym> Configuration</title>
|
|
|
|
|
|
|
|
<para>
|
2005-10-22 23:56:07 +02:00
|
|
|
There are several <acronym>WAL</>-related configuration parameters that
|
2001-01-25 00:15:19 +01:00
|
|
|
affect database performance. This section explains their use.
|
2004-08-08 06:34:43 +02:00
|
|
|
Consult <xref linkend="runtime-config"> for general information about
|
|
|
|
setting server configuration parameters.
|
2001-01-25 00:15:19 +01:00
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
2004-11-15 05:35:57 +01:00
|
|
|
<firstterm>Checkpoints</firstterm><indexterm><primary>checkpoint</></>
|
|
|
|
are points in the sequence of transactions at which it is guaranteed
|
2010-02-03 18:25:06 +01:00
|
|
|
that the heap and index data files have been updated with all information written before
|
2004-11-15 05:35:57 +01:00
|
|
|
the checkpoint. At checkpoint time, all dirty data pages are flushed to
|
2009-04-09 18:20:50 +02:00
|
|
|
disk and a special checkpoint record is written to the log file.
|
|
|
|
(The changes were previously flushed to the <acronym>WAL</acronym> files.)
|
2005-10-22 23:56:07 +02:00
|
|
|
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
|
2009-04-09 18:20:50 +02:00
|
|
|
data files before that point are guaranteed to be already on disk. Hence, after
|
|
|
|
a checkpoint, log segments preceding the one containing
|
2005-10-22 23:56:07 +02:00
|
|
|
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.)
|
2001-01-25 00:15:19 +01:00
|
|
|
</para>
|
|
|
|
|
2009-04-09 18:20:50 +02:00
|
|
|
<para>
|
|
|
|
The checkpoint requirement of flushing all dirty data pages to disk
|
|
|
|
can cause a significant I/O load. For this reason, checkpoint
|
|
|
|
activity is throttled so I/O begins at checkpoint start and completes
|
|
|
|
before the next checkpoint starts; this minimizes performance
|
|
|
|
degradation during checkpoints.
|
|
|
|
</para>
|
|
|
|
|
2001-01-25 00:15:19 +01:00
|
|
|
<para>
|
2010-02-03 18:25:06 +01:00
|
|
|
The server's background writer process automatically performs
|
2004-08-08 06:34:43 +02:00
|
|
|
a checkpoint every so often. A checkpoint is created every <xref
|
2004-03-09 17:57:47 +01:00
|
|
|
linkend="guc-checkpoint-segments"> log segments, or every <xref
|
|
|
|
linkend="guc-checkpoint-timeout"> seconds, whichever comes first.
|
2010-02-03 18:25:06 +01:00
|
|
|
The default settings are 3 segments and 300 seconds (5 minutes), respectively.
|
XLOG (and related) changes:
* Store two past checkpoint locations, not just one, in pg_control.
On startup, we fall back to the older checkpoint if the newer one
is unreadable. Also, a physical copy of the newest checkpoint record
is kept in pg_control for possible use in disaster recovery (ie,
complete loss of pg_xlog). Also add a version number for pg_control
itself. Remove archdir from pg_control; it ought to be a GUC
parameter, not a special case (not that it's implemented yet anyway).
* Suppress successive checkpoint records when nothing has been entered
in the WAL log since the last one. This is not so much to avoid I/O
as to make it actually useful to keep track of the last two
checkpoints. If the things are right next to each other then there's
not a lot of redundancy gained...
* Change CRC scheme to a true 64-bit CRC, not a pair of 32-bit CRCs
on alternate bytes. Polynomial borrowed from ECMA DLT1 standard.
* Fix XLOG record length handling so that it will work at BLCKSZ = 32k.
* Change XID allocation to work more like OID allocation. (This is of
dubious necessity, but I think it's a good idea anyway.)
* Fix a number of minor bugs, such as off-by-one logic for XLOG file
wraparound at the 4 gig mark.
* Add documentation and clean up some coding infelicities; move file
format declarations out to include files where planned contrib
utilities can get at them.
* Checkpoint will now occur every CHECKPOINT_SEGMENTS log segments or
every CHECKPOINT_TIMEOUT seconds, whichever comes first. It is also
possible to force a checkpoint by sending SIGUSR1 to the postmaster
(undocumented feature...)
* Defend against kill -9 postmaster by storing shmem block's key and ID
in postmaster.pid lockfile, and checking at startup to ensure that no
processes are still connected to old shmem block (if it still exists).
* Switch backends to accept SIGQUIT rather than SIGUSR1 for emergency
stop, for symmetry with postmaster and xlog utilities. Clean up signal
handling in bootstrap.c so that xlog utilities launched by postmaster
will react to signals better.
* Standalone bootstrap now grabs lockfile in target directory, as added
insurance against running it in parallel with live postmaster.
2001-03-13 02:17:06 +01:00
|
|
|
It is also possible to force a checkpoint by using the SQL command
|
2001-01-25 00:15:19 +01:00
|
|
|
<command>CHECKPOINT</command>.
|
|
|
|
</para>
|
|
|
|
|
2001-10-27 01:10:21 +02:00
|
|
|
<para>
|
2003-03-24 15:32:51 +01:00
|
|
|
Reducing <varname>checkpoint_segments</varname> and/or
|
2010-02-03 18:25:06 +01:00
|
|
|
<varname>checkpoint_timeout</varname> causes checkpoints to occur
|
2002-02-12 00:25:14 +01:00
|
|
|
more often. This allows faster after-crash recovery (since less work
|
|
|
|
will need to be redone). However, one must balance this against the
|
2007-08-02 00:45:09 +02:00
|
|
|
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
|
2005-10-13 19:32:42 +02:00
|
|
|
logging the entire page content. In that case,
|
|
|
|
a smaller checkpoint interval increases the volume of output to the WAL log,
|
2007-08-02 00:45:09 +02:00
|
|
|
partially negating the goal of using a smaller interval,
|
2005-10-13 19:32:42 +02:00
|
|
|
and in any case causing more disk I/O.
|
2001-10-27 01:10:21 +02:00
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
2004-08-08 06:34:43 +02:00
|
|
|
Checkpoints are fairly expensive, first because they require writing
|
|
|
|
out all currently dirty buffers, and second because they result in
|
|
|
|
extra subsequent WAL traffic as discussed above. It is therefore
|
|
|
|
wise to set the checkpointing parameters high enough that checkpoints
|
|
|
|
don't happen too often. As a simple sanity check on your checkpointing
|
|
|
|
parameters, you can set the <xref linkend="guc-checkpoint-warning">
|
|
|
|
parameter. If checkpoints happen closer together than
|
2007-08-02 00:45:09 +02:00
|
|
|
<varname>checkpoint_warning</> seconds,
|
|
|
|
a message will be output to the server log recommending increasing
|
2004-08-08 06:34:43 +02:00
|
|
|
<varname>checkpoint_segments</varname>. Occasional appearance of such
|
|
|
|
a message is not cause for alarm, but if it appears often then the
|
2005-10-13 19:32:42 +02:00
|
|
|
checkpoint control parameters should be increased. Bulk operations such
|
Update documentation on may/can/might:
Standard English uses "may", "can", and "might" in different ways:
may - permission, "You may borrow my rake."
can - ability, "I can lift that log."
might - possibility, "It might rain today."
Unfortunately, in conversational English, their use is often mixed, as
in, "You may use this variable to do X", when in fact, "can" is a better
choice. Similarly, "It may crash" is better stated, "It might crash".
Also update two error messages mentioned in the documenation to match.
2007-01-31 21:56:20 +01:00
|
|
|
as large <command>COPY</> transfers might cause a number of such warnings
|
2005-10-22 23:56:07 +02:00
|
|
|
to appear if you have not set <varname>checkpoint_segments</> high
|
|
|
|
enough.
|
2004-08-08 06:34:43 +02:00
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
2007-06-28 02:02:40 +02:00
|
|
|
To avoid flooding the I/O system with a burst of page writes,
|
|
|
|
writing dirty buffers during a checkpoint is spread over a period of time.
|
|
|
|
That period is controlled by
|
|
|
|
<xref linkend="guc-checkpoint-completion-target">, which is
|
|
|
|
given as a fraction of the checkpoint interval.
|
|
|
|
The I/O rate is adjusted so that the checkpoint finishes when the
|
|
|
|
given fraction of <varname>checkpoint_segments</varname> WAL segments
|
|
|
|
have been consumed since checkpoint start, or the given fraction of
|
|
|
|
<varname>checkpoint_timeout</varname> seconds have elapsed,
|
|
|
|
whichever is sooner. With the default value of 0.5,
|
|
|
|
<productname>PostgreSQL</> can be expected to complete each checkpoint
|
|
|
|
in about half the time before the next checkpoint starts. On a system
|
|
|
|
that's very close to maximum I/O throughput during normal operation,
|
|
|
|
you might want to increase <varname>checkpoint_completion_target</varname>
|
|
|
|
to reduce the I/O load from checkpoints. The disadvantage of this is that
|
|
|
|
prolonging checkpoints affects recovery time, because more WAL segments
|
|
|
|
will need to be kept around for possible use in recovery. Although
|
|
|
|
<varname>checkpoint_completion_target</varname> can be set as high as 1.0,
|
|
|
|
it is best to keep it less than that (perhaps 0.9 at most) since
|
|
|
|
checkpoints include some other activities besides writing dirty buffers.
|
|
|
|
A setting of 1.0 is quite likely to result in checkpoints not being
|
|
|
|
completed on time, which would result in performance loss due to
|
|
|
|
unexpected variation in the number of WAL segments needed.
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
|
|
|
There will always be at least one WAL segment file, and will normally
|
|
|
|
not be more than (2 + <varname>checkpoint_completion_target</varname>) * <varname>checkpoint_segments</varname> + 1
|
2004-08-08 06:34:43 +02:00
|
|
|
files. Each segment file is normally 16 MB (though this size can be
|
|
|
|
altered when building the server). You can use this to estimate space
|
|
|
|
requirements for <acronym>WAL</acronym>.
|
2003-03-24 15:32:51 +01:00
|
|
|
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
|
2007-06-28 02:02:40 +02:00
|
|
|
are more than 3 * <varname>checkpoint_segments</varname> + 1
|
2003-03-24 15:32:51 +01:00
|
|
|
segment files, the unneeded segment files will be deleted instead
|
|
|
|
of recycled until the system gets back under this limit.
|
2001-10-27 01:10:21 +02:00
|
|
|
</para>
|
|
|
|
|
2002-11-02 23:23:01 +01:00
|
|
|
<para>
|
2005-10-13 19:32:42 +02:00
|
|
|
There are two commonly used internal <acronym>WAL</acronym> functions:
|
2002-11-02 23:23:01 +01:00
|
|
|
<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>
|
2004-03-09 17:57:47 +01:00
|
|
|
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
|
Update documentation on may/can/might:
Standard English uses "may", "can", and "might" in different ways:
may - permission, "You may borrow my rake."
can - ability, "I can lift that log."
might - possibility, "It might rain today."
Unfortunately, in conversational English, their use is often mixed, as
in, "You may use this variable to do X", when in fact, "can" is a better
choice. Similarly, "It may crash" is better stated, "It might crash".
Also update two error messages mentioned in the documenation to match.
2007-01-31 21:56:20 +01:00
|
|
|
is worse, writing <acronym>WAL</acronym> buffers might also force the
|
2004-03-09 17:57:47 +01:00
|
|
|
creation of a new log segment, which takes even more
|
2002-11-02 23:23:01 +01:00
|
|
|
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
|
Update documentation on may/can/might:
Standard English uses "may", "can", and "might" in different ways:
may - permission, "You may borrow my rake."
can - ability, "I can lift that log."
might - possibility, "It might rain today."
Unfortunately, in conversational English, their use is often mixed, as
in, "You may use this variable to do X", when in fact, "can" is a better
choice. Similarly, "It may crash" is better stated, "It might crash".
Also update two error messages mentioned in the documenation to match.
2007-01-31 21:56:20 +01:00
|
|
|
with high log output, <function>LogFlush</function> requests might
|
2004-08-08 06:34:43 +02:00
|
|
|
not occur often enough to prevent <function>LogInsert</function>
|
|
|
|
from having to do writes. On such systems
|
2002-11-02 23:23:01 +01:00
|
|
|
one should increase the number of <acronym>WAL</acronym> buffers by
|
2004-03-09 17:57:47 +01:00
|
|
|
modifying the configuration parameter <xref
|
2004-08-08 06:34:43 +02:00
|
|
|
linkend="guc-wal-buffers">. The default number of <acronym>WAL</acronym>
|
|
|
|
buffers is 8. Increasing this value will
|
2007-08-02 00:45:09 +02:00
|
|
|
correspondingly increase shared memory usage. When
|
|
|
|
<xref linkend="guc-full-page-writes"> is set and the system is very busy,
|
|
|
|
setting this value higher will help smooth response times during the
|
2005-10-22 23:56:07 +02:00
|
|
|
period immediately following each checkpoint.
|
2002-11-15 03:44:57 +01:00
|
|
|
</para>
|
|
|
|
|
2001-01-25 00:15:19 +01:00
|
|
|
<para>
|
2004-03-09 17:57:47 +01:00
|
|
|
The <xref linkend="guc-commit-delay"> parameter defines for how many
|
2003-03-24 15:32:51 +01:00
|
|
|
microseconds the server process will sleep after writing a commit
|
2001-02-18 05:50:43 +01:00
|
|
|
record to the log with <function>LogInsert</function> but before
|
2001-01-25 00:15:19 +01:00
|
|
|
performing a <function>LogFlush</function>. This delay allows other
|
2003-03-24 15:32:51 +01:00
|
|
|
server processes to add their commit records to the log so as to have all
|
2003-09-20 22:12:05 +02:00
|
|
|
of them flushed with a single log sync. No sleep will occur if
|
2004-03-09 17:57:47 +01:00
|
|
|
<xref linkend="guc-fsync">
|
2010-02-03 18:25:06 +01:00
|
|
|
is not enabled, or if fewer than <xref linkend="guc-commit-siblings">
|
2003-09-20 22:12:05 +02:00
|
|
|
other sessions are currently in active transactions; this avoids
|
2003-03-24 15:32:51 +01:00
|
|
|
sleeping when it's unlikely that any other session will commit soon.
|
2001-02-26 01:50:08 +01:00
|
|
|
Note that on most platforms, the resolution of a sleep request is
|
2003-03-24 15:32:51 +01:00
|
|
|
ten milliseconds, so that any nonzero <varname>commit_delay</varname>
|
|
|
|
setting between 1 and 10000 microseconds would have the same effect.
|
2001-10-27 01:10:21 +02:00
|
|
|
Good values for these parameters are not yet clear; experimentation
|
|
|
|
is encouraged.
|
2001-01-25 00:15:19 +01:00
|
|
|
</para>
|
2001-03-16 06:44:33 +01:00
|
|
|
|
|
|
|
<para>
|
2004-03-09 17:57:47 +01:00
|
|
|
The <xref linkend="guc-wal-sync-method"> parameter determines how
|
2001-11-21 07:09:45 +01:00
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<productname>PostgreSQL</productname> will ask the kernel to force
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2007-08-02 00:45:09 +02:00
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<acronym>WAL</acronym> updates out to disk.
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2010-02-03 18:25:06 +01:00
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All the options should be the same in terms of reliability,
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2001-03-16 06:44:33 +01:00
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but it's quite platform-specific which one will be the fastest.
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2003-03-24 15:32:51 +01:00
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Note that this parameter is irrelevant if <varname>fsync</varname>
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2001-03-16 06:44:33 +01:00
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has been turned off.
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</para>
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<para>
|
2004-03-09 17:57:47 +01:00
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Enabling the <xref linkend="guc-wal-debug"> configuration parameter
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(provided that <productname>PostgreSQL</productname> has been
|
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|
compiled with support for it) will result in each
|
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<function>LogInsert</function> and <function>LogFlush</function>
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2005-10-22 23:56:07 +02:00
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<acronym>WAL</acronym> call being logged to the server log. This
|
Update documentation on may/can/might:
Standard English uses "may", "can", and "might" in different ways:
may - permission, "You may borrow my rake."
can - ability, "I can lift that log."
might - possibility, "It might rain today."
Unfortunately, in conversational English, their use is often mixed, as
in, "You may use this variable to do X", when in fact, "can" is a better
choice. Similarly, "It may crash" is better stated, "It might crash".
Also update two error messages mentioned in the documenation to match.
2007-01-31 21:56:20 +01:00
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option might be replaced by a more general mechanism in the future.
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2001-03-16 06:44:33 +01:00
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</para>
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2001-01-25 00:15:19 +01:00
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</sect1>
|
2003-03-24 15:32:51 +01:00
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|
|
<sect1 id="wal-internals">
|
2005-09-28 20:18:02 +02:00
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|
<title>WAL Internals</title>
|
2003-03-24 15:32:51 +01:00
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<para>
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<acronym>WAL</acronym> is automatically enabled; no action is
|
2004-08-08 06:34:43 +02:00
|
|
|
required from the administrator except ensuring that the
|
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|
|
disk-space requirements for the <acronym>WAL</acronym> logs are met,
|
2003-03-24 15:32:51 +01:00
|
|
|
and that any necessary tuning is done (see <xref
|
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|
|
linkend="wal-configuration">).
|
|
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|
</para>
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|
<para>
|
|
|
|
<acronym>WAL</acronym> logs are stored in the directory
|
|
|
|
<filename>pg_xlog</filename> under the data directory, as a set of
|
2008-05-02 21:52:37 +02:00
|
|
|
segment files, normally each 16 MB in size (but the size can be changed
|
|
|
|
by altering the <option>--with-wal-segsize</> configure option when
|
|
|
|
building the server). Each segment is divided into pages, normally
|
|
|
|
8 kB each (this size can be changed via the <option>--with-wal-blocksize</>
|
|
|
|
configure option). The log record headers are described in
|
2003-03-24 15:32:51 +01:00
|
|
|
<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
|
2010-02-03 18:25:06 +01:00
|
|
|
<filename>000000010000000000000000</filename>. The numbers do not wrap,
|
|
|
|
but it will take a very, very long time to exhaust the
|
2003-03-24 15:32:51 +01:00
|
|
|
available stock of numbers.
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
2010-02-03 18:25:06 +01:00
|
|
|
It is advantageous if the log is located on a different disk from the
|
|
|
|
main database files. This can be achieved by moving the
|
|
|
|
<filename>pg_xlog</filename> directory to another location (while the server
|
2003-03-24 15:32:51 +01:00
|
|
|
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>
|
2010-02-03 18:25:06 +01:00
|
|
|
The aim of <acronym>WAL</acronym> is to ensure that the log is
|
|
|
|
written before database records are altered, but this can be subverted by
|
2004-08-08 06:34:43 +02:00
|
|
|
disk drives<indexterm><primary>disk drive</></> that falsely report a
|
2007-08-02 00:45:09 +02:00
|
|
|
successful write to the kernel,
|
2004-08-08 06:34:43 +02:00
|
|
|
when in fact they have only cached the data and not yet stored it
|
2010-02-03 18:25:06 +01:00
|
|
|
on the disk. A power failure in such a situation might lead to
|
2003-03-24 15:32:51 +01:00
|
|
|
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
|
2010-02-03 18:25:06 +01:00
|
|
|
<filename>pg_control</filename>. Therefore, at the start of recovery,
|
|
|
|
the server first reads <filename>pg_control</filename> and
|
2003-03-24 15:32:51 +01:00
|
|
|
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
|
2009-01-15 01:34:25 +01:00
|
|
|
log on the first page modification after a checkpoint (assuming
|
|
|
|
<xref linkend="guc-full-page-writes"> is not disabled), all pages
|
2003-03-24 15:32:51 +01:00
|
|
|
changed since the checkpoint will be restored to a consistent
|
|
|
|
state.
|
|
|
|
</para>
|
|
|
|
|
|
|
|
<para>
|
2004-08-08 06:34:43 +02:00
|
|
|
To deal with the case where <filename>pg_control</filename> is
|
2010-02-03 18:25:06 +01:00
|
|
|
corrupt, we should support the possibility of scanning existing log
|
2004-11-15 07:32:15 +01:00
|
|
|
segments in reverse order — newest to oldest — in order to find the
|
2004-08-08 06:34:43 +02:00
|
|
|
latest checkpoint. This has not been implemented yet.
|
|
|
|
<filename>pg_control</filename> is small enough (less than one disk page)
|
|
|
|
that it is not subject to partial-write problems, and as of this writing
|
2010-02-03 18:25:06 +01:00
|
|
|
there have been no reports of database failures due solely to the inability
|
2004-08-08 06:34:43 +02:00
|
|
|
to read <filename>pg_control</filename> itself. So while it is
|
|
|
|
theoretically a weak spot, <filename>pg_control</filename> does not
|
|
|
|
seem to be a problem in practice.
|
2003-03-24 15:32:51 +01:00
|
|
|
</para>
|
|
|
|
</sect1>
|
2001-01-25 00:15:19 +01:00
|
|
|
</chapter>
|