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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>30.6. WAL Internals</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="wal-configuration.html" title="30.5. WAL Configuration" /><link rel="next" href="logical-replication.html" title="Chapter 31. Logical Replication" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">30.6. WAL Internals</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="wal-configuration.html" title="30.5. WAL Configuration">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="wal.html" title="Chapter 30. Reliability and the Write-Ahead Log">Up</a></td><th width="60%" align="center">Chapter 30. Reliability and the Write-Ahead Log</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="logical-replication.html" title="Chapter 31. Logical Replication">Next</a></td></tr></table><hr /></div><div class="sect1" id="WAL-INTERNALS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">30.6. WAL Internals</h2></div></div></div><a id="id-1.6.17.8.2" class="indexterm"></a><p>
<acronym class="acronym">WAL</acronym> is automatically enabled; no action is
required from the administrator except ensuring that the
disk-space requirements for the <acronym class="acronym">WAL</acronym> logs are met,
and that any necessary tuning is done (see <a class="xref" href="wal-configuration.html" title="30.5. WAL Configuration">Section 30.5</a>).
</p><p>
<acronym class="acronym">WAL</acronym> records are appended to the <acronym class="acronym">WAL</acronym>
logs as each new record is written. The insert position is described by
a Log Sequence Number (<acronym class="acronym">LSN</acronym>) that is a byte offset into
the logs, increasing monotonically with each new record.
<acronym class="acronym">LSN</acronym> values are returned as the datatype
<a class="link" href="datatype-pg-lsn.html" title="8.20. pg_lsn Type"><code class="type">pg_lsn</code></a>. Values can be
compared to calculate the volume of <acronym class="acronym">WAL</acronym> data that
separates them, so they are used to measure the progress of replication
and recovery.
</p><p>
<acronym class="acronym">WAL</acronym> logs are stored in the directory
<code class="filename">pg_wal</code> under the data directory, as a set of
segment files, normally each 16 MB in size (but the size can be changed
by altering the <code class="option">--wal-segsize</code> <span class="application">initdb</span> option). Each segment is
divided into pages, normally 8 kB each (this size can be changed via the
<code class="option">--with-wal-blocksize</code> configure option). The log record headers
are described in <code class="filename">access/xlogrecord.h</code>; 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
<code class="filename">000000010000000000000001</code>. The numbers do not wrap,
but it will take a very, very long time to exhaust the
available stock of numbers.
</p><p>
It is advantageous if the log is located on a different disk from the
main database files. This can be achieved by moving the
<code class="filename">pg_wal</code> directory 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.
</p><p>
The aim of <acronym class="acronym">WAL</acronym> is to ensure that the log is
written before database records are altered, but this can be subverted by
disk drives<a id="id-1.6.17.8.7.2" class="indexterm"></a> 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 might lead to
irrecoverable data corruption. Administrators should try to ensure
that disks holding <span class="productname">PostgreSQL</span>'s
<acronym class="acronym">WAL</acronym> log files do not make such false reports.
(See <a class="xref" href="wal-reliability.html" title="30.1. Reliability">Section 30.1</a>.)
</p><p>
After a checkpoint has been made and the log flushed, the
checkpoint's position is saved in the file
<code class="filename">pg_control</code>. Therefore, at the start of recovery,
the server first reads <code class="filename">pg_control</code> and
then the checkpoint record; then it performs the REDO operation by
scanning forward from the log location 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 (assuming
<a class="xref" href="runtime-config-wal.html#GUC-FULL-PAGE-WRITES">full_page_writes</a> is not disabled), all pages
changed since the checkpoint will be restored to a consistent
state.
</p><p>
To deal with the case where <code class="filename">pg_control</code> is
corrupt, we should support the possibility of scanning existing log
segments in reverse order — newest to oldest — in order to find the
latest checkpoint. This has not been implemented yet.
<code class="filename">pg_control</code> is small enough (less than one disk page)
that it is not subject to partial-write problems, and as of this writing
there have been no reports of database failures due solely to the inability
to read <code class="filename">pg_control</code> itself. So while it is
theoretically a weak spot, <code class="filename">pg_control</code> does not
seem to be a problem in practice.
</p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="wal-configuration.html" title="30.5. WAL Configuration">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="wal.html" title="Chapter 30. Reliability and the Write-Ahead Log">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="logical-replication.html" title="Chapter 31. Logical Replication">Next</a></td></tr><tr><td width="40%" align="left" valign="top">30.5. <acronym class="acronym">WAL</acronym> Configuration </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="40%" align="right" valign="top"> Chapter 31. Logical Replication</td></tr></table></div></body></html>
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