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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>29.5. 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 V1.79.1" /><link rel="prev" href="wal-configuration.html" title="29.4. WAL Configuration" /><link rel="next" href="logical-replication.html" title="Chapter 30. Logical Replication" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">29.5. WAL Internals</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="wal-configuration.html" title="29.4. WAL Configuration">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="wal.html" title="Chapter 29. Reliability and the Write-Ahead Log">Up</a></td><th width="60%" align="center">Chapter 29. Reliability and the Write-Ahead Log</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="logical-replication.html" title="Chapter 30. Logical Replication">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="WAL-INTERNALS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">29.5. WAL Internals</h2></div></div></div><a id="id-1.6.16.7.2" 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="29.4. WAL Configuration">Section 29.4</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> initdb 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-1.6.16.7.7.2" 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="29.1. Reliability">Section 29.1</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">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="wal.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="logical-replication.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">29.4. <acronym class="acronym">WAL</acronym> Configuration </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 30. Logical Replication</td></tr></table></div></body></html>
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