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diff --git a/doc/src/sgml/html/wal-internals.html b/doc/src/sgml/html/wal-internals.html new file mode 100644 index 0000000..10fb749 --- /dev/null +++ b/doc/src/sgml/html/wal-internals.html @@ -0,0 +1,70 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!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 15.4 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 15.4 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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