blob: b3b58002a59c11406fb4c8d3e24765bd8785b939 (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
|
<?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>69.3. Free Space Map</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="storage-toast.html" title="69.2. TOAST" /><link rel="next" href="storage-vm.html" title="69.4. Visibility Map" /></head><body id="docContent" class="container-fluid col-10"><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">69.3. Free Space Map</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="storage-toast.html" title="69.2. TOAST">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="storage.html" title="Chapter 69. Database Physical Storage">Up</a></td><th width="60%" align="center">Chapter 69. Database Physical Storage</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 13.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="storage-vm.html" title="69.4. Visibility Map">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="STORAGE-FSM"><div class="titlepage"><div><div><h2 class="title" style="clear: both">69.3. Free Space Map</h2></div></div></div><a id="id-1.10.22.5.2" class="indexterm"></a><a id="id-1.10.22.5.3" class="indexterm"></a><p>
Each heap and index relation, except for hash indexes, has a Free Space Map
(FSM) to keep track of available space in the relation. It's stored
alongside the main relation data in a separate relation fork, named after the
filenode number of the relation, plus a <code class="literal">_fsm</code> suffix. For example,
if the filenode of a relation is 12345, the FSM is stored in a file called
<code class="filename">12345_fsm</code>, in the same directory as the main relation file.
</p><p>
The Free Space Map is organized as a tree of <acronym class="acronym">FSM</acronym> pages. The
bottom level <acronym class="acronym">FSM</acronym> pages store the free space available on each
heap (or index) page, using one byte to represent each such page. The upper
levels aggregate information from the lower levels.
</p><p>
Within each <acronym class="acronym">FSM</acronym> page is a binary tree, stored in an array with
one byte per node. Each leaf node represents a heap page, or a lower level
<acronym class="acronym">FSM</acronym> page. In each non-leaf node, the higher of its children's
values is stored. The maximum value in the leaf nodes is therefore stored
at the root.
</p><p>
See <code class="filename">src/backend/storage/freespace/README</code> for more details on
how the <acronym class="acronym">FSM</acronym> is structured, and how it's updated and searched.
The <a class="xref" href="pgfreespacemap.html" title="F.26. pg_freespacemap">pg_freespacemap</a> module
can be used to examine the information stored in free space maps.
</p></div><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="storage-toast.html" title="69.2. TOAST">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="storage.html" title="Chapter 69. Database Physical Storage">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="storage-vm.html" title="69.4. Visibility Map">Next</a></td></tr><tr><td width="40%" align="left" valign="top">69.2. TOAST </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 13.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 69.4. Visibility Map</td></tr></table></div></body></html>
|
