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Minor editorialization on storage.sgml's documentation of free space

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maps.
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Tom Lane 2009-05-16 22:03:53 +00:00
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doc/src/sgml/storage.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/storage.sgml,v 1.27 2009/04/23 10:20:27 heikki Exp $ --> <!-- $PostgreSQL: pgsql/doc/src/sgml/storage.sgml,v 1.28 2009/05/16 22:03:53 tgl Exp $ -->
<chapter id="storage"> <chapter id="storage">
@ -131,12 +131,12 @@ there.
Each table and index is stored in a separate file, named after the table Each table and index is stored in a separate file, named after the table
or index's <firstterm>filenode</> number, which can be found in or index's <firstterm>filenode</> number, which can be found in
<structname>pg_class</>.<structfield>relfilenode</>. In addition to the <structname>pg_class</>.<structfield>relfilenode</>. In addition to the
main file (aka. main fork), a <firstterm>free space map</> (see main file (a/k/a main fork), each table and index has a <firstterm>free space
<xref linkend="storage-fsm">) that stores information about free space map</> (see <xref linkend="storage-fsm">), which stores information about free
available in the relation, is stored in a file named after the filenode space available in the relation. The free space map is stored in a file named
number, with the <literal>_fsm</> suffix. Tables also have a visibility map with the filenode number plus the suffix <literal>_fsm</>. Tables also have a
fork, with the <literal>_vm</> suffix, to track which pages are known to have visibility map fork, with the suffix <literal>_vm</>, to track which pages are
no dead tuples and therefore need no vacuuming. known to have no dead tuples and therefore need no vacuuming.
</para> </para>
<caution> <caution>
@ -157,6 +157,8 @@ This arrangement avoids problems on platforms that have file size limitations.
(Actually, 1 GB is just the default segment size. The segment size can be (Actually, 1 GB is just the default segment size. The segment size can be
adjusted using the configuration option <option>--with-segsize</option> adjusted using the configuration option <option>--with-segsize</option>
when building <productname>PostgreSQL</>.) when building <productname>PostgreSQL</>.)
In principle, free space map and visibility map forks could require multiple
segments as well, though this is unlikely to happen in practice.
The contents of tables and indexes are discussed further in The contents of tables and indexes are discussed further in
<xref linkend="storage-page-layout">. <xref linkend="storage-page-layout">.
</para> </para>
@ -193,7 +195,7 @@ if a tablespace other than <literal>pg_default</> is specified for them.
The name of a temporary file has the form The name of a temporary file has the form
<filename>pgsql_tmp<replaceable>PPP</>.<replaceable>NNN</></filename>, <filename>pgsql_tmp<replaceable>PPP</>.<replaceable>NNN</></filename>,
where <replaceable>PPP</> is the PID of the owning backend and where <replaceable>PPP</> is the PID of the owning backend and
<replaceable>NNN</> distinguishes different files of that backend. <replaceable>NNN</> distinguishes different temporary files of that backend.
</para> </para>
</sect1> </sect1>
@ -377,24 +379,24 @@ comparison table, in which all the HTML pages were cut down to 7 kB to fit.
<title>Free Space Map</title> <title>Free Space Map</title>
<indexterm> <indexterm>
<primary>Free Space Map</primary> <primary>Free Space Map</primary>
</indexterm> </indexterm>
<indexterm><primary>FSM</><see>Free Space Map</></indexterm> <indexterm><primary>FSM</><see>Free Space Map</></indexterm>
<para> <para>
A Free Space Map is stored with every heap and index relation, except for Each heap and index relation, except for hash indexes, has a Free Space Map
hash indexes, to keep track of available space in the relation. It's stored (FSM) to keep track of available space in the relation. It's stored
along the main relation data, in a separate FSM relation fork, named after alongside the main relation data in a separate relation fork, named after the
relfilenode of the relation, but with a <literal>_fsm</> suffix. For example, filenode number of the relation, plus a <literal>_fsm</> suffix. For example,
if the relfilenode of a relation is 12345, the FSM is stored in a file called if the filenode of a relation is 12345, the FSM is stored in a file called
<filename>12345_fsm</>, in the same directory as the main relation file. <filename>12345_fsm</>, in the same directory as the main relation file.
</para> </para>
<para> <para>
The Free Space Map is organized as a tree of <acronym>FSM</> pages. The The Free Space Map is organized as a tree of <acronym>FSM</> pages. The
bottom level <acronym>FSM</> pages stores the free space available on every bottom level <acronym>FSM</> pages store the free space available on each
heap (or index) page, using one byte to represent each heap page. The upper heap (or index) page, using one byte to represent each such page. The upper
levels aggregate information from the lower levels. levels aggregate information from the lower levels.
</para> </para>
@ -409,8 +411,8 @@ at the root.
<para> <para>
See <filename>src/backend/storage/freespace/README</> for more details on See <filename>src/backend/storage/freespace/README</> for more details on
how the <acronym>FSM</> is structured, and how it's updated and searched. how the <acronym>FSM</> is structured, and how it's updated and searched.
<xref linkend="pgfreespacemap"> contrib module can be used to view the The <filename>contrib/pg_freespacemap</> module can be used to examine the
information stored in free space maps. information stored in free space maps (see <xref linkend="pgfreespacemap">).
</para> </para>
</sect1> </sect1>