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

maps.
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Tom Lane 2009-05-16 22:03:53 +00:00
parent 2d6e2323a4
commit 03a5ff0d1a
1 changed files with 44 additions and 42 deletions
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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">
@ -131,12 +131,12 @@ there.
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
<structname>pg_class</>.<structfield>relfilenode</>. In addition to the
main file (aka. main fork), a <firstterm>free space map</> (see
<xref linkend="storage-fsm">) that stores information about free space
available in the relation, is stored in a file named after the filenode
number, with the <literal>_fsm</> suffix. Tables also have a visibility map
fork, with the <literal>_vm</> suffix, to track which pages are known to have
no dead tuples and therefore need no vacuuming.
main file (a/k/a main fork), each table and index has a <firstterm>free space
map</> (see <xref linkend="storage-fsm">), which stores information about free
space available in the relation. The free space map is stored in a file named
with the filenode number plus the suffix <literal>_fsm</>. Tables also have a
visibility map fork, with the suffix <literal>_vm</>, to track which pages are
known to have no dead tuples and therefore need no vacuuming.
</para>
<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
adjusted using the configuration option <option>--with-segsize</option>
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
<xref linkend="storage-page-layout">.
</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
<filename>pgsql_tmp<replaceable>PPP</>.<replaceable>NNN</></filename>,
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>
</sect1>
@ -383,18 +385,18 @@ comparison table, in which all the HTML pages were cut down to 7 kB to fit.
<indexterm><primary>FSM</><see>Free Space Map</></indexterm>
<para>
A Free Space Map is stored with every heap and index relation, except for
hash indexes, 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
relfilenode of the relation, but with a <literal>_fsm</> suffix. For example,
if the relfilenode of a relation is 12345, the FSM is stored in a file called
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 <literal>_fsm</> suffix. For example,
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.
</para>
<para>
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
heap (or index) page, using one byte to represent each heap page. The upper
bottom level <acronym>FSM</> 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.
</para>
@ -409,8 +411,8 @@ at the root.
<para>
See <filename>src/backend/storage/freespace/README</> for more details on
how the <acronym>FSM</> is structured, and how it's updated and searched.
<xref linkend="pgfreespacemap"> contrib module can be used to view the
information stored in free space maps.
The <filename>contrib/pg_freespacemap</> module can be used to examine the
information stored in free space maps (see <xref linkend="pgfreespacemap">).
</para>
</sect1>