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Commit03a5ff0

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Minor editorialization on storage.sgml's documentation of free space
maps.
1 parent2d6e232 commit03a5ff0

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‎doc/src/sgml/storage.sgml‎

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<!-- $PostgreSQL: pgsql/doc/src/sgml/storage.sgml,v 1.27 2009/04/23 10:20:27 heikki Exp $ -->
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<!-- $PostgreSQL: pgsql/doc/src/sgml/storage.sgml,v 1.28 2009/05/16 22:03:53 tgl Exp $ -->
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<chapter id="storage">
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@@ -33,7 +33,7 @@ these required items, the cluster configuration files
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<filename>postgresql.conf</filename>, <filename>pg_hba.conf</filename>, and
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<filename>pg_ident.conf</filename> are traditionally stored in
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<varname>PGDATA</> (although in <productname>PostgreSQL</productname> 8.0 and
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later, it is possible to keep them elsewhere).
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later, it is possible to keep them elsewhere).
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</para>
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<table tocentry="1" id="pgdata-contents-table">
@@ -74,7 +74,7 @@ Item
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<row>
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<entry><filename>pg_multixact</></entry>
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<entry>Subdirectory containing multitransaction status data
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(used for shared row locks)</entry>
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(used for shared row locks)</entry>
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</row>
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<row>
@@ -131,12 +131,12 @@ there.
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Each table and index is stored in a separate file, named after the table
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or index's <firstterm>filenode</> number, which can be found in
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<structname>pg_class</>.<structfield>relfilenode</>. In addition to the
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main file (aka. main fork), a <firstterm>free space map</> (see
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<xref linkend="storage-fsm">) that stores information about free space
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available in the relation,is stored in a file named after the filenode
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number, with the <literal>_fsm</> suffix. Tables also have a visibility map
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fork, with the <literal>_vm</> suffix, to track which pages are known to have
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no dead tuples and therefore need no vacuuming.
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main file (a/k/a main fork),each table and index hasa <firstterm>free space
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map</> (see <xref linkend="storage-fsm">), which stores information about free
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spaceavailable in the relation. The free space mapis stored in a file named
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with the filenodenumber plus thesuffix<literal>_fsm</>. Tables also have a
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visibility mapfork, with thesuffix<literal>_vm</>, to track which pages are
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known to haveno dead tuples and therefore need no vacuuming.
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</para>
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<caution>
@@ -157,6 +157,8 @@ This arrangement avoids problems on platforms that have file size limitations.
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(Actually, 1 GB is just the default segment size. The segment size can be
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adjusted using the configuration option <option>--with-segsize</option>
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when building <productname>PostgreSQL</>.)
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In principle, free space map and visibility map forks could require multiple
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segments as well, though this is unlikely to happen in practice.
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The contents of tables and indexes are discussed further in
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<xref linkend="storage-page-layout">.
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</para>
@@ -193,7 +195,7 @@ if a tablespace other than <literal>pg_default</> is specified for them.
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The name of a temporary file has the form
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<filename>pgsql_tmp<replaceable>PPP</>.<replaceable>NNN</></filename>,
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where <replaceable>PPP</> is the PID of the owning backend and
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<replaceable>NNN</> distinguishes different files of that backend.
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<replaceable>NNN</> distinguishes differenttemporaryfiles of that backend.
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</para>
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</sect1>
@@ -215,10 +217,10 @@ Oversized-Attribute Storage Technique).
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<para>
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<productname>PostgreSQL</productname> uses a fixed page size (commonly
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8 kB), and does not allow tuples to span multiple pages. Therefore, it is
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not possible to store very large field values directly. To overcome
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not possible to store very large field values directly. To overcome
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this limitation, large field values are compressed and/or broken up into
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multiple physical rows. This happens transparently to the user, with only
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small impact on most of the backend code. The technique is affectionately
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small impact on most of the backend code. The technique is affectionately
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known as <acronym>TOAST</> (or <quote>the best thing since sliced bread</>).
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</para>
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@@ -377,24 +379,24 @@ comparison table, in which all the HTML pages were cut down to 7 kB to fit.
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<title>Free Space Map</title>
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<indexterm>
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<primary>Free Space Map</primary>
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</indexterm>
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<indexterm><primary>FSM</><see>Free Space Map</></indexterm>
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<indexterm>
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<primary>Free Space Map</primary>
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</indexterm>
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<indexterm><primary>FSM</><see>Free Space Map</></indexterm>
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<para>
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A Free Space Map is stored with everyheap and index relation, except for
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hash indexes, to keep track of available space in the relation. It's stored
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along the main relation data, in a separateFSMrelation fork, named after
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relfilenodeof the relation,but with a <literal>_fsm</> suffix. For example,
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if therelfilenode of a relation is 12345, the FSM is stored in a file called
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Eachheap and index relation, except for hash indexes, has a Free Space Map
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(FSM) to keep track of available space in the relation. It's stored
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alongside the main relation data in a separate relation fork, named after the
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filenode numberof the relation,plus a <literal>_fsm</> suffix. For example,
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if thefilenode of a relation is 12345, the FSM is stored in a file called
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<filename>12345_fsm</>, in the same directory as the main relation file.
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</para>
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<para>
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The Free Space Map is organized as a tree of <acronym>FSM</> pages. The
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bottom level <acronym>FSM</> pagesstores the free space available onevery
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heap (or index) page, using one byte to represent eachheap page. The upper
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bottom level <acronym>FSM</> pagesstore the free space available oneach
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heap (or index) page, using one byte to represent eachsuch page. The upper
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levels aggregate information from the lower levels.
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</para>
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@@ -409,8 +411,8 @@ at the root.
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<para>
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See <filename>src/backend/storage/freespace/README</> for more details on
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how the <acronym>FSM</> is structured, and how it's updated and searched.
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<xref linkend="pgfreespacemap">contrib module can be used toview the
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information stored in free space maps.
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The <filename>contrib/pg_freespacemap</> module can be used toexamine the
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information stored in free space maps (see <xref linkend="pgfreespacemap">).
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</para>
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</sect1>
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and <structfield>pd_special</structfield>). These contain byte offsets
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from the page start to the start
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of unallocated space, to the end of unallocated space, and to the start of
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the special space.
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the special space.
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The next 2 bytes of the page header,
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<structfield>pd_pagesize_version</structfield>, store both the page size
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and a version indicator. Beginning with
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more than one page size in an installation.
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The last field is a hint that shows whether pruning the page is likely
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to be profitable: it tracks the oldest un-pruned XMAX on the page.
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</para>
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<table tocentry="1" id="pageheaderdata-table">
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<title>PageHeaderData Layout</title>
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<titleabbrev>PageHeaderData Layout</titleabbrev>
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<tgroup cols="4">
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<tgroup cols="4">
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<thead>
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<row>
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<row>
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<entry>Field</entry>
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<entry>Type</entry>
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<entry>Length</entry>
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</para>
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<para>
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The items themselves are stored in space allocated backwards from the end
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of unallocated space. The exact structure varies depending on what the
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table is to contain. Tables and sequences both use a structure named
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<type>HeapTupleHeaderData</type>, described below.
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</para>
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<para>
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The final section is the <quote>special section</quote> which can
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contain anything the access method wishes to store. For example,
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b-tree indexes store links to the page's left and right siblings,
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as well as some other data relevant to the index structure.
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Ordinary tables do not use a special section at all (indicated by setting
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<structfield>pd_special</> to equal the page size).
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</para>
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<para>
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All table rows are structured in the same way. There is a fixed-size
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<structfield>t_hoff</> a MAXALIGN multiple will appear between the null
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bitmap and the object ID. (This in turn ensures that the object ID is
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suitably aligned.)
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</para>
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<table tocentry="1" id="heaptupleheaderdata-table">
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<title>HeapTupleHeaderData Layout</title>
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<titleabbrev>HeapTupleHeaderData Layout</titleabbrev>
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<tgroup cols="4">
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<tgroup cols="4">
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<thead>
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<row>
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<row>
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<entry>Field</entry>
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<entry>Type</entry>
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<entry>Length</entry>
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</para>
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<para>
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Interpreting the actual data can only be done with information obtained
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from other tables, mostly <structname>pg_attribute</structname>. The
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key values needed to identify field locations are
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null values. All this trickery is wrapped up in the functions
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<firstterm>heap_getattr</firstterm>, <firstterm>fastgetattr</firstterm>
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and <firstterm>heap_getsysattr</firstterm>.
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</para>
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<para>
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value and some flag bits. Depending on the flags, the data can be either
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inline or in a <acronym>TOAST</> table;
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it might be compressed, too (see <xref linkend="storage-toast">).
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</para>
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</sect1>
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