Movatterモバイル変換

[0]ホーム
URL:

Skip to content

Navigation Menu

Sign in
Appearance settings

Search code, repositories, users, issues, pull requests...

Provide feedback

We read every piece of feedback, and take your input very seriously.

Saved searches

Use saved searches to filter your results more quickly

 Sign up
Appearance settings

Commit03a5ff0

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

File tree

1 file changed

+44
-42
lines changed

1 file changed

+44
-42
lines changed

‎doc/src/sgml/storage.sgml

Lines changed: 44 additions & 42 deletions
Original file line numberDiff line numberDiff line change
@@ -1,4 +1,4 @@
1-
<!-- $PostgreSQL: pgsql/doc/src/sgml/storage.sgml,v 1.27 2009/04/23 10:20:27 heikki Exp $ -->
1+
<!-- $PostgreSQL: pgsql/doc/src/sgml/storage.sgml,v 1.28 2009/05/16 22:03:53 tgl Exp $ -->
22

33
<chapter id="storage">
44

@@ -33,7 +33,7 @@ these required items, the cluster configuration files
3333
<filename>postgresql.conf</filename>, <filename>pg_hba.conf</filename>, and
3434
<filename>pg_ident.conf</filename> are traditionally stored in
3535
<varname>PGDATA</> (although in <productname>PostgreSQL</productname> 8.0 and
36-
later, it is possible to keep them elsewhere).
36+
later, it is possible to keep them elsewhere).
3737
</para>
3838

3939
<table tocentry="1" id="pgdata-contents-table">
@@ -74,7 +74,7 @@ Item
7474
<row>
7575
<entry><filename>pg_multixact</></entry>
7676
<entry>Subdirectory containing multitransaction status data
77-
(used for shared row locks)</entry>
77+
(used for shared row locks)</entry>
7878
</row>
7979

8080
<row>
@@ -131,12 +131,12 @@ there.
131131
Each table and index is stored in a separate file, named after the table
132132
or index's <firstterm>filenode</> number, which can be found in
133133
<structname>pg_class</>.<structfield>relfilenode</>. In addition to the
134-
main file (aka. main fork), a <firstterm>free space map</> (see
135-
<xref linkend="storage-fsm">) that stores information about free space
136-
available in the relation,is stored in a file named after the filenode
137-
number, with the <literal>_fsm</> suffix. Tables also have a visibility map
138-
fork, with the <literal>_vm</> suffix, to track which pages are known to have
139-
no dead tuples and therefore need no vacuuming.
134+
main file (a/k/a main fork),each table and index hasa <firstterm>free space
135+
map</> (see <xref linkend="storage-fsm">), which stores information about free
136+
spaceavailable in the relation. The free space mapis stored in a file named
137+
with the filenodenumber plus thesuffix<literal>_fsm</>. Tables also have a
138+
visibility mapfork, with thesuffix<literal>_vm</>, to track which pages are
139+
known to haveno dead tuples and therefore need no vacuuming.
140140
</para>
141141

142142
<caution>
@@ -157,6 +157,8 @@ This arrangement avoids problems on platforms that have file size limitations.
157157
(Actually, 1 GB is just the default segment size. The segment size can be
158158
adjusted using the configuration option <option>--with-segsize</option>
159159
when building <productname>PostgreSQL</>.)
160+
In principle, free space map and visibility map forks could require multiple
161+
segments as well, though this is unlikely to happen in practice.
160162
The contents of tables and indexes are discussed further in
161163
<xref linkend="storage-page-layout">.
162164
</para>
@@ -193,7 +195,7 @@ if a tablespace other than <literal>pg_default</> is specified for them.
193195
The name of a temporary file has the form
194196
<filename>pgsql_tmp<replaceable>PPP</>.<replaceable>NNN</></filename>,
195197
where <replaceable>PPP</> is the PID of the owning backend and
196-
<replaceable>NNN</> distinguishes different files of that backend.
198+
<replaceable>NNN</> distinguishes differenttemporaryfiles of that backend.
197199
</para>
198200

199201
</sect1>
@@ -215,10 +217,10 @@ Oversized-Attribute Storage Technique).
215217
<para>
216218
<productname>PostgreSQL</productname> uses a fixed page size (commonly
217219
8 kB), and does not allow tuples to span multiple pages. Therefore, it is
218-
not possible to store very large field values directly. To overcome
220+
not possible to store very large field values directly. To overcome
219221
this limitation, large field values are compressed and/or broken up into
220222
multiple physical rows. This happens transparently to the user, with only
221-
small impact on most of the backend code. The technique is affectionately
223+
small impact on most of the backend code. The technique is affectionately
222224
known as <acronym>TOAST</> (or <quote>the best thing since sliced bread</>).
223225
</para>
224226

@@ -377,24 +379,24 @@ comparison table, in which all the HTML pages were cut down to 7 kB to fit.
377379

378380
<title>Free Space Map</title>
379381

380-
<indexterm>
381-
<primary>Free Space Map</primary>
382-
</indexterm>
383-
<indexterm><primary>FSM</><see>Free Space Map</></indexterm>
382+
<indexterm>
383+
<primary>Free Space Map</primary>
384+
</indexterm>
385+
<indexterm><primary>FSM</><see>Free Space Map</></indexterm>
384386

385387
<para>
386-
A Free Space Map is stored with everyheap and index relation, except for
387-
hash indexes, to keep track of available space in the relation. It's stored
388-
along the main relation data, in a separateFSMrelation fork, named after
389-
relfilenodeof the relation,but with a <literal>_fsm</> suffix. For example,
390-
if therelfilenode of a relation is 12345, the FSM is stored in a file called
388+
Eachheap and index relation, except for hash indexes, has a Free Space Map
389+
(FSM) to keep track of available space in the relation. It's stored
390+
alongside the main relation data in a separate relation fork, named after the
391+
filenode numberof the relation,plus a <literal>_fsm</> suffix. For example,
392+
if thefilenode of a relation is 12345, the FSM is stored in a file called
391393
<filename>12345_fsm</>, in the same directory as the main relation file.
392394
</para>
393395

394396
<para>
395397
The Free Space Map is organized as a tree of <acronym>FSM</> pages. The
396-
bottom level <acronym>FSM</> pagesstores the free space available onevery
397-
heap (or index) page, using one byte to represent eachheap page. The upper
398+
bottom level <acronym>FSM</> pagesstore the free space available oneach
399+
heap (or index) page, using one byte to represent eachsuch page. The upper
398400
levels aggregate information from the lower levels.
399401
</para>
400402

@@ -409,8 +411,8 @@ at the root.
409411
<para>
410412
See <filename>src/backend/storage/freespace/README</> for more details on
411413
how the <acronym>FSM</> is structured, and how it's updated and searched.
412-
<xref linkend="pgfreespacemap">contrib module can be used toview the
413-
information stored in free space maps.
414+
The <filename>contrib/pg_freespacemap</> module can be used toexamine the
415+
information stored in free space maps (see <xref linkend="pgfreespacemap">).
414416
</para>
415417

416418
</sect1>
@@ -515,7 +517,7 @@ data. Empty in ordinary tables.</entry>
515517
and <structfield>pd_special</structfield>). These contain byte offsets
516518
from the page start to the start
517519
of unallocated space, to the end of unallocated space, and to the start of
518-
the special space.
520+
the special space.
519521
The next 2 bytes of the page header,
520522
<structfield>pd_pagesize_version</structfield>, store both the page size
521523
and a version indicator. Beginning with
@@ -530,15 +532,15 @@ data. Empty in ordinary tables.</entry>
530532
more than one page size in an installation.
531533
The last field is a hint that shows whether pruning the page is likely
532534
to be profitable: it tracks the oldest un-pruned XMAX on the page.
533-
535+
534536
</para>
535-
537+
536538
<table tocentry="1" id="pageheaderdata-table">
537539
<title>PageHeaderData Layout</title>
538540
<titleabbrev>PageHeaderData Layout</titleabbrev>
539-
<tgroup cols="4">
541+
<tgroup cols="4">
540542
<thead>
541-
<row>
543+
<row>
542544
<entry>Field</entry>
543545
<entry>Type</entry>
544546
<entry>Length</entry>
@@ -627,25 +629,25 @@ data. Empty in ordinary tables.</entry>
627629
</para>
628630

629631
<para>
630-
632+
631633
The items themselves are stored in space allocated backwards from the end
632634
of unallocated space. The exact structure varies depending on what the
633635
table is to contain. Tables and sequences both use a structure named
634636
<type>HeapTupleHeaderData</type>, described below.
635637

636638
</para>
637-
639+
638640
<para>
639-
641+
640642
The final section is the <quote>special section</quote> which can
641643
contain anything the access method wishes to store. For example,
642644
b-tree indexes store links to the page's left and right siblings,
643645
as well as some other data relevant to the index structure.
644646
Ordinary tables do not use a special section at all (indicated by setting
645647
<structfield>pd_special</> to equal the page size).
646-
648+
647649
</para>
648-
650+
649651
<para>
650652

651653
All table rows are structured in the same way. There is a fixed-size
@@ -669,15 +671,15 @@ data. Empty in ordinary tables.</entry>
669671
<structfield>t_hoff</> a MAXALIGN multiple will appear between the null
670672
bitmap and the object ID. (This in turn ensures that the object ID is
671673
suitably aligned.)
672-
674+
673675
</para>
674-
676+
675677
<table tocentry="1" id="heaptupleheaderdata-table">
676678
<title>HeapTupleHeaderData Layout</title>
677679
<titleabbrev>HeapTupleHeaderData Layout</titleabbrev>
678-
<tgroup cols="4">
680+
<tgroup cols="4">
679681
<thead>
680-
<row>
682+
<row>
681683
<entry>Field</entry>
682684
<entry>Type</entry>
683685
<entry>Length</entry>
@@ -743,7 +745,7 @@ data. Empty in ordinary tables.</entry>
743745
</para>
744746

745747
<para>
746-
748+
747749
Interpreting the actual data can only be done with information obtained
748750
from other tables, mostly <structname>pg_attribute</structname>. The
749751
key values needed to identify field locations are
@@ -753,7 +755,7 @@ data. Empty in ordinary tables.</entry>
753755
null values. All this trickery is wrapped up in the functions
754756
<firstterm>heap_getattr</firstterm>, <firstterm>fastgetattr</firstterm>
755757
and <firstterm>heap_getsysattr</firstterm>.
756-
758+
757759
</para>
758760
<para>
759761

@@ -767,7 +769,7 @@ data. Empty in ordinary tables.</entry>
767769
value and some flag bits. Depending on the flags, the data can be either
768770
inline or in a <acronym>TOAST</> table;
769771
it might be compressed, too (see <xref linkend="storage-toast">).
770-
772+
771773
</para>
772774
</sect1>
773775

0 commit comments

Comments
 (0)
[8]ページ先頭
©2009-2025 Movatter.jp