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Improve documentation about multixact IDs.

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Per gripe from Josh Berkus.
This commit is contained in:
Tom Lane 2014-02-17 12:20:57 -05:00
parent 6ef325429c
commit e7f409756d
2 changed files with 17 additions and 12 deletions
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21
doc/src/sgml/maintenance.sgml
View File

@ -616,13 +616,16 @@ HINT: Stop the postmaster and vacuum that database in single-user mode.
</indexterm>
<para>
<firstterm>Multixacts</> are used to implement row locking by
multiple transactions: since there is limited space in the tuple
header to store lock information, that information is stored as a
multixact separately in the <filename>pg_multixact</> subdirectory,
and only its ID is in the <structfield>xmax</> field
in the tuple header.
Similar to transaction IDs, multixact IDs are implemented as a
<firstterm>Multixact IDs</> are used to support row locking by
multiple transactions. Since there is only limited space in a tuple
header to store lock information, that information is encoded as
a <quote>multiple transaction ID</>, or multixact ID for short,
whenever there is more than one transaction concurrently locking a
row. Information about which transaction IDs are included in any
particular multixact ID is stored separately in
the <filename>pg_multixact</> subdirectory, and only the multixact ID
appears in the <structfield>xmax</> field in the tuple header.
Like transaction IDs, multixact IDs are implemented as a
32-bit counter and corresponding storage, all of which requires
careful aging management, storage cleanup, and wraparound handling.
</para>
@ -634,8 +637,8 @@ HINT: Stop the postmaster and vacuum that database in single-user mode.
is replaced by a different value, which can be the zero value, a single
transaction ID, or a newer multixact ID. For each table,
<structname>pg_class</>.<structfield>relminmxid</> stores the oldest
possible value still stored in any tuple of that table. Every time this
value is older than
possible multixact ID still appearing in any tuple of that table.
If this value is older than
<xref linkend="guc-vacuum-multixact-freeze-table-age">, a whole-table
scan is forced. Whole-table <command>VACUUM</> scans, regardless of
what causes them, enable advancing the value for that table.

8
doc/src/sgml/release-9.3.sgml
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@ -64,9 +64,11 @@ Branch: REL9_3_STABLE [8e9a16ab8] 2013-12-16 11:29:51 -0300
</para>
<para>
The method for tuple freezing was unable to handle some cases
involving freezing of <quote>multixact</> IDs, with the practical
effect that shared row-level locks might be forgotten once old enough.
The logic for tuple freezing was unable to handle some cases involving
freezing of
<link linkend="vacuum-for-multixact-wraparound"><firstterm>multixact</>
IDs</link>, with the practical effect that shared row-level locks
might be forgotten once old enough.
</para>
<para>