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