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Teach autovacuum about multixact member wraparound.

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The logic introduced in commit b69bf30b9b
and repaired in commits 669c7d20e6 and
7be47c56af helps to ensure that we don't
overwrite old multixact member information while it is still needed,
but a user who creates many large multixacts can still exhaust the
member space (and thus start getting errors) while autovacuum stands
idly by.

To fix this, progressively ramp down the effective value (but not the
actual contents) of autovacuum_multixact_freeze_max_age as member space
utilization increases.  This makes autovacuum more aggressive and also
reduces the threshold for a manual VACUUM to perform a full-table scan.

This patch leaves unsolved the problem of ensuring that emergency
autovacuums are triggered even when autovacuum=off.  We'll need to fix
that via a separate patch.

Thomas Munro and Robert Haas
This commit is contained in:
Robert Haas
2015-05-08 12:09:14 -04:00
parent 195fbd4012
commit 53bb309d2d
5 changed files with 130 additions and 11 deletions
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8
doc/src/sgml/maintenance.sgml
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@ -628,6 +628,9 @@ HINT: Stop the postmaster and vacuum that database in single-user mode.
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.
There is a separate storage area which holds the list of members in
each multixact, which also uses a 32-bit counter and which must also
be managed.
</para>
<para>
@ -656,7 +659,10 @@ HINT: Stop the postmaster and vacuum that database in single-user mode.
As a safety device, a whole-table vacuum scan will occur for any table
whose multixact-age is greater than
<xref linkend="guc-autovacuum-multixact-freeze-max-age">.
This will occur even if autovacuum is nominally disabled.
This will occur even if autovacuum is nominally disabled. Whole-table
vacuum scans will also occur progressively for all tables, starting with
those that have the oldest multixact-age, if the amount of used member
storage space exceeds the amount 25% of the addressible storage space.
</para>
</sect3>
</sect2>