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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 83fbd9b599
commit 596fb5aa73
5 changed files with 130 additions and 11 deletions
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16
src/backend/commands/vacuum.c
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@@ -426,6 +426,7 @@ vacuum_set_xid_limits(int freeze_min_age,
{
int freezemin;
int mxid_freezemin;
int effective_multixact_freeze_max_age;
TransactionId limit;
TransactionId safeLimit;
MultiXactId mxactLimit;
@@ -482,17 +483,24 @@ vacuum_set_xid_limits(int freeze_min_age,
*freezeLimit = limit;
/*
* Compute the multixact age for which freezing is urgent. This is
* normally autovacuum_multixact_freeze_max_age, but may be less if we
* are short of multixact member space.
*/
effective_multixact_freeze_max_age = MultiXactMemberFreezeThreshold();
/*
* Determine the minimum multixact freeze age to use: as specified by
* caller, or vacuum_multixact_freeze_min_age, but in any case not more
* than half autovacuum_multixact_freeze_max_age, so that autovacuums to
* than half effective_multixact_freeze_max_age, so that autovacuums to
* prevent MultiXact wraparound won't occur too frequently.
*/
mxid_freezemin = multixact_freeze_min_age;
if (mxid_freezemin < 0)
mxid_freezemin = vacuum_multixact_freeze_min_age;
mxid_freezemin = Min(mxid_freezemin,
autovacuum_multixact_freeze_max_age / 2);
effective_multixact_freeze_max_age / 2);
Assert(mxid_freezemin >= 0);
/* compute the cutoff multi, being careful to generate a valid value */
@@ -501,7 +509,7 @@ vacuum_set_xid_limits(int freeze_min_age,
mxactLimit = FirstMultiXactId;
safeMxactLimit =
ReadNextMultiXactId() - autovacuum_multixact_freeze_max_age;
ReadNextMultiXactId() - effective_multixact_freeze_max_age;
if (safeMxactLimit < FirstMultiXactId)
safeMxactLimit = FirstMultiXactId;
@@ -556,7 +564,7 @@ vacuum_set_xid_limits(int freeze_min_age,
if (freezetable < 0)
freezetable = vacuum_multixact_freeze_table_age;
freezetable = Min(freezetable,
autovacuum_multixact_freeze_max_age * 0.95);
effective_multixact_freeze_max_age * 0.95);
Assert(freezetable >= 0);
/*