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Commitddebd21

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Increase threshold for multixact member emergency autovac to 50%.
Analysis by Noah Misch shows that the 25% threshold set by commit53bb309 is lower than any other,similar autovac threshold. While we don't know exactly what valuewill be optimal for all users, it is better to err a little on thehigh side than on the low side. A higher value increases the riskthat users might exhaust the available space and start seeing errorsbefore autovacuum can clean things up sufficiently, but a user whohits that problem can compensate for it by reducingautovacuum_multixact_freeze_max_age to a value dependent on theiraverage multixact size. On the flip side, if the emergency capimposed by that patch kicks in too early, the user will experienceexcessive wraparound scanning and will be unable to mitigate thatproblem by configuration. The new value will hopefully reduce therisk of such bad experiences while still providing enough headroomto avoid multixact member exhaustion for most users.Along the way, adjust the documentation to reflect the effects ofcommit04e6d3b, which taughtautovacuum to run for multixact wraparound even when autovacuumis configured off.
1 parent543fbec commitddebd21

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‎doc/src/sgml/maintenance.sgml

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@@ -655,11 +655,12 @@ HINT: Stop the postmaster and use a standalone backend to VACUUM in "mydb".
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<para>
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As a safety device, a whole-table vacuum scan will occur for any table
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whose multixact-age is greater than
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<xref linkend="guc-autovacuum-multixact-freeze-max-age">.
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This will occur even if autovacuum is nominally disabled. Whole-table
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<xref linkend="guc-autovacuum-multixact-freeze-max-age">. Whole-table
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vacuum scans will also occur progressively for all tables, starting with
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those that have the oldest multixact-age, if the amount of used member
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storage space exceeds the amount 25% of the addressible storage space.
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storage space exceeds the amount 50% of the addressible storage space.
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Both of these kinds of whole-table scans will occur even if autovacuum is
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nominally disabled.
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</para>
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</sect3>
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</sect2>

‎src/backend/access/transam/multixact.c

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@@ -167,7 +167,7 @@
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((xid) % MULTIXACT_MEMBERS_PER_MEMBERGROUP) * sizeof(TransactionId))
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/* Multixact members wraparound thresholds. */
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#defineMULTIXACT_MEMBER_SAFE_THRESHOLD(MaxMultiXactOffset /4)
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#defineMULTIXACT_MEMBER_SAFE_THRESHOLD(MaxMultiXactOffset /2)
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#defineMULTIXACT_MEMBER_DANGER_THRESHOLD\
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(MaxMultiXactOffset - MaxMultiXactOffset / 4)
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