Re: Re: BUG #12990: Missing pg_multixact/members files (appears to have wrapped, then truncated)

Robert Haas wrote:

> So here's a new patch, based on your latest version, which looks
> reasonably committable to me.

I think this code should also reduce the multixact_freeze_min_age value
at the same time as multixact_freeze_table_age.  If the table age is
reduced but freeze_min_age remains high, old multixacts might still
remain in the table.  The default value for freeze min age is 5 million,
but users may change it.  Perhaps freeze min age should be set to
Min(modified freeze table age, freeze min age) so that old multixacts
are effectively frozen whenever a full table scan requested.

> 1. Should we be installing one or more GUCs to control this behavior?
> I've gone back to hard-coding things so that at 25% we start
> triggering autovacuum and by 75% we zero out the freeze ages, because
> the logic you proposed in your last version looks insanely complicated
> to me.  (I do realize that I suggested the approach, but that was
> before I realized the full complexity of the problem.)  I now think
> that if we want to make this tunable, we need to create and expose
> GUCs for it.  I'm hoping we can get by without that, but I'm not sure.

I think things are complicated enough; I vote for no additional GUCs at
this point.

> 2. Doesn't the code that sets MultiXactState->multiVacLimit also need
> to use what I'm now calling MultiXactMemberFreezeThreshold() - or some
> similar logic?  Otherwise, a user with autovacuum=off won't get
> emergency autovacuums for member exhaustion, even though they will get
> them for offset exhaustion.

Yeah, it looks like it does.

--
Álvaro Herrera                http://www.2ndQuadrant.com/
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