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

On Wed, May 6, 2015 at 10:34 AM, Alvaro Herrera
<alvherre(at)2ndquadrant(dot)com> 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.

I think it does that. It sets the min age to half the value it sets
for the table age, which I think is consistent with what we do
elsewhere.

>> 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.

That's fine with me for now.

>> 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.

OK, I'm not clear how to do that correctly, exactly, but hopefully one
of us can figure that out.

--
Robert Haas
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