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

On Wed, May 6, 2015 at 6:26 AM, Thomas Munro
<thomas(dot)munro(at)enterprisedb(dot)com> wrote:
> On Wed, May 6, 2015 at 2:29 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> + * Based on the assumption that there is no reasonable way for an end user to
>> + * configure the thresholds for this, we define the safe member count to be
>> + * half of the member address space, and the dangerous level to be
>>
>> but:
>>
>> + const MultiXactOffset safe_member_count = MaxMultiXactOffset / 4;
>>
>> Those don't match. [...]
>
> Fixed/obsoleted in the attached patch. It has a dynamic
> safe_member_count based on scaling the GUC as described in my earlier
> email with the v7 patch; the behaviour with the default GUC value
> works out to a similar safe_member_count value, but this way it can be
> changed if needed, and we don't introduce any new GUCs. Also, since
> the GUC used in determining safe_member_count is either
> autovacuum_multixact_freeze_max_age or vacuum_multixact_freeze_max_age
> depending on which kind of vacuum it is, that is now a parameter
> passed into MultiXactCheckMemberUsage, so safe_member_count is no
> longer a constant.

To be honest, now that you've pointed out that the fraction of the
multixact members space that is in use will always be larger,
generally much larger, than the fraction of the offset space that is
in use, I've kind of lost all enthusiasm for making the
safe_member_count stuff dependent on
autovacuum_multixact_freeze_max_age. I'm inclined to go back to 25%,
the way you had it before.

We could think about adding a new GUC in master, but I'm actually
leaning toward the view that we should just hard-code 25% for now and
consider revising it later if that proves inadequate.

--
Robert Haas
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