Re: multixacts woes

On Fri, May 8, 2015 at 2:27 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2015-05-08 14:15:44 -0400, Robert Haas wrote:
>> Apparently, we have been hanging our hat since the release of 9.3.0 on
>> the theory that the average multixact won't ever have more than two
>> members, and therefore the members SLRU won't overwrite itself and
>> corrupt data.
>
> It's essentially a much older problem - it has essentially existed since
> multixacts were introduced (8.1?). The consequences of it were much
> lower before 9.3 though.

OK, I wasn't aware of that. What exactly were the consequences before 9.3?

> I'm not inclined to backport it at this stage. Maybe if we get some
> field reports about too many anti-wraparound vacuums due to this, *and*
> the code has been tested in 9.5.

That was about what I was thinking, too.

>> Another thought that occurs to me is that if we had a freeze map, it
>> would radically decrease the severity of this problem, because
>> freezing would become vastly cheaper. I wonder if we ought to try to
>> get that into 9.5, even if it means holding up 9.5
>
> I think that's not realistic. Doing this right isn't easy. And doing it
> wrong can lead to quite bad results, i.e. data corruption. Doing it
> under the pressure of delaying a release further and further seems like
> recipe for disaster.

Those are certainly good things to worry about.

> FWIW, I intend to either work on this myself, or help whoever seriously
> tackles this, in the next cycle.

That would be great. I'll investigate what resources EnterpriseDB can
commit to this.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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