Re: pgsql: Fix a couple of bugs in MultiXactId freezing

On Tue, Dec 3, 2013 at 7:20 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Magnus Hagander <magnus(at)hagander(dot)net> writes:
> > On Tue, Dec 3, 2013 at 7:11 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> >> Maybe we should just bite the bullet and change the WAL format for
> >> heap_freeze (inventing an all-new record type, not repurposing the old
> >> one, and allowing WAL replay to continue to accept the old one). The
> >> implication for users would be that they'd have to update slave servers
> >> before the master when installing the update; which is unpleasant, but
> >> better than living with a known data corruption case.
>
> > Agreed. It may suck, but it sucks less.
>
> > How badly will it break if they do the upgrade in the wrong order though.
> > Will the slaves just stop (I assume this?) or is there a risk of a
> > wrong-order upgrade causing extra breakage?
>
> I assume what would happen is the slave would PANIC upon seeing a WAL
> record code it didn't recognize. Installing the updated version should
> allow it to resume functioning. Would be good to test this, but if it
> doesn't work like that, that'd be another bug to fix IMO. We've always
> foreseen the possible need to do something like this, so it ought to
> work reasonably cleanly.
>

Yeah, as long as that's tested and actually works, that sounds like an
acceptable thing to deal with.

> > I assume we'd then get rid of the old record type completely in 9.4,
> right?
>
> Yeah, we should be able to drop it in 9.4, since we'll surely have other
> WAL format changes anyway.
>

And even if not, there's no point in keeping it unless we actually support
replication from 9.3 -> 9.4, I think, and I don't believe anybody has even
considered working on that yet :)

--
Magnus Hagander
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