On Thu, Mar 5, 2020 at 11:20 PM Tomas Vondra <tomas.von...@2ndquadrant.com> wrote: > > On Wed, Mar 04, 2020 at 10:28:32AM +0530, Amit Kapila wrote: > > > > Sure, there's a lot to discuss. And it's possible (likely) it's not > feasible to get this into PG13. But I think it's still worth discussing > it, instead of just punting it into the next CF right away. >
That makes sense to me. > >> There's been a tremendous amount of work done since I last > >> worked on it, and a lot was discussed on this thread, so it'll take a > >> while to get familiar with the new code ... > >> > >> The first thing I realized that WAL-logging of assignments in v12 does > >> both the "old" logging (using dedicated message) and "new" with > >> toplevel-XID embedded in the first message. Yes, the patch was wrong, > >> because it eliminated all calls to ProcArrayApplyXidAssignment() and so > >> it was trivial to crash the replica due to KnownAssignedXids overflow. > >> But I don't think re-introducing XLOG_XACT_ASSIGNMENT message is the > >> right fix. > >> > >> I actually proposed doing this (having both ways to log assignments) so > >> that there's no regression risk with (wal_level < logical). But IIRC > >> Andres objected to it, argumenting that we should not log the same piece > >> of information in two very different ways at the same time (IIRC it was > >> discussed on the FOSDEM dev meeting, so I don't have a link to share). > >> And I do agree with him ... > >> > > > >So, aren't we worried about the overhead of the amount of WAL and > >performance impact for the transactions? We might want to check the > >pgbench read-write test to see if that will add any significant > >overhead. > > > > Well, sure. I agree we need to see how this affects performance, and > I'll do some benchmarks (I think I did that when submitting the patch, > but I don't recall the numbers / details). > > Isn't it a bit strange to log stuff twice, though, if we worry about > performance? Surely that's more expensive than logging it just once. Of > course, it might be useful if most systems need just the "old" way. > > I know it's going to be a bit hand-wavy, but I think embedding the > assignments into existing WAL messages is about the cheapest way to log > this. I would not expect this to be mesurably more expensive than what > we have now, but I might be wrong. > I agree that this shouldn't be much expensive, but it is better to be sure in that regard. -- With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com