Re: pg_upgrade < 9.3 -> >=9.3 misses a step around multixacts
От | Andres Freund |
---|---|
Тема | Re: pg_upgrade < 9.3 -> >=9.3 misses a step around multixacts |
Дата | |
Msg-id | 20140720223010.GF5974@alap3.anarazel.de обсуждение исходный текст |
Ответ на | Re: pg_upgrade < 9.3 -> >=9.3 misses a step around multixacts (Tom Lane <tgl@sss.pgh.pa.us>) |
Ответы |
Re: pg_upgrade < 9.3 -> >=9.3 misses a step around multixacts
(Tom Lane <tgl@sss.pgh.pa.us>)
|
Список | pgsql-bugs |
On 2014-07-20 18:16:51 -0400, Tom Lane wrote: > Andres Freund <andres@2ndquadrant.com> writes: > > On 2014-07-20 17:43:04 -0400, Tom Lane wrote: > >> No, I don't think so. Truncation is driven off oldestMultiXid from > >> pg_control, not from relminmxid. The only thing in-the-future values of > >> those will do to us is prevent autovacuum from thinking it must do a full > >> table scan. (In particular, in-the-future values do not cause > >> oldestMultiXid to get advanced, because we're always looking for the > >> oldest value not the newest.) > > > Right. But that's the problem. If oldestMulti is set to, say, 3000000000 > > by pg_resetxlog during pg_upgrade but *minmxid = 1 those tables won't be > > full tables scanned because of multixacts. But vac_truncate_clog() will > > SetMultiXactIdLimit(minMulti, minmulti_datoid); > > regardless. > > > Note that it'll not notice the limit of other databases in this case > > because vac_truncate_clog() will effectively use the in memory > > GetOldestMultiXactId() and check if other databases are before that. But > > there won't be any because they all appear in the future. Due to that > > the next checkpoint will tru6ncate the clog to the cutoff multi xid used > > by the last vacuum. > > Right. > > > Am I missing something? > > My point is that the cutoff multi xid won't be new enough to remove > non-LOCKED_ONLY (ie, post-9.3) mxids. Why not? Afaics this will continue to happen until multixacts are wrapped around once? So the cutoff multi will be new enough for that at some point after the pg_upgrade? Luckily in most cases full table vacuums triggered due to normal xids will prevent bad problems though. There have been a couple reports where people included pg_controldata output indicating crazy rates of multixid consumption but I think none of those were crazy enough to burn multis so fast that important ones get truncated before a full table vacuum occurs due to normal xids. > >> But in any case, we both agree that setting relminmxid to equal nextMulti > >> is completely unsafe in a 9.3 cluster that's already been up. So the > >> proposed fix instructions are certainly wrong. > > > Right. I'm pondering what to do about it instead. The best idea I have > > is something like: > > 1) Jot down pg_controldata|grep NextMultiXactId > > 2) kill/wait for all existing transactions to end > > 3) vacuum all databases with vacuum_multixact_freeze_min_age=0. That'll > > get rid of all old appearing multis > > 4) Update pg_class to set relminmxid=value from 1), same with > > pg_database > > > But that sucks and doesn't deal with all the problems :( > > Yeah. At this point I'm of the opinion that we should not recommend any > manual corrective actions for this issue. They're likely to do more harm > than good, especially if the user misses or fat-fingers any steps. I don't really see us coming up with something robust in time :/. It's a bid sad, but maybe we should recommend contacting the mailing list if pg_upgrade has been used and nextMulti is above 2^31? Btw, we really should have txid_current() equivalent for multis... > I'm also thinking that the lack of any complaints suggests there are few > or no existing installations with nextMulti past 2^31, anyhow. If it were > even past 400000000 (default autovacuum_multixact_freeze_max_age), we'd > have been hearing howls of anguish about full-database freezing scans > occurring immediately after a pg_upgrade (thanks to minmxid = 1 being old > enough to trigger that). I think people just chalk that up to 'crazy pg vacuuming behaviour' and not investigate further. At least that's my practical experience :( Greetings, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
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