On 9 November 2012 23:24, Tom Lane <t...@sss.pgh.pa.us> wrote: > During normal running, operations such as btree page splits are > extremely careful about the order in which they acquire and release > buffer locks, if they're doing something that concurrently modifies > multiple pages. > > During WAL replay, that all goes out the window. Even if an individual > WAL-record replay function does things in the right order for "standard" > cases, RestoreBkpBlocks has no idea what it's doing. So if one or more > of the referenced pages gets treated as a full-page image, we are left > with no guarantee whatsoever about what order the pages are restored in. > That never mattered when the code was originally designed, but it sure > matters during Hot Standby when other queries might be able to see the > intermediate states. > > I can't prove that this is the cause of bug #7648, but it's fairly easy > to see that it could explain the symptom. You only need to assume that > the page-being-split had been handled as a full-page image, and that the > new right-hand page had gotten allocated by extending the relation. > Then there will be an interval just after RestoreBkpBlocks does its > thing where the updated left-hand sibling is in the index and is not > locked in any way, but its right-link points off the end of the index. > If a few indexscans come along before the replay process gets to > continue, you'd get exactly the reported errors. > > I'm inclined to think that we need to fix this by getting rid of > RestoreBkpBlocks per se, and instead having the per-WAL-record restore > routines dictate when each full-page image is restored (and whether or > not to release the buffer lock immediately). That's not going to be a > small change unfortunately :-(
No, but it looks like a clear bug scenario and a clear resolution also. I'll start looking at it. -- Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
