(I originally posted this to pgsql-admin and was pointed to here instead.)
Folks-
I'm doing a postmortem on a corruption event we had. I have an idea on what happened, but not sure. I figure I'd share what happened and see if I'm close to right here.
Event: Running 9.1.6 with hot-standby, archiving 4 months of wal files, and even a nightly pg_dump all. 50G database. Trying to update or delete a row in a small (21 row, but heavily used table) would lock up completely. Never finish. Removed all clients, restarted the db instance, no joy. Check pg_stat_activity, and nothing that wasn't idle.... run the delete, locked up.
Ran (SELECT*FROM pg_class JOIN pg_locks ON pg_locks.relation = pg_class.oid;) with no clients touching this row, fresh restart of the db, and saw virtualtransactions against this same table where the values would be -1/nnnnn were nnnnn was a huge number. Turns out we had about 159 entries from different tables in the database. Checked hot-standby and, of course, no locks or anything. Switched to hot-standby.
Hot-standby instantly gained these locks, Also noticed that 2 prepared transactions migrated to the hot-standby. Binary upgraded to 9.1.8, locks still existed. Ended up executing the one thing we knew would work. Take the site down, pg_dumpall to fresh instance. Everything is fine.
A little more background: We were running 9.1.4 back when 9.1.6 came out. We saw there was possible corruption issues and did a binary upgrade and reindexing. Everything seemed to be fine, but I wonder if we really had problems back then. We rebuilt the hot-standby after the binary upgrade via normal restore and wal-file replays. I should also note that this row that had the lock on it that would not go away, was created by an app server that was killed (via -9) since it was non-responsive, and the row 'create date' (in db and logs) is the exact time the app server was killed.
I was wondering if a) these virtualtransactions that start with '-1/' indicate a problem, b) if this could have happened from pre 9.1.6 corruption that was fixed in 9.1.6. Or, could this have occurred when we killed that app server? Or.... am I looking in the wrong place.
I do still have the old data directories so I can start them up and check out the dataset. Any advice?
Currently running environment: CentOS 5.x
Used the http://yum.postgresql.org/9.1 repo...
Currently using PostgreSQL 9.1.8 on x86_64-unknown-linux-gnu, compiled by gcc (GCC) 4.1.2 20080704 (Red Hat 4.1.2-52), 64-bit
but was on 9.1.6 at failure time.
The hardware on both primary and secondary servers are in good condition, raid5 via a hardware card with no issues. Each a 2x4x2 (2 hyperthreaded Xeon E5620 2.4Ghz, 4 cores each, 16 threads total) 32G Ram.
Data from pg_dumpall checked out fine; no data loss occurred that we could tell. Just this transaction lock and what seemed like invalid backend ids listed in the virtualtransaction from the pg_locks table/view. (-1/verylargenumber)
Virtually, Ned Wolpert
"Settle thy studies, Faustus, and begin..." --Marlowe
