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?
--
Virtually, Ned Wolpert
"Settle thy studies, Faustus, and begin..." --Marlowe