Re: [HACKERS] Stats Collector Oddity
[EMAIL PROTECTED] (Tom Lane) writes: Chris Browne [EMAIL PROTECTED] writes: We're getting a bit of an anomaly relating to pg_stat_activity... ... That PID has been dead for several days, but this connection is marked as being open, still, after lo many days. This probably just means that the backend termination stats message got dropped due to heavy load. That's expected behavior in all pre-8.2 releases: the stats system was never intended to provide guaranteed-exactly-correct status. PG 8.2 has reimplemented the pg_stat_activity view to make it more trustworthy. (The other stuff is still probabilistic, but being just event counters, message loss isn't so obvious.) That seems a *bit* surprising; the system wasn't expected to be under particularly heavy load during the period in question; I would have expected particularly light load. No matter; there may have been some brief heavy load to cause this. There isn't any way, short of restarting the postmaster, to get rid of that PID, is there? -- cbbrowne,@,linuxdatabases.info http://cbbrowne.com/info/linuxdistributions.html High-level languages are a pretty good indicator that all else is seldom equal. - Tim Bradshaw, comp.lang.lisp ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [HACKERS] Stats Collector Oddity
Chris Browne [EMAIL PROTECTED] writes: There isn't any way, short of restarting the postmaster, to get rid of that PID, is there? The entry will get overwritten when that BackendId slot gets re-used, so just starting enough concurrent backends should do it. (Since incoming backends always take the lowest free slot, the fact that the dead entry has persisted awhile means that it must have a number higher than your normal number of concurrent sessions ... which is evidence in favor of the idea that it happened during a load spike ...) regards, tom lane ---(end of broadcast)--- TIP 5: don't forget to increase your free space map settings
Re: [HACKERS] Stats Collector Oddity
A long time ago, in a galaxy far, far away, [EMAIL PROTECTED] (Tom Lane) wrote: Chris Browne [EMAIL PROTECTED] writes: There isn't any way, short of restarting the postmaster, to get rid of that PID, is there? The entry will get overwritten when that BackendId slot gets re-used, so just starting enough concurrent backends should do it. (Since incoming backends always take the lowest free slot, the fact that the dead entry has persisted awhile means that it must have a number higher than your normal number of concurrent sessions ... which is evidence in favor of the idea that it happened during a load spike ...) Cool. I started up a nice little bunch of psql sessions in the background, and then once they were all up, shut down my shell session, thereby eliminating them. And that did, indeed, clear out that pg_stat_activity entry. ... And five minutes later, Nagios sent me message indicating that node had recovered from having an ancient open connection. I'll re-add a few gratuitous details here in the hopes that that makes this easily findable if anyone else should search for the issue... The Problem: - pg_stat_activity was reporting an elderly transaction in progress - that backend process wasn't running anymore - pg_stat_activity *was* reporting other legitimate activity; this was not the scenario where it had gotten deranged (normally due to excessive load) - Per Tom's comments, there evidently *was* some load spike where the closing of this particular connection did not get logged by the stats collector The Solution: - We needed to roll the stats collector through a bunch of its slots in order to clean the apparently-still-populated entry out. - Ran, in a shell: for i in `seq 100`; do psql done That left 100 psql sessions in the background, all connected to the database backend. - Closed the shell. That then HUPped the 100 psql sessions. That got the offending pg_stat_activity entry cleared out. -- let name=cbbrowne and tld=gmail.com in name ^ @ ^ tld;; http://linuxdatabases.info/info/finances.html Temporary tattoos are a CRITICAL ELEMENT of our security strategy. To suggest otherwise is sheer lunacy. -- Reid Fleming, cDc ---(end of broadcast)--- TIP 6: explain analyze is your friend
[HACKERS] Stats Collector Oddity
We're getting a bit of an anomaly relating to pg_stat_activity... oxrstld=# SELECT * from pg_stat_activity where current_query 'IDLE'; datid| datname | procpid | usesysid | usename | current_query | query_start +-+-+--+-+---+--- 1347729970 | oxrstld | 893094 | 122 | tldepp | commit| 2006-12-16 19:34:08.583978+00 (1 row) oxrstld=# select version(); version -- PostgreSQL 7.4.12 on powerpc-ibm-aix5.3.0.0, compiled by GCC gcc (GCC) 4.0.1 (1 row) That PID has been dead for several days, but this connection is marked as being open, still, after lo many days. This has *traditionally* been a sign that the stats collector has been clobbered, at which point pg_stat_activity becomes useless. That's not the case here; a less restricted query on pg_stat_activity shows other more recent data that keeps changing, seemingly consistent with system activity. I can track most of the lifecycle of that PID; the connection was established at 2006-12-16 18:46:38, and I see a few errors associated with the PID (violations of unique constraints; usual business stuff). I see nothing in the logs at around 19:34 which would suggest a reason for any strange behaviour. There are no records about that PID after 19:34... I'd like to get rid of this entry, if I can; it's blowing up tests that warn us about elderly transactions, causing a false positive... -- output = reverse(moc.enworbbc @ enworbbc) http://www3.sympatico.ca/cbbrowne/ Rules of the Evil Overlord #212. I will not send out battalions composed wholly of robots or skeletons against heroes who have qualms about killing living beings. http://www.eviloverlord.com/ ---(end of broadcast)--- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to [EMAIL PROTECTED] so that your message can get through to the mailing list cleanly
Re: [HACKERS] Stats Collector Oddity
Chris Browne [EMAIL PROTECTED] writes: We're getting a bit of an anomaly relating to pg_stat_activity... ... That PID has been dead for several days, but this connection is marked as being open, still, after lo many days. This probably just means that the backend termination stats message got dropped due to heavy load. That's expected behavior in all pre-8.2 releases: the stats system was never intended to provide guaranteed-exactly-correct status. PG 8.2 has reimplemented the pg_stat_activity view to make it more trustworthy. (The other stuff is still probabilistic, but being just event counters, message loss isn't so obvious.) regards, tom lane ---(end of broadcast)--- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
