On Fri, 2007-10-12 at 11:44 +0200, Michael Paesold wrote: > Simon Riggs wrote: > > On Fri, 2007-10-12 at 01:24 -0400, Alvaro Herrera wrote: > >> Yes, I think it is easy to mark the "is for xid wraparound" bit in the > >> WorkerInfo struct and have the cancel work only if it's off. > >> > >> However, what I think should happen is that the signal handler for > >> SIGINT in a worker for xid wraparound should not cancel the current > >> vacuum. Instead turn it into a no-op, if possible. That way we also > >> disallow a user from cancelling vacuums for xid wraparound. I think he > >> can do that with pg_cancel_backend, and it could be dangerous. > > > > I think that is dangerous too because the user may have specifically > > turned AV off. That anti-wraparound vacuum might spring up right in a > > busy period and start working its way through many tables, all of which > > cause massive writes to occur. That's about as close to us causing an > > outage as I ever want to see. We need a way through that to allow the > > user to realise his predicament and find a good time to VACUUM. I never > > want to say to anybody "nothing you can do, just sit and watch, your > > production system will be working again in no time. Restart? no that > > won't work either." > > You are probably right that VACUUM going full-steam is a bad idea in most > situations. Except for anti-wraparound vacuum, cancellation seems the most > reasonable thing to do. Because autovacuum will usually pickup the table in > time again.
Yeh, if we do have to do the second emergency anti-wraparound, then that should be at full speed, since there's nothing else to do at that point. > The only problem I would see is if someone has an application that does a > lot of schema changes (doesn't sound like a good idea anyway). In that case > they would better issue manual vacuums on such tables. I can't see a use case for regular DDL as part of an application, on an otherwise integral table (lots of updates and deletes). -- Simon Riggs 2ndQuadrant http://www.2ndQuadrant.com ---------------------------(end of broadcast)--------------------------- TIP 6: explain analyze is your friend
