Re: First steps with 8.3 and autovacuum launcher

On Fri, 2007-10-12 at 01:24 -0400, Alvaro Herrera wrote:
> Michael Paesold escribió:
> > Simon Riggs wrote:
>
> > Hmm, I am not sure we are there, yet. Autovacuum does take extra care to
> > vacuum tables nearing xid wrap-around, right? It even does so when
> > autovacuum is disabled in the configuration.
> >
> > So in case a vacuum is needed for that very reason, the vacuum should *not*
> > be canceled, of course. So we don't really need the information, whether
> > the AV worker is doing VACUUM or ANALYZE, but whether it is critical
> > against xid wrap-around. Could that be done as easily as in Alvaro's patch
> > for distinguishing vacuum/analyze? Alvaro?
>
> 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."

--
Simon Riggs
2ndQuadrant http://www.2ndQuadrant.com