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

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