Re: How about a option to disable autovacuum cancellation on lock conflict?

Robert Haas wrote:
> On Sat, Nov 29, 2014 at 11:46 PM, Jim Nasby <Jim.Nasby@bluetreble.com> wrote:
> > What do you mean by "never succeed"? Is it skipping a large number of pages?
> > Might re-trying the locks within the same vacuum help, or are the user locks
> > too persistent?
> 
> You are confused.  He's talking about the relation-level lock that
> vacuum attempts to take before doing any work at all on a given table,
> not the per-page cleanup locks that it takes while processing each
> page.  If the relation-level lock can't be acquired, the whole table
> is skipped.

Almost there.  Autovacuum takes the relation-level lock, starts
processing.  Some time later, another process wants a lock that
conflicts with the one autovacuum has.  This is flagged by the deadlock
detector, and a signal is sent to autovacuum, which commits suicide.

If the table is large, the time window for this to happen is large also;
there might never be a time window large enough between two lock
acquisitions for one autovacuum run to complete in a table.  This
starves the table from vacuuming completely, until things are bad enough
that an emergency vacuum is forced.  By then, the bloat is disastrous.

I think it's that suicide that Andres wants to disable.

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
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