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/
PostgreSQL Development, 24x7 Support, Training & Services
