Re: [HACKERS] [PATCH] Vacuum: Update FSM more frequently

Claudio Freire <klaussfreire@gmail.com> writes:
> On Wed, Mar 28, 2018 at 6:54 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> After 0001,
>> there's no reason to assume that vacuum is particularly likely to get
>> cancelled between having made cleanups and having updated the upper FSM
>> levels.  (Maybe the odds are a bit more for the no-indexes case, but
>> that doesn't seem like it's common enough to justify a special mechanism
>> either.)

> Why not?

> Any kind of DDL (even those that don't rewrite the heap) would cancel
> autovacuum.

> You might think DDL isn't common enough to worry about, but I've seen
> cases where regular reindex were required to keep index bloat in check
> (and were cron'd), and those cancel autovacuum all the time.

If you've got a situation where every vacuum gets canceled partway
through, you've got bloat problems regardless, because the heap tuples are
never getting removed in the first place; worrying about whether the FSM
is up to date is pretty pointless.  The 0001 patch basically updates the
FSM as soon as it can after the tuples are actually deleted, so I think
we've made the window as small as practical, and I don't really see a need
to do extra work (and add substantial extra complexity) to deal with
missed cleanup a bit sooner.

People who are dealing with this sort of scenario a lot might be well
advised to reduce autovacuum's maintenance_work_mem, so that the cleanup
cycles happen more often.  That's kind of costly in terms of the number
of index scans, but it reduces the amount of cleanup work that can be
lost to a cancel.

(I'd also argue that a setup such as you describe is very possibly making
things worse not better.  Perhaps the 0001 patch will go some way towards
making it less necessary to do that.)

I've pushed 0001 and a replacement for 0003.  I have to go do something
else right now, but I'll take a closer look at 0004 later.

			regards, tom lane