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

Claudio Freire <klaussfreire(at)gmail(dot)com> writes:
> On Sat, Mar 24, 2018 at 4:17 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> I hadn't paid any attention to this patch previously, so maybe I'm
>> missing something ... but this sure seems like a very bizarre approach
>> to the problem. If the idea is to fix the FSM's upper levels after
>> vacuuming a known sub-range of the table, why would you not proceed
>> by teaching FreeSpaceMapVacuum to recurse only within that sub-range
>> of page numbers? This setup with a threshold seems entirely Rube
>> Goldbergian. It's dependent on a magic threshold number that you can
>> only select by trial and error, and it's inevitably going to spend time
>> updating segments of the FSM tree that have nothing to do with the part
>> that's been vacuumed.

> Well, the point is to not only update the range we know we've
> vacuumed, but also to finish the updates done by a potential
> previously cancelled autovacuum.

I think that's not an important consideration, or at least would stop
being one after a patch like this. The reason it's a problem now is
precisely that we don't try to vacuum the FSM till the very end; if
we did FSM cleanup every X pages --- in particular, before not after
the final relation-truncation attempt --- there wouldn't be risk of
skipping so much FSM work that we need to worry about forcing the
issue just in case there'd been a prior cancellation.

(Of course, you'd still need to do something after the truncation
step to truncate the FSM, but I'm arguing it should *only* respond
to that, not have to clean up all the rest of the FSM state.)

regards, tom lane