Re: Foreground vacuum and buffer access strategy

On Mon, Aug 12, 2013 at 11:47 PM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
> Reviving a very old thread, because I've run into the issue again.
> On Tue, May 29, 2012 at 11:58 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>> On Fri, May 25, 2012 at 4:06 PM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
>>> If I invoke vacuum manually and do so with VacuumCostDelay == 0, I
>>> have basically declared my intentions to get this pain over with as
>>> fast as possible even if it might interfere with other processes.
>>>
>>> Under that condition, shouldn't it use BAS_BULKWRITE rather than
>>> BAS_VACUUM? The smaller ring size leads to a lot of synchronous WAL
>>> flushes which I think can slow the vacuum down a lot.
>>
>> Of course, an autovacuum of a really big table could run too slowly,
>> too, even though it's not a foreground task.
>
> True. But almost by definition, an autovacuum is not trying to run
> inside a maintenance window.
>
> Would it be reasonable to upgrade the ring buffer size whenever
> VacuumCostDelay is zero, regardless of whether it is a manual or an
> auto vac? One thing I worry about is that many people may have
> changed autovacuum_vacuum_cost_delay from 20 directly to 0 or -1, and
> the accidental throttling on WAL syncs might be the only thing
> preventing their system from falling over each time autovac of a large
> table kicks in.

I'm not sure what the right thing to do here is, but I definitely
agree there's a problem. There are definitely cases where people want
or indeed need to vacuum as fast as possible, and using a small ring
buffer is not the way to do that. Now, tying that to VacuumCostDelay
doesn't seem right, because setting that to 0 shouldn't suddenly
change the behavior in other ways, as well.

In general, the approach we've taken so far has been to try to hide
the ring-buffer behavior from users and not make it tunable, but I'm
not sure we can really get away with that in this case. Increasing
the ring-buffer size has system-wide performance implications which
could be very good (less bloat) or very bad (I/O starvation of
concurrent activity). I don't think the system knows enough to guess
which one will be better in any particular case.

--
Robert Haas
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