Re: reloption to prevent VACUUM from truncating empty pages at the end of relation

Fujii Masao <masao.fujii@gmail.com> writes:
> When VACUUM tries to truncate the trailing empty pages, it scans shared_buffers
> to invalidate the pages-to-truncate during holding an AccessExclusive lock on
> the relation. So if shared_buffers is huge, other transactions need to wait for
> a very long time before accessing to the relation. Which would cause the
> response-time spikes, for example, I observed such spikes several times on
> the server with shared_buffers = 300GB while running the benchmark.
> Therefore, I'm thinking to propose $SUBJECT and enable it to avoid such spikes
> for that relation.

I think that the real problem here is having to do a scan of all of shared
buffers.  VACUUM's not the only thing that has to do that, there's also
e.g. DROP and TRUNCATE.  So rather than a klugy solution that only fixes
VACUUM (and not very well, requiring user intervention and an unpleasant
tradeoff), we ought to look at ways to avoid needing a whole-pool scan to
find the pages belonging to one relation.  In the past we've been able to
skate by without a decent solution for that because shared buffers were
customarily not all that big.  But if we're going to start considering
huge buffer pools to be a case we want to have good performance for,
that's got to change.

			regards, tom lane