Re: [HACKERS] Autovacuum Improvements

Alvaro Herrera wrote:
> Bruce Momjian wrote:
> >
> > Yep, agreed on the random I/O issue. The larger question is if you have
> > a huge table, do you care to reclaim 3% of the table size, rather than
> > just vacuum it when it gets to 10% dirty? I realize the vacuum is going
> > to take a lot of time, but vacuuming to relaim 3% three times seems like
> > it is going to be more expensive than just vacuuming the 10% once. And
> > vacuuming to reclaim 1% ten times seems even more expensive. The
> > partial vacuum idea is starting to look like a loser to me again.
>
> But if the partial vacuum is able to clean the busiest pages and reclaim
> useful space, currently-running transactions will be able to use that
> space and thus not have to extend the table. Not that extension is a
> problem on itself, but it'll keep your working set smaller.

Yes, but my point is that if you are trying to avoid vacuuming the
table, I am afraid the full index scan is going to be painful too. I
can see corner cases where partial vacuum is a win (I only have 4 hours
of idle I/O), but for the general case I am still worried that partial
vacuum will not be that useful as long as we have to scan the indexes.

--
Bruce Momjian bruce(at)momjian(dot)us
EnterpriseDB http://www.enterprisedb.com

+ If your life is a hard drive, Christ can be your backup. +