Re: perf pb solved only after pg_dump and restore - Mailing list pgsql-performance

Guillaume,

Thanks for your help.

> On 28 Aug 2006 11:43:16 +0200, Guillaume Cottenceau <gc@mnc.ch> wrote:
> >  max_fsm_pages is 20000
> >  max_fsm_relations is 1000
> > Do they look low?
>
> Yes they are probably too low if you don't run VACUUM on a regular
> basis and you have a lot of UPDATE/DELETE activity. FSM doesn't take a
> lot of memory so it's usually recommended to have a confortable value
> for it.

Normally, we run VACUUM ANALYZE overnight. I'd say we have low
DELETE activity, kinda high SELECT/INSERT activity, and UPDATE
would be in the middle of that.

> I usually recommend to read:
> http://www.pervasive-postgres.com/instantkb13/article.aspx?id=10116&cNode=5K1C3W
> http://www.pervasive-postgres.com/instantkb13/article.aspx?id=10087&cNode=5K1C3W
> to understand better what VACUUM and FSM mean.

Thanks for the pointer, will read that.

> > Can you elaborate? I have created a couple of indexes (according
> > to multiple models of use in our application) and they do take up
> > quite some disk space (table dump is 600M but after restore it
> > takes up 1.5G on disk) but I thought they could only do good or
> > never be used, not impair performance..
>
> Index slow downs write activity (you have to maintain them). It's not
> always a good idea to create them.

Of course. How newbie did I look :/. The thing is that I once did
a few measurements and noticed no (measurable) impact in INSERT
with a supplementary index, so I (wrongly) forgot about this.

> > > Maybe a VACUUM FULL fullowed by a REINDEX will have solved your problem.
> >
> > So these would have reordered the data for faster sequential
> > access which is not the case of VACUUM ANALYZE?
>
> VACUUM ANALYZE won't help you if your database is completely bloated.

What do you mean exactly by bloated? If you mean that there is a
lot of (unused) data, the thing is that our trim removed most of
it. I was kinda hoping that after analyzing the database, the old
data would exit the whole picture, which obviously wasn't the
case.

About REINDEX: is it ok to consider that REINDEX is to indexes
what VACUUM FULL is to table data, because it cleans up unused
index pages?

> And AFAICS you're not running it on a regular basis so your database
> was probably completely bloated which means:
> - bloated indexes,
> - bloated tables (ie a lot of fragmentation in the pages which means
> that you need far more pages to store the same data).

I suppose that table fragmentation occurs when DELETE are
interleaved with INSERT?

> The only ways to solve this situation is either to dump/restore or run
> a VACUUM FULL ANALYZE (VERBOSE is better to keep a log), and
> eventually reindex any bloated index (depends on your situation).

Ok.

> > > When the free_space_map is to low, VACUUM ANALYZE should have told you
> > > via a warning (at least, if your logging is set appropriately).
> >
> > Unfortunately, we didn't keep the logs of VACUUM ANALYZE, so I
> > can't be sure :/
>
> You should really run VACUUM ANALYZE VERBOSE on a regular basis and
> analyze the logs to be sure your VACUUM strategy and FSM settings are
> OK.

VACUUM ANALYZE is normally run overnight (each night). Is it not
regular enough? There can be hundreds of thousands of statements
a day.

--
Guillaume Cottenceau
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