Re: Additional stats for Relations

On Sat, 2006-10-14 at 11:32 +0530, NikhilS wrote:

> On 10/13/06, Jim C. Nasby <jim(at)nasby(dot)net> wrote:

> I'm also not sure if this metric is what you actually want,
> since a
> single page can be returned many times from the FSM even
> between
> vacuums. Tracking how many pages for a relation have been put
> into the
> FSM might be more useful...
>
> <Nikhils>
> Pages might be put into the FSM, but by this metric don't we get the
> actual usage of the pages from the FSM? Agreed a single page can be
> returned multiple times, but since it serves a new tuple, shouldn't we
> track it?
> <Nikhils>

This makes sense for indexes, but only makes sense for heaps when we
know that the backend will keep re-accessing the block until it is full
- so only of interest in steady-state workloads.

IMHO Jim's proposal makes more sense for general use.

> > heap_blks_extend: The number of times file extend was
> invoked on the
> > relation

Sounds good

> > heap_blks_truncate: The total number of blocks that have
> been truncated due
> > to vacuum activity e.g.

Sounds good

> > As an addendum to the truncate stats above, we can also have
> the additional
> > following stats:
> >
> > heap_blks_maxtruncate: The max block of buffers truncated in
> one go
> >
> > heap_blks_ntruncate: The number of times truncate was called
> on this
> > relation

Those last 2 sound too complex for normal use and ntruncate is most
likely the same as number of vacuums anyway. Hmmm...Perhaps nvacuums is
a more interesting metric? We've got last vacuum date, but no indication
of how frequently a vacuum has run.

> Do you have a use-case for this info? I can see where it might
> be neat
> to know, but I'm not sure how you'd actually use it in the
> real world.
>
> <Nikhils>
> The use-case according to me is that these stats help prove the
> effectiveness of autovacuum/vacuum operations. By varying some autovac
> guc variables, and doing subsequent (pgbench e.g.) runs, one can find
> out the optimum values for these variables using these stats.
> <Nikhils>

This should be useful for tuning space allocation/deallocation. If we
get this patch in early it should help get feedback on this area.

--
Simon Riggs
EnterpriseDB http://www.enterprisedb.com