Re: Additional stats for Relations - Mailing list pgsql-hackers

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

> On 10/13/06, Jim C. Nasby <jim@nasby.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