Re: PoC: history of recent vacuum/checkpoint runs (using new hooks)

Hi,

On Wed, Dec 25, 2024 at 06:25:50PM +0100, Tomas Vondra wrote:
> Hi,
>
> On 12/23/24 07:35, wenhui qiu wrote:
> > Hi Tomas 
> >      This is a great feature.  
> > + /*
> > + * Define (or redefine) custom GUC variables.
> > + */
> > + DefineCustomIntVariable("stats_history.size",
> > + "Sets the amount of memory available for past events.",
> > + NULL,
> > + &statsHistorySizeMB,
> > + 1,
> > + 1,
> > + 128,
> > + PGC_POSTMASTER,
> > + GUC_UNIT_MB,
> > + NULL,
> > + NULL,
> > + NULL);
> > +
> > RAM is in terabytes now, the statsHistorySize is 128MB ,I think can
> > increase to store more history record ?
> >
>
> Maybe, the 128MB is an arbitrary (and conservative) limit - it's enough
> for ~500k events, which seems good enough for most systems. Of course,
> on systems with many relations might need more space, not sure.
>
> I was thinking about specifying the space in more natural terms, either
> as amount of time ("keep 1 day of history") or number of entries ("10k
> entries"). That would probably mean the memory can't be allocated as
> fixed size.
>
> But maybe it'd be possible to just write the entries to a file. We don't
> need random access to past entries (unlike e.g. pg_stat_statements), and
> people won't query that very often either.

Thanks for working on this!

Another idea regarding the storage of those metrics: I think that one would
want to see "precise" data for recent metrics but would probably be fine with some
level of aggregation for historical ones. Something like being able to retrieve
"1 day of raw data" and say one year of data aggregated by day (average, maximum,
minimum , standard deviation and maybe some percentiles) could be fine too.

Regards,

--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com