Re: Add pg_stat_vfdcache view for VFD cache statistics

Hello,
On Tue, Mar 24, 2026 at 3:09 PM David Geier <geidav(dot)pg(at)gmail(dot)com> wrote:

> On 24.03.2026 00:36, KAZAR Ayoub wrote:
>
> >> I've looked at struct vfd and some simple changes to the struct would
> >> already cut memory consumption in half. I can look into that.
> >>
> >> Thoughts?
> >
> > Looking forward to this.
>
> I try to come up with something the next days.
>
> > What also bothers me in that space is if a backend allocates 100K entries
> > in VFD cache, that cache is never shrank ever again,
> > the cache only grows (if it needs more than its lifetime maximum) until
> the
> > backend dies, although this is useful as entries are reused if free
> instead
> > of
> > allocating entries, whether a spike in files openings effects a long
> living
> > backend to keep holding a useless amount of
> > cache size it will need in the future, i don't imagine this to be common
> > though, what do you think about this issue from your experience ?
>
> Currently the cache is directly mapped by the VFD index. That means we
> could only resize down to the maximum used VFD index.
>
> Being able to resize independently of the maximum VFD index would
> require changing to a hash map like simplehash.h. I can take a look how
> invasive such a change would be.

> --
> David Geier
>
I've implemented the recommended global stats view on vfd cache, the
implementation should be also straightforward as it follows the same
cumulative shared statistics infrastructure like pgstat_bgwriter and others
do.

Attached is v2 patch also contains what David suggested for global cache
size and entries in the view.

Kind regards,
Ayoub