Re: Add pg_stat_vfdcache view for VFD cache statistics

On Wed, Sep 9, 2026 at 9:22 AM David Geier <geidav(dot)pg(at)gmail(dot)com> wrote:

> >>> Yes of course, I’d be happy to take a look.
> >> Attached is the patch. It's pretty small and passes regress tests.
> >>
> > Nice one.
> > Few comments on it:
> > #1:
> > + oldcontext = MemoryContextSwitchTo(VfdCxt);
> > + newVfdCache = repalloc_array(VfdCache, Vfd, newCacheSize);
> > + MemoryContextSwitchTo(oldcontext);
> >
> > Can't we just do this?
> > + newVfdCache = repalloc_array(VfdCache, Vfd, newCacheSize);
> >
> > because repalloc doesn't need CurrentMemoryContext.
>
> Yes. You can then make it even simpler and get rid of newVfdCache via
>
> VfdCache = repalloc_array(VfdCache, Vfd, newCacheSize);
>
> > #2:
> > + newDescs = MemoryContextAllocExtended(VfdCxt,
> > + newMax * sizeof(AllocateDesc), MCXT_ALLOC_NO_OOM);
> > if (newDescs == NULL)
> > return false;
> > + memcpy(newDescs, allocatedDescs, maxAllocatedDescs *
> > sizeof(AllocateDesc));
> > + pfree(allocatedDescs);
> >
> > We can also just replace it with:
> > + newDescs = repalloc_array_extended(allocatedDescs, AllocateDesc,
> > + newMax, MCXT_ALLOC_NO_OOM);
> > Correct?
>
> Yes.
>
> v7-0001: looks good to me.
>
> v7-0003: I'm wondering if we still want cache_bytes in pg_stat_vfdcache,
> now where it's exposed via pg_backend_memory_contexts. It seems to me
> that other stats functionality also doesn't expose memory info that is
> accessible via pg_backend_memory_contexts. But I'm not completely sure
> what's best here.
>
pg_backend_memory_contexts is local to current backend session, so AFAIK
there's no other way of getting cluster-wide vfd cache memory usage.
Therefore we need cache_bytes for this?

Regards,
Ayoub