Re: Add pg_stat_vfdcache view for VFD cache statistics

On Wed, Sep 9, 2026 at 4:06 PM Ayoub Kazar <kazarayoub2004(at)gmail(dot)com> wrote:

> 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);
>>
> Done. Attached is the change.

>
>> > #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
>