Re: pg_stat_bgwriter.buffers_backend is pretty meaningless (and more?)

I've gone ahead and implemented option 1 (commented below).

On Thu, Oct 6, 2022 at 6:23 PM Melanie Plageman
<melanieplageman@gmail.com> wrote:
>
> v31 failed in CI, so
> I've attached v32 which has a few issues fixed:
> - addressed some compiler warnings I hadn't noticed locally
> - autovac launcher and worker do indeed use bulkread strategy if they
>   end up starting before critical indexes have loaded and end up doing a
>   sequential scan of some catalog tables, so I have changed the
>   restrictions on BackendTypes allowed to track IO Operations in
>   IOCONTEXT_BULKREAD
> - changed the name of the column "fsynced" to "files_synced" to make it
>   more clear what unit it is in (and that the unit differs from that of
>   the "unit" column)
>
> In an off-list discussion with Andres, he mentioned that he thought
> buffers reused by a BufferAccessStrategy should be split from buffers
> "acquired" and that "acquired" should be renamed "clocksweeps".
>
> I have started doing this, but for BufferAccessStrategy IO there are a
> few choices about how we want to count the clocksweeps:
>
> Currently the following situations are counted under the following
> IOContexts and IOOps:
>
> IOCONTEXT_[VACUUM,BULKREAD,BULKWRITE], IOOP_ACQUIRE
> - reuse a buffer from the ring
>
> IOCONTEXT_SHARED, IOOP_ACQUIRE
> - add a buffer to the strategy ring initially
> - add a new shared buffer to the ring when all the existing buffers in
>   the ring are pinned
>
> And in the new paradigm, I think these are two good options:
>
> 1)
> IOCONTEXT_[VACUUM,BULKREAD,BULKWRITE], IOOP_CLOCKSWEEP
> - add a buffer to the strategy ring initially
> - add a new shared buffer to the ring when all the existing buffers in
>   the ring are pinned
>
> IOCONTEXT_[VACUUM,BULKREAD,BULKWRITE], IOOP_REUSE
> - reuse a buffer from the ring
>

I've implemented this option in attached v33.

> 2)
> IOCONTEXT_[VACUUM,BULKREAD,BULKWRITE], IOOP_CLOCKSWEEP
> - add a buffer to the strategy ring initially
>
> IOCONTEXT_[VACUUM,BULKREAD,BULKWRITE], IOOP_REUSE
> - reuse a buffer from the ring
>
> IOCONTEXT SHARED, IOOP_CLOCKSWEEP
> - add a new shared buffer to the ring when all the existing buffers in
>   the ring are pinned


- Melanie