Re: Option to not use ringbuffer in VACUUM, using it in failsafe mode

Hi,

On 2023-01-11 16:18:34 -0500, Tom Lane wrote:
> Peter Geoghegan <pg(at)bowt(dot)ie> writes:
> > On Wed, Jan 11, 2023 at 11:18 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
> >> I don't like that - it's also quite useful to disable use of ringbuffers when
> >> you actually need to clean up indexes. Especially when we have a lot of dead
> >> tuples we'll rescan indexes over and over...
>
> > That's a fair point.
>
> > My vote goes to "REUSE_BUFFERS", then.
>
> I wonder whether it could make sense to allow a larger ringbuffer size,
> rather than just the limit cases of "on" and "off".

I can see that making sense, particularly if we were to later extend this to
other users of ringbuffers. E.g. COPYs us of the ringbuffer makes loading of
data > 16MB but also << s_b vastly slower, but it can still be very important
to use if there's lots of parallel processes loading data.

Maybe BUFFER_USAGE_LIMIT, with a value from -1 to N, with -1 indicating the
default value, 0 preventing use of a buffer access strategy, and 1..N
indicating the size in blocks?

Would we want to set an upper limit lower than implied by the memory limit for
the BufferAccessStrategy allocation?

Greetings,

Andres Freund