Re: hint in determining effective_io_concurrency

On Thu, Apr 22, 2021 at 03:52:32PM -0400, Bruce Momjian wrote:
> On Thu, Apr 22, 2021 at 09:45:15PM +0200, Luca Ferrari wrote:
> > Hi all,
> > I'm unable to find (apparently) a way to find out a possible value to
> > start with for effective_io_concurrency.
> > I suspect that benchmarking, e.g., using bonnie++ or sysbench and
> > testing with different values of concurrency could help to determine
> > the max number of concurrent request, (tps, lower latency, ecc.).
> > Is thjs correct or is there another suggested way?
>
> I recommend 256 for SSDs or other RAM-like fsync systems, and maybe
> maybe 16 for magnetic.

Note that the interpretation of this GUC changed in v13.
https://www.postgresql.org/docs/13/release-13.html
|Change the way non-default effective_io_concurrency values affect concurrency (Thomas Munro)
|Previously, this value was adjusted before setting the number of concurrent requests. The value is now used directly. Conversion of old values to new ones can be done using:
|SELECT round(sum(OLDVALUE / n::float)) AS newvalue FROM generate_series(1, OLDVALUE) s(n);

--
Justin