Re: Use fadvise in wal replay

>
> > On 21 Jun 2022, at 16:59, Jakub Wartak <jakub(dot)wartak(at)tomtom(dot)com> wrote:
> Oh, wow, your benchmarks show really impressive improvement.
>

FWIW I was trying to speedup long sequential file reads in Postgres using
fadvise hints. I've found no detectable improvements.
Then I've written 1Mb - 1Gb sequential read test with both fadvise
POSIX_FADV_WILLNEED
and POSIX_FADV_SEQUENTIAL in Linux. The only improvement I've found was

1. when the size of read was around several Mb and fadvise len also around
several Mb.
2. when before fdavice and the first read there was a delay (which was
supposedly used by OS for reading into prefetch buffer)
3. If I read sequential blocks i saw speedup only on first ones. Overall
read speed of say 1Gb file remained unchanged no matter what.

I became convinced that if I read something long, OS does necessary
speedups automatically (which is also in agreement with fadvise manual/code
comments).
Could you please elaborate how have you got the results with that big
difference? (Though I don't against fadvise usage, at worst it is expected
to be useless).

--
Best regards,
Pavel Borisov

Postgres Professional: http://postgrespro.com <http://www.postgrespro.com>