Re: BitmapHeapScan streaming read user and prelim refactoring

On Sat, Mar 30, 2024 at 12:40 PM Tomas Vondra
<tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
> Sorry, I meant the prefetch (readahead) built into ZFS. I may be wrong
> but I don't think the regular RA (in linux kernel) works for ZFS, right?

Right, it separate page cache ("ARC") and prefetch settings:

https://openzfs.github.io/openzfs-docs/Performance%20and%20Tuning/Module%20Parameters.html

That's probably why Linux posix_fadvise didn't affect it, well that
and the fact, at a wild guess, that Solaris didn't have that system
call...

> I was wondering if we could use this (posix_fadvise) to improve that,
> essentially by issuing fadvise even for sequential patterns. But now
> that I think about that, if posix_fadvise works since 2.2, maybe RA
> works too now?)

It should work fine. I am planning to look into this a bit some day
soon -- I think there may be some interesting interactions between
systems with big pages/records like ZFS/BTRFS/... and io_combine_limit
that might offer interesting optimisation tweak opportunities, but
first things first...