Re: WIP: WAL prefetch (another approach)

On Wed, Apr 13, 2022 at 8:05 AM Thomas Munro <thomas.munro@gmail.com> wrote:
> On Wed, Apr 13, 2022 at 3:57 AM Dagfinn Ilmari Mannsåker
> <ilmari@ilmari.org> wrote:
> > Simon Riggs <simon.riggs@enterprisedb.com> writes:
> > > This is a nice feature if it is safe to turn off full_page_writes.

> > > When is it safe to do that? On which platform?
> > >
> > > I am not aware of any released software that allows full_page_writes
> > > to be safely disabled. Perhaps something has been released recently
> > > that allows this? I think we have substantial documentation about
> > > safety of other settings, so we should carefully document things here
> > > also.
> >
> > Our WAL reliability docs claim that ZFS is safe against torn pages:
> >
> > https://www.postgresql.org/docs/current/wal-reliability.html:
> >
> >     If you have file-system software that prevents partial page writes
> >     (e.g., ZFS), you can turn off this page imaging by turning off the
> >     full_page_writes parameter.
>
> Unfortunately, posix_fadvise(WILLNEED) doesn't do anything on ZFS
> right now :-(.

Update: OpenZFS now has this working in its master branch (Linux only
for now), so fingers crossed for the next release.