Re: Enable data checksums by default

On Thu, Aug 15, 2024 at 09:49:04AM +0200, Jakub Wartak wrote:
> On Wed, Aug 7, 2024 at 4:18 PM Greg Sabino Mullane <htamfids(at)gmail(dot)com> wrote:
> >
> > On Wed, Aug 7, 2024 at 4:43 AM Michael Banck <mbanck(at)gmx(dot)net> wrote:
> >>
> >> I think the last time we dicussed this the consensus was that
> >> computational overhead of computing the checksums is pretty small for
> >> most systems (so the above change seems warranted regardless of whether
> >> we switch the default), but turning on wal_compression also turns on
> >> wal_log_hints, which can increase WAL by quite a lot. Maybe this is
> [..]
> >
> >
> > Yeah, that seems something beyond this patch? Certainly we should
> > mention wal_compression in the release notes if the default changes.
> > I mean, I feel wal_log_hints should probably default to on as well,
> > but I've honestly never really given it much thought because my
> > fingers are trained to type "initdb -k". I've been using data
> > checksums for roughly a decade now. I think the only time I've NOT
> > used checksums was when I was doing checksum overhead measurements,
> > or hacking on the pg_checksums program.
>
> Maybe I don't understand something, but just to be clear:
> wal_compression (mentioned above) is not turning wal_log_hints on,
> just the wal_log_hints needs to be on when using data checksums
> (implicitly, by the XLogHintBitIsNeeded() macro). I suppose Michael
> was thinking about the wal_log_hints earlier (?)

Uh, I am pretty sure I meant to say "turning on data_checksums als turns
on wal_log_hints", sorry about the confusion.

I guess the connection is that if you turn on wal_lot_hints (either
directly or via data_checksums) then the number FPIs goes up (possibly
signficantly), and enabling wal_compression could (partly) remedy that.
But I agree with Greg that such a discussion is probably out-of-scope
for this default change.

Michael