On Mon, 2013-03-04 at 23:22 +0200, Heikki Linnakangas wrote:
> On 04.03.2013 23:00, Jeff Davis wrote:
> > On Mon, 2013-03-04 at 22:27 +0200, Heikki Linnakangas wrote:
> >> Yeah, fragmentation will certainly hurt some workloads. But how badly,
> >> and which workloads, and how does that compare with the work that
> >> PostgreSQL has to do to maintain the checksums? I'd like to see some
> >> data on those things.
> >
> > I think we all would. Btrfs will be a major filesystem in a few years,
> > and we should be ready to support it.
>
> Perhaps we should just wait a few years? If we suspect that this becomes
> obsolete in a few years
I do not expect it to be obsolete, even if btrfs is stable and fast
today.
Consider this hypothetical scenario: what if btrfs performs acceptably
well today, but they tune it away from our needs later and it tanks
performance? Then, when we complain, the btrfs people say "for DB
workloads, you should turn off COW, or use ext4 or XFS". And then we say
"but we want checksums". And then they tell us that real databases do
their own checksums.
Then what?
I don't think that scenario is very outlandish. Postgres is essentially
a COW system (for tuples), and stacking COW on top of COW does not seem
like a good idea (neither for filesystems nor actual cows). So it may be
within reason for the filesystem folks to say we're doing the wrong
thing, and then checksums are our problem again. Additionally, I don't
have a lot of faith that linux will address all of our btrfs complaints
(even legitimate ones) in a reasonable amount of time, if ever.
Regards,Jeff Davis