Re: [PING] fallocate() causes btrfs to never compress postgresql files

On Fri, May 30, 2025 at 3:58 AM Dimitrios Apostolou <jimis(at)gmx(dot)net> wrote:
> All I'm saying is that this is a regression for PostgreSQL users that keep
> tablespaces on compressed Btrfs. What could be done from postgres, is to
> provide a runtime setting for avoiding fallocate(), going instead through
> the old code path. Idelly this would be an option per tablespace, but even
> a global one is better than nothing.

Here's an initial sketch of such a setting. Better name, design,
words welcome. Would need a bit more work to cover temp tables too.
It's slightly tricky to get smgr to behave differently because of the
contents of a system catalogue! I couldn't think of a better way than
exposing it as a flag that the buffer manager layer has to know about
and compute earlier, but that also seems a bit strange, as fallocate
is a highly md.c specific concern. Hmm.

I suppose something like the 0001 part could be back-patched if this
is considered a serious enough problem without other workarounds, so I
did this in two steps. I wonder if there are good reasons to want to
change the number on other file systems. I suppose it at least allows
experimentation.