Re: fallocate / posix_fallocate for new WAL file creation (etc...)

On Sun, 2013-06-30 at 18:55 -0400, Greg Smith wrote:
> This makes platform level testing a lot easier, thanks. Attached is an
> updated copy of that program with some error checking. If the files it
> creates already existed, the code didn't notice, and a series of write
> errors happened. If you set the test up right it's not a problem, but
> it's better if a bad setup is caught. I wrapped the whole test with a
> shell script, also attached, which insures the right test sequence and
> checks.

Thank you.

> That's glibc helpfully converting your call to posix_fallocate into
> small writes, because the OS doesn't provide a better way in that
> kernel. It's not hard to imagine this being slower than what the WAL
> code is doing right now. I'm not worried about correctness issues
> anymore, but my gut paranoia about this not working as expected on older
> systems was justified. Everyone who thought I was just whining owes me
> a cookie.

So your theory is that it may be slower because there are twice as many
syscalls (one per 4K page rather than one per 8K page)? Interesting
observation.

> This is what I plan to benchmark specifically next.

In the interest of keeping this patch moving forward, do you have an
estimate for when this testing will be complete?

> If the
> posix_fallocate approach is actually slower than what's done now when
> it's not getting kernel acceleration, which is the case on RHEL5 era
> kernels, we might need to make the configure time test more complicated.
> Whether posix_fallocate is defined isn't sensitive enough; on Linux it
> may be the case that this only is usable when fallocate() is also there.

I'd say that if posix_fallocate is slower than the existing code on
pretty much any platform, we shouldn't commit the patch at all. I would
be quite surprised if that was the case, however.

Regards,
Jeff Davis