Re: 9.4 regression

Jon Nelson <jnelson+pgsql(at)jamponi(dot)net> writes:
> At this point I'm convinced that the issue is a pathological case in
> ext4. The performance impact disappears as soon as the unwritten
> extent(s) are written to with real data. Thus, even though allocating
> files with posix_fallocate is - frequently - orders of magnitude
> quicker than doing it with write(2), the subsequent re-write can be
> more expensive. At least, that's what I'm gathering from the various
> threads. Why this issue didn't crop up in earlier testing and why I
> can't seem to make test_fallocate do it (even when I modify
> test_fallocate to write to the newly-allocated file in a mostly-random
> fashion) has me baffled.

Does your test program use all the same writing options that the real
WAL writes do (like O_DIRECT)?

> Should this feature be reconsidered?

Well, ext4 isn't the whole world, but it's sure a big part of it.
The real point though is that obviously we didn't do enough performance
testing, so we'd better do more before deciding what to do.

regards, tom lane