On Thu, Aug 8, 2013 at 2:50 PM, Hannu Krosing <hannu@2ndquadrant.com> wrote:
> On 08/08/2013 05:28 PM, Jon Nelson wrote:
...
> Just an idea - can you check if using a fillfactor different form 100
> changes anything
>
> pgbench -s 20 -p 54320 -d pgb -F 90 -i
>
>
>> pgbench -j 80 -c 80 -T 120 -p 54320 pgb
>> pg_ctl -D tt -w stop
>
> That is, does extending tables and indexes to add updated tuples play
> any role here
I gave it a go - it didn't make any difference at all.
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. Should this feature be reconsidered?
--
Jon
