Re: fsync method checking

Bruce Momjian <pgman@candle.pha.pa.us> writes:
> Tom Lane wrote:
>> It's what tested out as the best bet.  I think we were using pgbench
>> as the test platform, which as you know I have doubts about, but at
>> least it is testing one actual write/sync pattern Postgres can generate.

> I assume pgbench has so much variance that trying to see fsync changes
> in there would be hopeless.

The results were fairly reproducible, as I recall; else we'd have looked
for another test method.  You may want to go back and consult the
pghackers archives.

>> * Some of the test cases count open()/close() overhead, some don't.

> The only one I saw that had an extra open() was the fsync after close
> test.  I add a do-nothing open/close to the previous test so they are
> the same.

Why is it sensible to include open/close overhead in the "simple write"
case and not in the "o_sync write" cases, for instance?  Doesn't seem
like a fair comparison to me.  Adding the open overhead to all cases
might make it "fair", but it would also make it not what we want to
measure.

>> * The program is claimed to test whether you can write from one process
>> and fsync from another, but it does no such thing AFAICS.

> It really just shows whether the fsync fater the close has similar
> timing to the one before the close.  That was the best way I could think
> to test it.

Sure, but where's the "separate process" part?  What this seems to test
is whether a single process can sync its own writes through a different
file descriptor; which is interesting but by no means the only thing we
need to be sure of if we want to make the bgwriter handle syncing.

            regards, tom lane