Re: [HACKERS] fsync method checking

On 22 Mar, Tom Lane wrote:
> markw(at)osdl(dot)org writes:
>> I could certainly do some testing if you want to see how DBT-2 does.
>> Just tell me what to do. ;)
>
> Just do some runs that are identical except for the wal_sync_method
> setting. Note that this should not have any impact on SELECT
> performance, only insert/update/delete performance.

Ok, here are the results I have from my 4-way xeon system, a 14 disk
volume for the log and a 52 disk volume for everything else:
http://developer.osdl.org/markw/pgsql/wal_sync_method.html

7.5devel-200403222

wal_sync_method metric
default (fdatasync) 1935.28
fsync 1613.92

# ./test_fsync -f /opt/pgdb/dbt2/pg_xlog/test.out
Simple write timing:
write 0.018787

Compare fsync times on write() and non-write() descriptor:
(If the times are similar, fsync() can sync data written
on a different descriptor.)
write, fsync, close 13.057781
write, close, fsync 13.311313

Compare one o_sync write to two:
one 16k o_sync write 6.515122
two 8k o_sync writes 12.455124

Compare file sync methods with one 8k write:
(o_dsync unavailable)
open o_sync, write 6.270724
write, fdatasync 13.275225
write, fsync, 13.359847

Compare file sync methods with 2 8k writes:
(o_dsync unavailable)
open o_sync, write 12.479563
write, fdatasync 13.651709
write, fsync, 14.000240