Re: Determine optimal fdatasync/fsync, O_SYNC/O_DSYNC options

Bruce Momjian wrote:

> Have you seen /src/tools/fsync?
>

Now that the argument is already open, why postgres choose
on linux fdatasync? I'm understanding from other posts that
on this platform open_sync is better than fdatasync.

However I choose open_sync. During initdb why don't detect
this parameter ?

Regards
Gaetano Mendola

These are my times:

kernel 2.4.9-e.24smp ( RAID SCSI ):

Simple write timing:
write 0.011544

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 1.233312
write, close, fsync 1.242086

Compare one o_sync write to two:
one 16k o_sync write 0.517633
two 8k o_sync writes 0.824603

Compare file sync methods with one 8k write:
(o_dsync unavailable)
open o_sync, write 0.438580
write, fdatasync 1.239377
write, fsync, 1.178017

Compare file sync methods with 2 8k writes:
(o_dsync unavailable)
open o_sync, write 0.818720
write, fdatasync 1.395602
write, fsync, 1.351214

kernel 2.4.22-1.2199.nptlsmp (single EIDE disk):

Simple write timing:
write 0.023697

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 0.688765
write, close, fsync 0.702166

Compare one o_sync write to two:
one 16k o_sync write 0.498296
two 8k o_sync writes 0.543956

Compare file sync methods with one 8k write:
(o_dsync unavailable)
open o_sync, write 0.259664
write, fdatasync 0.971712
write, fsync, 1.006096

Compare file sync methods with 2 8k writes:
(o_dsync unavailable)
open o_sync, write 0.536882
write, fdatasync 1.160347
write, fsync, 1.189699