Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > I have been poking around with our fsync default options to see if I can
> > improve them. One issue is that we never default to O_SYNC, but default
> > to O_DSYNC if it exists, which seems strange.
>
> As I recall, that was based on testing on some different platforms.
> It's not particularly "strange": O_SYNC implies writing at least two
> places on the disk (file and inode). O_DSYNC or fdatasync should
> theoretically be the fastest alternatives, O_SYNC and fsync the worst.
But why perfer O_DSYNC over fdatasync if you don't prefer O_SYNC over
fsync?
>
> > Compare fsync before and after write's close:
> > write, fsync, close 0.000707
> > write, close, fsync 0.000808
>
> What does that mean? You can't fsync a closed file.
You reopen and fsync.
> > This shows terrible O_SYNC performance for 2 8k writes, but is faster
> > for a single 8k write. Strange.
>
> I'm not sure I believe these numbers at all... my experience is that
> getting trustworthy disk I/O numbers is *not* easy.
These numbers were reproducable on all the platforms I tested.
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 359-1001
+ If your life is a hard drive, | 13 Roberts Road
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073
