Re: [HACKERS] fsync method checking

Tom Lane wrote:
> Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)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.

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