Re: fsync or fdatasync

Ragnar Kj�rstad wrote:
> On Tue, Sep 10, 2002 at 05:07:30PM -0400, Bruce Momjian wrote:
> > > Anyway - I'm sure this is not enough to convince you, so I'll have to
> > > set up a test instead. But not tonight.
> >
> > Again, that is a test case for only one OS.  It is helpful if we are
> > going to start doing per-OS defaults, which is something we have talked
> > about.
>
> Oh; I assumed that was already the case.
>
> > What would be great is a test program we can run on different
> > OS's to find out which is more efficient.
>
> Yes. Bare in mind though, that this is as much a filesystem issue as a
> kernel issue. Two different filesystems on the same kernel may behave
> very differently.
>
> Of course one can't distribute seperate postgresql in different versions
> optimized for differet filesystems, so perhaps it's just as good to
> leave the default as it is and rather put some info (e.g. benchmarks for
> different setting on different filesystems on different operating
> systems, and the benchmark-script itself so people can do their own
> tests). This way the default is allright, and users that need to tweek a
> little extra have the info they need.
>

What we could do ideally is set the default by running some test during
initdb perhaps.

> > Remember, in most cases, we are fsync'ing only one block so there is no
> > _gathering_ to do.
>
> Yes, I know you said so. But if that's the case for only most cases
> there are some cases were it's not - so there is still some potential.

Yes, but the cases are so rare it is probably not worth bothering about
especially since O_SYNC has to be set on file open so you can't switch
between that and fdatasync depending on how many blocks you have.

--
  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