Re: [HACKERS] win32 performance - fsync question

["Michael Paesold" <mpaesold(at)gmx(dot)at>]

Bruce Momjian wrote:

> Michael Paesold wrote:
> > Magnus Hagander wrote:
[snip]

> Michael, I am not sure why you come to the conclusion that open_sync
> requires turning off the disk write cache.  I saw nothing to indicate
> that in the thread:

I was just seeing his error message below...

> http://archives.postgresql.org/pgsql-hackers-win32/2005-02/msg00035.php
>
> I read the following:
>
> > > > * Win32, with fsync, write-cache disabled: no data corruption
> > > > * Win32, with fsync, write-cache enabled: no data corruption
> > > > * Win32, with osync, write cache disabled: no data corruption
> > > > * Win32, with osync, write cache enabled: no data corruption. Once I
> > > > got:
> > > > 2005-02-24 12:19:54 LOG:  could not open file "C:/Program
> > > > Files/PostgreSQL/8.0/data/pg_xlog/000000010000000000000010"
> > > (log file
> > > > 0, segment 16): No such file or directory
> > > >   but the data in the database was consistent.

A missing xlog file does not strike me as "very save". Perhaps someone can 
explain what happened, but I would not feel good about this. Again this note 
(from Tom Lane) in combination with the above error would tell me, we don't 
fully understand the risk here.

> > > It disturbs me that you couldn't produce data corruption in
> > > the cases where it theoretically should occur.  Seems like
> > > this is an indication that your test was insufficiently
> > > severe, or that there is something going on we don't understand.
> >
> > The Windows driver knows abotu the write cache, and at least fsync()
> > pushes through the write cache even if it's there. This seems to
> > indicate taht O_SYNC at least partiallyi does this as well. This is why
> > there is no performance difference at all on fsync() with write cache on
> > or off.
> >
> > I don't know if this is true for all IDE disks. COuld be that my disk is
> > particularly well-behaved.
>
> This indicated to me that open_sync did not require any additional
> changes than our current fsync.

We both based our understanding on the same evidence. It seems we just have 
a different level of paranoia. ;-)

Best Regards,
Michael Paesold