Re: [Qemu-devel] [PATCH] raw-posix: Linearize direct I/O on Linux NFS

[Anthony Liguori]

On 04/15/2011 11:23 AM, Badari Pulavarty wrote:
> On Fri, 2011-04-15 at 17:34 +0200, Christoph Hellwig wrote:
> > On Fri, Apr 15, 2011 at 04:26:41PM +0100, Stefan Hajnoczi wrote:
> > > On Fri, Apr 15, 2011 at 4:05 PM, Christoph Hellwig<address@hidden>  wrote:
> > > > NAK. ?Just wait for the bloody NFS client fix to get in instead of
> > > > adding crap like that.
> > > That's totally fine if NFS client will be fixed in the near future but
> > > this doesn't seem likely:
> > >
> > > http://www.spinics.net/lists/linux-nfs/msg20462.html
> > The code to use preadv/pwritev has been in qemu for over 2 years,
> > and it took people to notice the NFS slowdown until now, so don't
> > expect it to be fixed three days layer.
> True. That brings up a different question - whether we are doing
> enough testing on mainline QEMU :(

The issue here is NFS, not QEMU.  Moreover, the real problem is that 
we're using O_DIRECT.  O_DIRECT seems to result in nothing but problems 
and it never seems to be tested well on any file system.

I think the fundamental problem we keep running into really boils down 
to O_DIRECT being a second class interface within Linux.

Regards,

Anthony Liguori