Re: [Qemu-devel] Default cache mode

[Christoph Hellwig]

On Wed, Jun 29, 2011 at 06:17:02PM +0400, Michael Tokarev wrote:
> > Honestly, I don't know. Usually the problem is resolved with setting a
> > different cache option, so nobody bothers to ask for details. I'd guess
> > that it's ext4 in most cases.
> 
> Extremly poor performance also happens on raw devices --
> be it lvm volumes or plain partitions, so that's w/o
> any filesystem.

Depends on your setup.  If you have ATA devices that have WCE=1
it will suck due to the non-queueable FLUSH command.  If you have
an older kernel with the draining barriers semantics it will to.

If you run on a SAS disk or FC array with WCE=0 performance will
be quite good and close to native performance on these.

That's all assuming you use O_DIRECT.  using cache=writhrough
as-is will suck everywher.