Re: [Qemu-devel] Default cache mode

[Michael Tokarev]

29.06.2011 18:50, Christoph Hellwig wrote:
> 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.

The whole question (sorry I cut a bit more details than needed)
was about cache=writhrough :)

O_DIRECT on consumer drives with WCE=1 works fairy well too.

/mjt