Re: [Qemu-devel] [PATCH] block-raw: Make cache=off default again

[Kevin Wolf <kwolf@redhat.com>]

Christoph Hellwig schrieb:
> On Mon, Jun 22, 2009 at 01:18:03PM +0200, Kevin Wolf wrote:
>> Since the patch that suggested it woudl revert qcow2 to writeback caching, in
>> fact none of the drivers has had cache=off as default any more. This patch
>> restores the desired behaviour.
> 
> Looks good to me.  I still quite hate that BDRV_O_CACHE_DEF thing, just
> makes it too complicated for drivers to get it right.

Actually it's just raw that must get it right. All the other drivers
access their image file through raw.

> Btw, what is the performance for qcow2 without write-back caching with
> all your improvements?

I haven't done real benchmarks for some time, but my simple test cases
like "run mke2fs on an empty image" have shown great improvement (at
least on IDE, for virtio I haven't understood yet what influences the
size of its requests - you should have received a private mail from Dor
about this). Possibly we could even enable cache=off for qcow2 now.

IIUC, you have a nice setup to measure block performance now. Maybe
you'd like to let it run with qcow2 and cache=wb/none?

Kevin