Re: [Qemu-devel] [PATCH] qemu-img: Add cache command line option

[Federico Simoncelli <fsimonce@redhat.com>]

----- Original Message -----
> From: "Christoph Hellwig" <hch@lst.de>
> To: "Federico Simoncelli" <fsimonce@redhat.com>
> Cc: kvm@vger.kernel.org, kwolf@redhat.com, qemu-devel@nongnu.org, avi@redhat.com
> Sent: Thursday, June 16, 2011 4:28:09 PM
> Subject: Re: [Qemu-devel] [PATCH] qemu-img: Add cache command line option
> On Wed, Jun 15, 2011 at 09:46:10AM -0400, Federico Simoncelli wrote:
> > qemu-img currently writes disk images using writeback and filling
> > up the cache buffers which are then flushed by the kernel preventing
> > other processes from accessing the storage.
> > This is particularly bad in cluster environments where time-based
> > algorithms might be in place and accessing the storage within
> > certain timeouts is critical.
> > This patch adds the option to choose a cache method when writing
> > disk images.
> 
> Allowing to chose the mode is of course fine, but what about also
> choosing a good default? writethrough doesn't really make any sense
> for qemu-img, given that we can trivially flush the cache at the end
> of the operations. I'd also say that using the buffer cache doesn't
> make sense either, as there is little point in caching these
> operations.

I totally agree with you, I didn't change the default to increase the
chances for my patch to be accepted.
If we want cache=none as default we can easily change:

-#define BDRV_DEFAULT_CACHE "writeback"
+#define BDRV_DEFAULT_CACHE "none"

-- 
Federico