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Date: Thu, 16 Jun 2011 16:43:23 +0200
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] qemu-img: Add cache command line option
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To: Christoph Hellwig <hch@lst.de>
Cc: Federico Simoncelli <fsimonce@redhat.com>, qemu-devel@nongnu.org, kvm@vger.kernel.org, avi@redhat.com

Am 16.06.2011 16:28, schrieb Christoph Hellwig:
> 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.

Right, we need to keep the defaults as they are. That is, for convert
unsafe and for everything else writeback. The patch seems to make
writeback the default for everything.

Kevin