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Date: Mon, 24 Feb 2014 12:50:01 +0100
From: Peter Lieven <pl@kamp.de>
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Subject: Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with
	bdrv_write_zeroes
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To: Paolo Bonzini <pbonzini@redhat.com>, Kevin Wolf <kwolf@redhat.com>
Cc: qemu-devel@nongnu.org, stefanha@redhat.com, mreitz@redhat.com

On 24.02.2014 11:38, Paolo Bonzini wrote:
> Il 24/02/2014 11:26, Peter Lieven ha scritto:
>>
>> I personally do not need this for QCOW2 but for iSCSI. Here the optimization
>> is basically saved bandwidth since a zero write becomes a WRITESAME.
>
> It saves bandwidth, but at the potential cost of extra host CPU utilization.  I would be fine with having this automatically, but drv->bdrv_co_write_zeroes is not the right check because it is true for qcow2 and raw formats.  Something using 
> bdrv_get_info is probably better, because it would have fewer or no false positives.
>
>> In all cases if unmap=on it would additionally save disk space.
>
> It would also cause worse performance though.  I think the automatic addition BDRV_REQ_MAY_UNMAP is what should be a separate option. Perhaps you can have a three-state option, detect-zeros=no/yes/unmap.
I think this is the best option to start with and default it to no for the beginning.

Peter