Re: [Qemu-devel] [RFC PATCH] block: optimize zero writes with bdrv_write_zeroes

[Peter Lieven]

Am 25.02.2014 14:41, schrieb Kevin Wolf:
> Am 24.02.2014 um 14:01 hat Peter Lieven geschrieben:
>> 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.
>> What would be the desired way to store this flag in the BlockDriverState?
> Some new enum field?
Is there an already implemented example where I can copy from? Its quite 
diffucult to search through all the involved
functions when doing it for the first time.

Peter