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

[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?

Kevin