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
