On Mon, Oct 7, 2013 at 11:42 AM, Paolo Bonzini <pbonz...@redhat.com> wrote: > Il 07/10/2013 10:42, Stefan Hajnoczi ha scritto: >> Could you make bdrv_co_write_zeroes() always use UNMAP, if possible, and >> avoid adding the new BDRV_REQ_MAY_UNMAP flag? While reading the first >> few patches in this series I wondered why there is a need to expose >> flags at all... >> >> Sometimes it is useful to distinguish between zeroing at the image >> format level from discarding at the device level, but I don't think we >> make use of that yet. I'd prefer to keep the interface simple for now >> and add flags later, if necessary. >> >> Or maybe I just missed something ;) > > The flag is needed to implement the right semantics for the SCSI WRITE > SAME command, which are: > > - if the UNMAP bit is off, always write the sectors (that's > bdrv_aio_write_zeroes without BDRV_REQ_MAY_UNMAP if the payload is zero, > otherwise it's emulated with bdrv_aio_writev) > > - if the target can "discard and write the specified payload", you can > discard, else you must write the sectors with the correct payload > (that's bdrv_aio_write_zeroes with BDRV_REQ_MAY_UNMAP). > > Contrast this with the UNMAP command, which does not make any guarantee > on the content of the sectors after the command is completed (a few > months ago we agreed that, even if you have discard_zeroes=true in the > target, it is fine for UNMAP to do nothing).
Okay, then let's keep the patches to expose the flag. Stefan
