Re: [PATCH v2] virtio-blk: add DISCARD support to virtio-blk driver

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jul 04, 2017 at 11:24:01AM +0200, Paolo Bonzini wrote:
> 
> 
> On 05/07/2017 10:44, Changpeng Liu wrote:
> > Currently virtio-blk driver does not provide discard feature flag, so the
> > filesystems which built on top of the block device will not send discard
> > command. This is okay for HDD backend, but it will impact the performance
> > for SSD backend.
> > 
> > Add a feature flag VIRTIO_BLK_F_DISCARD and command VIRTIO_BLK_T_DISCARD
> > to extend exist virtio-blk protocol, define 16 bytes discard descriptor
> > for each discard segment, the discard segment defination aligns with
> > SCSI or NVM Express protocols, virtio-blk driver will support multi-range
> > discard request as well.
> > 
> > Signed-off-by: Changpeng Liu <changpeng.liu@intel.com>
> 
> Please include a patch for the specification.


Most importantly, please remember to copy virtio-dev@lists.oasis-open.org
on anything that changes the host/guest interface.

> Since we are at it, I
> would like to have three operations defined using the same descriptor:
> 
> - discard (SCSI UNMAP)
> 
> - write zeroes (SCSI WRITE SAME without UNMAP flag)
> 
> - write zeroes and possibly discard (SCSI WRITE SAME with UNMAP flag)
> 
> The last two can use the same command VIRTIO_BLK_T_WRITE_ZEROES, using
> the reserved field as a flags field.
> 
> Paolo

> > ---
> >  drivers/block/virtio_blk.c      | 76 +++++++++++++++++++++++++++++++++++++++--
> >  include/uapi/linux/virtio_blk.h | 19 +++++++++++
> >  2 files changed, 92 insertions(+), 3 deletions(-)