RE: [virtio-dev] Re: [RFC] virtio-blk: add discard and write zeroes features to specification

["Liu, Changpeng" <changpeng.liu@intel.com>]

> -----Original Message-----
> From: Paolo Bonzini [mailto:pbonzini@redhat.com]
> Sent: Tuesday, February 27, 2018 4:42 PM
> To: Liu, Changpeng <changpeng.liu@intel.com>; virtio-dev@lists.oasis-open.org;
> virtio@lists.oasis-open.org
> Cc: cavery@redhat.com; stefanha@redhat.com; Wodkowski, PawelX
> <pawelx.wodkowski@intel.com>; Harris, James R <james.r.harris@intel.com>
> Subject: Re: [virtio-dev] Re: [RFC] virtio-blk: add discard and write zeroes features
> to specification
> 
> On 27/02/2018 06:45, Liu, Changpeng wrote:
> >> Perhaps we can put a bit here saying whether write_zeroes will ever look
> >> at the \field{unmap} bit?  Like:
> >>
> >> 	u8   write_zeroes_may_unmap;
> >
> > I do take this into consideration, if write zeroes is supported and discard is not
> supported,
> > even the OS send write zeroes command with unmap bit enabled, the backend
> will also
> > ignore this bit, so the configuration parameter seems useless.
> 
> The reason is that it lets the guest OS know if WRITE ZEROES with UNMAP
> will be fast or not.  It's not super important, but it is easy enough to
> add and also easy enough to fill in for the host.
Ok, I will add it.
> 
> Paolo