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.

Paolo