Re: [Qemu-devel] [PATCH] qcow2: Support BDRV_REQ_MAY_UNMAP

[Paolo Bonzini]

On 29/09/2016 04:21, Fam Zheng wrote:
> On Wed, 09/28 18:11, Max Reitz wrote:
>> Note that BDRV_REQ_MAY_UNMAP does not mean "Yes, please discard" but
>> just "You may discard if it's easier for you". But it's actually not
>> easier for us, so I don't see why we're doing it.
>>
>> As far as I can guess you actually want some way to tell a block driver
>> to actually make an effort to discard clusters as long they then read
>> back as zero (which is why you cannot simply use bdrv_pdiscard()).
>> However, I think this would require a new flag called
>> BDRV_REQ_SHOULD_UNMAP (which should imply BDRV_REQ_MAY_UNMAP).
> 
> This flag doesn't make sense to me, if the protocol doesn't know how to unmap,
> it can ignore BDRV_REQ_MAY_UNMAP, but not BDRV_REQ_SHOULD_UNMAP. It just
> complicates things a little.

I don't think we actually have a use for a "MAY" unmap flag.  Either we
keep the not-so-perfect name or we replace MAY_UNMAP with "should" or
"want" or "would_like" unmap...  But Fam's patch does do what was
intended for the flag (which is the equivalent of the UNMAP bit in the
SCSI WRITE SAME command).

Paolo