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

[Fam Zheng]

On Thu, 09/29 09:58, Paolo Bonzini wrote:
> 
> 
> 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).

After reading rfc2119, now I agree that "SHOULD" is better. :)

Fam