Re: [Qemu-devel] [PATCH 4/4] qemu-img: conditionally discard target on convert

[Paolo Bonzini <pbonzini@redhat.com>]

Il 18/07/2013 13:04, Peter Lieven ha scritto:
>> But if you set BDRV_DISCARD_WRITE_ZEROES, then you always need a
>> fallback to bdrv_write_zeroes.  Why not just call bdrv_write_zeroes to
>> begin with?  That's why extending bdrv_write_zeroes is preferable.
> In this case wo do not need a flag to the function at all. If the
> driver sets bdi->write_zeroes_w_discard = 1 then bdrv_write_zeroes
> can use bdrv_discard to write zeroes and the driver has to
> ensure that all is zero afterwards.

Peter, you removed exactly the part of the email where I explained the
wrong part of your reasoning:

   you cannot do that [discard in bdrv_write_zeroes] unconditionally.
   Some operations can use it, some cannot.  Think of SCSI disk
   emulation: it must not discard is WRITE SAME is sent without the
   UNMAP bit!

> If the driver would have a better method of writing zeroes than
> discard it simply should not set bdi->write_zeroes_w_discard = 1.

If the driver had a better method of writing zeroes than discard, it
simply should ignore the BDRV_MAY_UNMAP (or BDRV_MAY_DISCARD) flag in
its bdrv_write_zeros implementation.

With Kevin's proposal, there is no reason to add a flag to bdi.  (Sorry
it took a few iterations to get it right).

Paolo