Re: [Qemu-devel] [Qemu-block] [PATCH v3] block: Turn on "unmap" in active commit

[Fam Zheng <famz@redhat.com>]

On Thu, 09/29 13:41, Stefan Hajnoczi wrote:
> On Tue, Sep 27, 2016 at 07:14:52PM +0800, Fam Zheng wrote:
> > We already specified BDRV_O_UNMAP when opening images in 'qemu-img
> > commit', but didn't turn on the "unmap" in the active commit job. This
> > patch fixes that so that zeroed clusters in top image can be discarded
> > which is desired in the virt-sparsify use case, where a temporary
> > overlay is created and fstrim'ed before commiting back, to free space in
> > the original image.
> > 
> > This also enables it for block-commit.
> > 
> > Signed-off-by: Fam Zheng <famz@redhat.com>
> > ---
> > v3: Change the right parameter.
> > v2: Add "unmap" to block-commit as well. [Kevin]
> > ---
> >  block/mirror.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/block/mirror.c b/block/mirror.c
> > index f9d1fec..8847ec5 100644
> > --- a/block/mirror.c
> > +++ b/block/mirror.c
> > @@ -1042,7 +1042,7 @@ void commit_active_start(const char *job_id, BlockDriverState *bs,
> >  
> >      mirror_start_job(job_id, bs, base, NULL, speed, 0, 0,
> >                       MIRROR_LEAVE_BACKING_CHAIN,
> > -                     on_error, on_error, false, cb, opaque, &local_err,
> > +                     on_error, on_error, true, cb, opaque, &local_err,
> >                       &commit_active_job_driver, false, base, auto_complete);
> >      if (local_err) {
> >          error_propagate(errp, local_err);
> 
> Why is unmap an option at all?
> 
> What's wrong with using BDRV_REQ_MAY_UNMAP on all
> blk_aio_pwrite_zeroes() calls?

Because unmap is an QMP option of drive-backup. I think in the drive-mirror
context, it mitigates the limitation that we have no control over target's
BDRV_O_UNMAP (always inherited from source).

Fam