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

[Kevin Wolf <kwolf@redhat.com>]

Am 30.09.2016 um 04:12 hat Fam Zheng geschrieben:
> 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).

Wouldn't the more straightforward implementation then be if
qmp_drive_mirror() set BDRV_O_UNMAP for the target depending on the flag
rather than passing the flag down to the mirror job?

Hm... And should BDRV_O_UNMAP really be a BlockBackend option rather
than a BDS one? We already enable it unconditionally on non-root nodes
and it seems to make sense to me to allow discard e.g. from a block job,
but not from the guest.

Kevin