Re: [PATCH for-5.1 1/2] qcow2: Implement v2 zero writes with discard if possible

[Kevin Wolf <kwolf@redhat.com>]

Am 22.07.2020 um 19:01 hat Maxim Levitsky geschrieben:
> On Mon, 2020-07-20 at 15:18 +0200, Kevin Wolf wrote:
> > qcow2 version 2 images don't support the zero flag for clusters, so for
> > write_zeroes requests, we return -ENOTSUP and get explicit zero buffer
> > writes. If the image doesn't have a backing file, we can do better: Just
> > discard the respective clusters.
> > 
> > This is relevant for 'qemu-img convert -O qcow2 -n', where qemu-img has
> > to assume that the existing target image may contain any data, so it has
> > to write zeroes. Without this patch, this results in a fully allocated
> > target image, even if the source image was empty.
> > 
> > Reported-by: Nir Soffer <nsoffer@redhat.com>
> > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > ---
> >  block/qcow2-cluster.c | 9 ++++++++-
> >  1 file changed, 8 insertions(+), 1 deletion(-)
> > 
> > diff --git a/block/qcow2-cluster.c b/block/qcow2-cluster.c
> > index 4b5fc8c4a7..a677ba9f5c 100644
> > --- a/block/qcow2-cluster.c
> > +++ b/block/qcow2-cluster.c
> > @@ -1797,8 +1797,15 @@ int qcow2_cluster_zeroize(BlockDriverState *bs, uint64_t offset,
> >      assert(QEMU_IS_ALIGNED(end_offset, s->cluster_size) ||
> >             end_offset >= bs->total_sectors << BDRV_SECTOR_BITS);
> >  
> > -    /* The zero flag is only supported by version 3 and newer */
> > +    /*
> > +     * The zero flag is only supported by version 3 and newer. However, if we
> > +     * have no backing file, we can resort to discard in version 2.
> > +     */
> >      if (s->qcow_version < 3) {
> > +        if (!bs->backing) {
> > +            return qcow2_cluster_discard(bs, offset, bytes,
> > +                                         QCOW2_DISCARD_REQUEST, false);
> > +        }
> >          return -ENOTSUP;
> >      }
> >  
> 
> From my knowelege of nvme, I remember that discard doesn't have to zero the blocks.
> There is special namespace capability the indicates the contents of the discarded block.
> (Deallocate Logical Block Features)
> 
> If and only if the discard behavier flag indicates that discarded areas are zero,
> then the write-zero command can have special 'deallocate' flag that hints the controller
> to discard the sectors.
> 
> So woudn't discarding the clusters have theoretical risk of introducing garbage there?

No, qcow2_cluster_discard() has a defined behaviour. For v2 images, it
unallocates the cluster in the L2 table (this is only safe without a
backing file), for v3 images it converts them to zero clusters.

Kevin