Re: [PATCH v5 4/9] qcow2: Support BDRV_REQ_ZERO_WRITE for truncate

[Kevin Wolf <kwolf@redhat.com>]

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Am 23.04.2020 um 12:53 hat Max Reitz geschrieben:
> On 22.04.20 17:21, Kevin Wolf wrote:
> > If BDRV_REQ_ZERO_WRITE is set and we're extending the image, calling
> > qcow2_cluster_zeroize() with flags=0 does the right thing: It doesn't
> > undo any previous preallocation, but just adds the zero flag to all
> > relevant L2 entries. If an external data file is in use, a write_zeroes
> > request to the data file is made instead.
> > 
> > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > ---
> >  block/qcow2.c | 30 ++++++++++++++++++++++++++++++
> >  1 file changed, 30 insertions(+)
> > 
> > diff --git a/block/qcow2.c b/block/qcow2.c
> > index 9cfbdfc939..bd632405d1 100644
> > --- a/block/qcow2.c
> > +++ b/block/qcow2.c
> 
> [...]
> 
> > @@ -4214,6 +4215,35 @@ static int coroutine_fn qcow2_co_truncate(BlockDriverState *bs, int64_t offset,
> >          g_assert_not_reached();
> >      }
> >  
> > +    if ((flags & BDRV_REQ_ZERO_WRITE) && offset > old_length) {
> > +        uint64_t zero_start = QEMU_ALIGN_UP(old_length, s->cluster_size);
> > +        uint64_t zero_end = QEMU_ALIGN_UP(offset, s->cluster_size);
> > +
> > +        /* Use zero clusters as much as we can */
> > +        ret = qcow2_cluster_zeroize(bs, zero_start, zero_end - zero_start, 0);
> 
> It’s kind of a pity that this changes the cluster mappings that were
> established when using falloc/full preallocation already (i.e., they
> become preallocated zero clusters then, so when writing to them, we need
> COW again).
> 
> But falloc/full preallocation do not guarantee that the new data is
> zero, so I suppose this is the only thing we can reasonably do.

If we really want, I guess we could make full preallocation first try
passing BDRV_REQ_ZERO_WRITE to the protocol layer and if that succeeds,
we could skip setting the zero cluster flag at the qcow2 level.

Feels like a separate patch, though.

Kevin

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