On 05/17/2016 03:15 AM, Denis V. Lunev wrote: > They are used once only. This makes code more compact. > > The patch also improves comments in the code. > > Signed-off-by: Denis V. Lunev <d...@openvz.org> > CC: Kevin Wolf <kw...@redhat.com> > --- > block/qcow2.c | 39 +++++++++++++++------------------------ > 1 file changed, 15 insertions(+), 24 deletions(-) >
> @@ -2439,20 +2418,32 @@ static coroutine_fn int
> qcow2_co_write_zeroes(BlockDriverState *bs,
> nb_sectors);
>
> if (head != 0 || tail != 0) {
> - int64_t cl_start = sector_num - head;
> + BlockDriverState *file;
> + uint64_t off;
> + int nr;
> +
> + int64_t cl_start = sector_num - head, res;
>
> assert(cl_start + s->cluster_sectors >= sector_num + nb_sectors);
>
> sector_num = cl_start;
> nb_sectors = s->cluster_sectors;
>
> - if (!is_zero_cluster(bs, sector_num)) {
> + /* check that the cluster is zeroed taking into account entire
> + backing chain */
> + nr = s->cluster_sectors;
> + res = bdrv_get_block_status_above(bs, NULL, cl_start,
> + s->cluster_sectors, &nr, &file);
> + if (res < 0 || !(res & BDRV_BLOCK_ZERO)) {
> return -ENOTSUP;
> }
This is somewhat pessimistic in certain cases. Suppose I have the
following cluster (borrowing from Kevin's nice notation in 154):
4k: -- -- XX --
and issue a 'write -z 2k 1k'
As written, the code will see that bdrv_get_block_status_above() does
NOT have all ZERO for the cluster, so it will return -ENOTSUP; we will
then call into the fallbacks and write explicit zeroes, so that the top
file now has an allocated cluster full of zero. But if we were a bit
smarter, we would only check the allocation status of the backing
sectors that do not overlap with our write_zeroes request, because the
remaining sectors are about to be overwritten; in this case, we can
specifically optimize to write a zero cluster without allocation.
You'll also need to rebase this series on top of my pending work to
rewrite bdrv_co_write_zeroes to instead be bdrv_co_pwrite_zeroes with a
byte interface (patches to be posted later today), if mine gets reviewed
first.
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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