| Message ID | 1463476543-3087-6-git-send-email-den@openvz.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
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 <den@openvz.org> > CC: Kevin Wolf <kwolf@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.
diff --git a/block/qcow2.c b/block/qcow2.c index 05beb64..feaf146 100644 --- a/block/qcow2.c +++ b/block/qcow2.c @@ -2405,27 +2405,6 @@ finish: } -static bool is_zero_cluster(BlockDriverState *bs, int64_t start) -{ - BDRVQcow2State *s = bs->opaque; - int nr; - BlockDriverState *file; - int64_t res = bdrv_get_block_status_above(bs, NULL, start, - s->cluster_sectors, &nr, &file); - return res >= 0 && (res & BDRV_BLOCK_ZERO); -} - -static bool is_zero_cluster_top_locked(BlockDriverState *bs, int64_t start) -{ - BDRVQcow2State *s = bs->opaque; - int nr = s->cluster_sectors; - uint64_t off; - int ret; - - ret = qcow2_get_cluster_offset(bs, start << BDRV_SECTOR_BITS, &nr, &off); - return ret == QCOW2_CLUSTER_UNALLOCATED || ret == QCOW2_CLUSTER_ZERO; -} - static coroutine_fn int qcow2_co_write_zeroes(BlockDriverState *bs, int64_t sector_num, int nb_sectors, BdrvRequestFlags flags) { @@ -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; } qemu_co_mutex_lock(&s->lock); /* We can have new write after previous check */ - if (!is_zero_cluster_top_locked(bs, sector_num)) { + nr = s->cluster_sectors; + ret = qcow2_get_cluster_offset(bs, cl_start << BDRV_SECTOR_BITS, + &nr, &off); + if (ret != QCOW2_CLUSTER_UNALLOCATED && ret != QCOW2_CLUSTER_ZERO) { qemu_co_mutex_unlock(&s->lock); return -ENOTSUP; }
They are used once only. This makes code more compact. The patch also improves comments in the code. Signed-off-by: Denis V. Lunev <den@openvz.org> CC: Kevin Wolf <kwolf@redhat.com> --- block/qcow2.c | 39 +++++++++++++++------------------------ 1 file changed, 15 insertions(+), 24 deletions(-)