Commit f29fbf7c authored by Kevin Wolf's avatar Kevin Wolf
Browse files

qcow2: Avoid COW during metadata preallocation 



Limiting the allocation to INT_MAX bytes isn't particularly clever
because it means that the final cluster will be a partial cluster which
will be completed through a COW operation. This results in unnecessary
data read and write requests which lead to an unwanted non-sparse
filesystem block for metadata preallocation.

Align the maximum allocation size down to the cluster size to avoid this
situation.

Cc: qemu-stable@nongnu.org
Signed-off-by: default avatarKevin Wolf <kwolf@redhat.com>
Reviewed-by: default avatarEric Blake <eblake@redhat.com>
parent de38b500

block/qcow2.c

+1 −1
Original line number Diff line number Diff line
@@ -2734,7 +2734,7 @@ static int coroutine_fn preallocate_co(BlockDriverState *bs, uint64_t offset, 
    bytes = new_length - offset;



    while (bytes) {

        cur_bytes = MIN(bytes, INT_MAX);

        cur_bytes = MIN(bytes, QEMU_ALIGN_DOWN(INT_MAX, s->cluster_size));

        ret = qcow2_alloc_cluster_offset(bs, offset, &cur_bytes,

                                         &host_offset, &meta);

        if (ret < 0) {