On 5/25/20 1:08 PM, Alberto Garcia wrote:
This works now at the subcluster level and pwrite_zeroes_alignment is updated accordingly.qcow2_cluster_zeroize() is turned into qcow2_subcluster_zeroize() with the following changes: - The request can now be subcluster-aligned. - The cluster-aligned body of the request is still zeroized using zero_in_l2_slice() as before. - The subcluster-aligned head and tail of the request are zeroized with the new zero_l2_subclusters() function. There is just one thing to take into account for a possible future improvement: compressed clusters cannot be partially zeroized so zero_l2_subclusters() on the head or the tail can return -ENOTSUP. This makes the caller repeat the *complete* request and write actual zeroes to disk. This is sub-optimal because 1) if the head area was compressed we would still be able to use the fast path for the body and possibly the tail. 2) if the tail area was compressed we are writing zeroes to the head and the body areas, which are already zeroized.
Is this true? The block layer tries hard to break zero requests up so that any non-cluster-aligned requests do not cross cluster boundaries. In practice, that means that when you have an unaligned request, the head and tail cluster will be the same cluster, and there is no body in play, so that returning -ENOTSUP is correct because there really is no other work to do and repeating the entire request (which is less than a cluster in length) is the right approach.
Signed-off-by: Alberto Garcia <be...@igalia.com> --- block/qcow2.h | 4 +-- block/qcow2-cluster.c | 80 +++++++++++++++++++++++++++++++++++++++---- block/qcow2.c | 27 ++++++++------- 3 files changed, 90 insertions(+), 21 deletions(-)
Reviewed-by: Eric Blake <ebl...@redhat.com> -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org
