On 5/25/20 1:08 PM, Alberto Garcia wrote:
Two things need to be taken into account here:1) With full_discard == true the L2 entry must be cleared completely. This also includes the L2 bitmap if the image has extended L2 entries. 2) With full_discard == false we have to make the discarded cluster read back as zeroes. With normal L2 entries this is done with the QCOW_OFLAG_ZERO bit, whereas with extended L2 entries this is done with the individual 'all zeroes' bits for each subcluster. Note however that QCOW_OFLAG_ZERO is not supported in v2 qcow2 images so, if there is a backing file, discard cannot guarantee that the image will read back as zeroes. If this is important for the caller it should forbid it as qcow2_co_pdiscard() does (see 80f5c01183 for more details). Signed-off-by: Alberto Garcia <[email protected]> --- block/qcow2-cluster.c | 52 +++++++++++++++++++------------------------ 1 file changed, 23 insertions(+), 29 deletions(-)
Reviewed-by: Eric Blake <[email protected]> -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org
