Re: [PATCH v7 23/32] qcow2: Add subcluster support to discard_in_l2_slice()

[Eric Blake]

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 <address@hidden>
> ---
>   block/qcow2-cluster.c | 52 +++++++++++++++++++------------------------
>   1 file changed, 23 insertions(+), 29 deletions(-)

Reviewed-by: Eric Blake <address@hidden>

--
Eric Blake, Principal Software Engineer
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