Re: [PATCH v7 20/32] qcow2: Add subcluster support to calculate_l2_meta()

[Eric Blake]

On 5/25/20 1:08 PM, Alberto Garcia wrote:
> If an image has subclusters then there are more copy-on-write
> scenarios that we need to consider. Let's say we have a write request
> from the middle of subcluster #3 until the end of the cluster:
>
> 1) If we are writing to a newly allocated cluster then we need
>     copy-on-write. The previous contents of subclusters #0 to #3 must
>     be copied to the new cluster. We can optimize this process by
>     skipping all leading unallocated or zero subclusters (the status of
>     those skipped subclusters will be reflected in the new L2 bitmap).
>
> 2) If we are overwriting an existing cluster:
>
>     2.1) If subcluster #3 is unallocated or has the all-zeroes bit set
>          then we need copy-on-write (on subcluster #3 only).
>
>     2.2) If subcluster #3 was already allocated then there is no need
>          for any copy-on-write. However we still need to update the L2
>          bitmap to reflect possible changes in the allocation status of
>          subclusters #4 to #31. Because of this, this function checks
>          if all the overwritten subclusters are already allocated and
>          in this case it returns without creating a new QCowL2Meta
>          structure.

Quite the mouthful!  But the description looks correct, and the code 
appears to match it.

> After all these changes l2meta_cow_start() and l2meta_cow_end()
> are not necessarily cluster-aligned anymore. We need to update the
> calculation of old_start and old_end in handle_dependencies() to
> guarantee that no two requests try to write on the same cluster.
>
> Signed-off-by: Alberto Garcia <address@hidden>
> ---
>   block/qcow2-cluster.c | 163 +++++++++++++++++++++++++++++++++---------
>   1 file changed, 131 insertions(+), 32 deletions(-)

Reviewed-by: Eric Blake <address@hidden>

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org