Re: [PATCH v3 1/2] quorum: Implement bdrv_co_block_status()

[Max Reitz <mreitz@redhat.com>]

On 11.11.20 17:53, Alberto Garcia wrote:
> The quorum driver does not implement bdrv_co_block_status() and
> because of that it always reports to contain data even if all its
> children are known to be empty.
> 
> One consequence of this is that if we for example create a quorum with
> a size of 10GB and we mirror it to a new image the operation will
> write 10GB of actual zeroes to the destination image wasting a lot of
> time and disk space.
> 
> Since a quorum has an arbitrary number of children of potentially
> different formats there is no way to report all possible allocation
> status flags in a way that makes sense, so this implementation only
> reports when a given region is known to contain zeroes
> (BDRV_BLOCK_ZERO) or not (BDRV_BLOCK_DATA).
> 
> If all children agree that a region contains zeroes then we can return
> BDRV_BLOCK_ZERO using the smallest size reported by the children
> (because all agree that a region of at least that size contains
> zeroes).
> 
> If at least one child disagrees we have to return BDRV_BLOCK_DATA.
> In this case we use the largest of the sizes reported by the children
> that didn't return BDRV_BLOCK_ZERO (because we know that there won't
> be an agreement for at least that size).
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> Tested-by: Tao Xu <tao3.xu@intel.com>
> ---
>   block/quorum.c             |  52 +++++++++++++
>   tests/qemu-iotests/312     | 148 +++++++++++++++++++++++++++++++++++++
>   tests/qemu-iotests/312.out |  67 +++++++++++++++++
>   tests/qemu-iotests/group   |   1 +
>   4 files changed, 268 insertions(+)
>   create mode 100755 tests/qemu-iotests/312
>   create mode 100644 tests/qemu-iotests/312.out

Reviewed-by: Max Reitz <mreitz@redhat.com>