Re: [PATCH v4 4/5] qapi: blockdev-backup: add discard-source parameter

[Markus Armbruster <armbru@redhat.com>]

Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru> writes:

> Add a parameter that enables discard-after-copy. That is mostly useful
> in "push backup with fleecing" scheme, when source is snapshot-access
> format driver node, based on copy-before-write filter snapshot-access
> API:
>
> [guest]      [snapshot-access] ~~ blockdev-backup ~~> [backup target]
>    |            |
>    | root       | file
>    v            v
> [copy-before-write]
>    |             |
>    | file        | target
>    v             v
> [active disk]   [temp.img]
>
> In this case discard-after-copy does two things:
>
>  - discard data in temp.img to save disk space
>  - avoid further copy-before-write operation in discarded area
>
> Note that we have to declare WRITE permission on source in
> copy-before-write filter, for discard to work. Still we can't take it
> unconditionally, as it will break normal backup from RO source. So, we
> have to add a parameter and pass it thorough bdrv_open flags.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> Reviewed-by: Fiona Ebner <f.ebner@proxmox.com>
> Tested-by: Fiona Ebner <f.ebner@proxmox.com>

[...]

> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index 1874f880a8..2ef52ae9a7 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -1610,6 +1610,9 @@
>  #     node specified by @drive.  If this option is not given, a node
>  #     name is autogenerated.  (Since: 4.2)
>  #
> +# @discard-source: Discard blocks on source which are already copied

"have been copied"?

> +#     to the target.  (Since 9.1)
> +#
>  # @x-perf: Performance options.  (Since 6.0)
>  #
>  # Features:
> @@ -1631,6 +1634,7 @@
>              '*on-target-error': 'BlockdevOnError',
>              '*auto-finalize': 'bool', '*auto-dismiss': 'bool',
>              '*filter-node-name': 'str',
> +            '*discard-source': 'bool',
>              '*x-perf': { 'type': 'BackupPerf',
>                           'features': [ 'unstable' ] } } }

QAPI schema
Acked-by: Markus Armbruster <armbru@redhat.com>