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

[Vladimir Sementsov-Ogievskiy]

On 13.03.24 19:08, Markus Armbruster wrote:
> 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"?

Oh, right

> > +#     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>

Thanks!

--
Best regards,
Vladimir