Re: [PATCH v3 4/5] qapi: blockdev-backup: add discard-source parameter

[Fiona Ebner <f.ebner@proxmox.com>]

Am 28.02.24 um 15:15 schrieb Vladimir Sementsov-Ogievskiy:
> 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>