Re: [PATCH v4 17/18] qapi: backup: add immutable-source parameter

[Hanna Reitz]

On 16.02.22 20:46, Vladimir Sementsov-Ogievskiy wrote:
> We are on the way to implement internal-backup with fleecing scheme,
> which includes backup job copying from fleecing block driver node
> (which is target of copy-before-write filter) to final target of
> backup. This job doesn't need own filter, as fleecing block driver node
> is a kind of snapshot, it's immutable from reader point of view.
>
> Let's add a parameter for backup to not insert filter but instead
> unshare writes on source. This way backup job becomes a simple copying
> process.
>
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   qapi/block-core.json      | 11 ++++++-
>   include/block/block_int.h |  1 +
>   block/backup.c            | 61 +++++++++++++++++++++++++++++++++++----
>   block/replication.c       |  2 +-
>   blockdev.c                |  1 +
>   5 files changed, 69 insertions(+), 7 deletions(-)

I’m not really technically opposed to this, but I wonder what the actual 
benefit of this is.  It sounds like the only benefit is that we don’t 
need a filter driver, but what’s the problem with such a filter driver?

(And if we just want to copy data off of a immutable node, I personally 
would go for the mirror job instead, but it isn’t like I could give good 
technical reasons for that personal bias.)