Re: [PATCH v4 3/3] qapi: deprecate drive-backup

[Eric Blake <eblake@redhat.com>]

On Thu, Nov 04, 2021 at 09:58:11AM +0100, Markus Armbruster wrote:
> From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> 
> Modern way is using blockdev-add + blockdev-backup, which provides a
> lot more control on how target is opened.
> 
> As example of drive-backup problems consider the following:
> 
> User of drive-backup expects that target will be opened in the same
> cache and aio mode as source. Corresponding logic is in
> drive_backup_prepare(), where we take bs->open_flags of source.
> 
> It works rather bad if source was added by blockdev-add. Assume source
> is qcow2 image. On blockdev-add we should specify aio and cache options
> for file child of qcow2 node. What happens next:
> 
> drive_backup_prepare() looks at bs->open_flags of qcow2 source node.
> But there no BDRV_O_NOCAHE neither BDRV_O_NATIVE_AIO: BDRV_O_NOCAHE is
> places in bs->file->bs->open_flags, and BDRV_O_NATIVE_AIO is nowhere,
> as file-posix parse options and simply set s->use_linux_aio.
> 
> The documentation is updated in a minimal way, so that drive-backup is
> noted only as a deprecated command, and blockdev-backup used in most of
> places.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> Reviewed-by: Kashyap Chamarthy <kchamart@redhat.com>
> Signed-off-by: Markus Armbruster <armbru@redhat.com>
> ---

Reviewed-by: Eric Blake <eblake@redhat.com>

I think this is appropriate for inclusion in 6.2, even if it did not
quite make soft freeze, so that we aren't delaying the deprecation for
an entire cycle.

>  docs/about/deprecated.rst              | 11 ++++++
>  docs/interop/live-block-operations.rst | 47 +++++++++++++++++---------
>  qapi/block-core.json                   |  5 ++-
>  qapi/transaction.json                  |  6 +++-
>  4 files changed, 51 insertions(+), 18 deletions(-)
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
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