Re: [PATCH] qapi: deprecate drive-backup

[Daniel P . Berrangé]

On Fri, Apr 23, 2021 at 03:59:00PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> 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.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
> 
> Hi all! I remember, I suggested to deprecate drive-backup some time ago,
> and nobody complain.. But that old patch was inside the series with
> other more questionable deprecations and it did not landed.
> 
> Let's finally deprecate what should be deprecated long ago.
> 
> We now faced a problem in our downstream, described in commit message.
> In downstream I've fixed it by simply enabling O_DIRECT and linux_aio
> unconditionally for drive_backup target. But actually this just shows
> that using drive-backup in blockdev era is a bad idea. So let's motivate
> everyone (including Virtuozzo of course) to move to new interfaces and
> avoid problems with all that outdated option inheritance.
> 
>  docs/system/deprecated.rst | 5 +++++
>  qapi/block-core.json       | 5 ++++-
>  2 files changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/docs/system/deprecated.rst b/docs/system/deprecated.rst
> index 80cae86252..b6f5766e17 100644
> --- a/docs/system/deprecated.rst
> +++ b/docs/system/deprecated.rst
> @@ -186,6 +186,11 @@ Use the more generic commands ``block-export-add`` and 
> ``block-export-del``
>  instead.  As part of this deprecation, where ``nbd-server-add`` used a
>  single ``bitmap``, the new ``block-export-add`` uses a list of ``bitmaps``.
>  
> +``drive-backup`` (since 6.0)

NB, it'll need to be 6.1 by time this patch has a chance to merge

> +''''''''''''''''''''''''''''
> +
> +Use ``blockdev-backup`` in pair with ``blockdev-add`` instead.
> +
>  System accelerators
>  -------------------
>  
> diff --git a/qapi/block-core.json b/qapi/block-core.json
> index 6d227924d0..8e2c6e1622 100644
> --- a/qapi/block-core.json
> +++ b/qapi/block-core.json
> @@ -1642,6 +1642,9 @@
>  # The operation can be stopped before it has completed using the
>  # block-job-cancel command.
>  #
> +# Features:
> +# @deprecated: This command is deprecated. Use @blockdev-backup instead.
> +#
>  # Returns: - nothing on success
>  #          - If @device is not a valid block device, GenericError
>  #
> @@ -1657,7 +1660,7 @@
>  #
>  ##
>  { 'command': 'drive-backup', 'boxed': true,
> -  'data': 'DriveBackup' }
> +  'data': 'DriveBackup', 'features': ['deprecated'] }
>  
>  ##
>  # @blockdev-backup:
> -- 
> 2.29.2
> 
> 

Regards,
Daniel
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