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Re: [Qemu-devel] [PATCH v4 6/8] blockdev: add DriveBackup transaction

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v4 6/8] blockdev: add DriveBackup transaction
Date: Thu, 16 May 2013 13:21:19 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130402 Thunderbird/17.0.5

On 05/16/2013 02:36 AM, Stefan Hajnoczi wrote:
> This patch adds a transactional version of the drive-backup QMP command.
> It allows atomic snapshots of multiple drives along with automatic
> cleanup if there is a failure to start one of the backup jobs.
> Note that QMP events are emitted for block job completion/cancellation
> and the block job will be listed by query-block-jobs.
> @DriveBackup
> @device: the name of the device whose writes should be mirrored.
> @target: the target of the new image. If the file exists, or if it
>          is a device, the existing file/device will be used as the new
>          destination.  If it does not exist, a new file will be created.
> @format: #optional the format of the new destination, default is to
>          probe if @mode is 'existing', else the format of the source
> @mode: #optional whether and how QEMU should create a new image, default is
>        'absolute-paths'.
> @speed: #optional the maximum speed, in bytes per second
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
>  blockdev.c       | 53 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>  qapi-schema.json | 26 +++++++++++++++++++++++++-
>  2 files changed, 78 insertions(+), 1 deletion(-)

Reviewed-by: Eric Blake <address@hidden>

Hmm, I wonder if we can simplify patch 3/8, by hoisting the DriveBackup
definition into that patch, and writing:

{ 'command': 'drive-backup',
  'data': 'DriveBackup' }

instead of the current open-coding repetition of the arguments for the
standalone command in comparison to the transaction action.  So far, all
uses of { 'command':'str', 'data':{...} } in the qapi-schema.json use a
direct object instead of a named type, but if we could fix the qapi code
generation to honor dictionary types in place of an open-coded type, it
might make our interface file more compact.

> +
> +static void drive_backup_commit(BlkTransactionState *common)
> +{
> +    /* Block job has started, nothing to do here */
> +}

Given this implementation, should we modify the extensible transaction
series to allow for a NULL commit callback, and merely insist only that
at least one of commit/abort is non-NULL (rather than the current
insistence that commit is mandatory and abort is optional)?

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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