On Sat, Jan 30, 2016 at 01:56:30PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> Add qmp command to query dirty bitmap contents. This is needed for
> external backup.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>  block/dirty-bitmap.c         | 55 +++++++++++++++++++++++++++++++++++++++
>  blockdev.c                   | 62 ++++++++++++++++++++++++++++++++++++++++++++
>  include/block/dirty-bitmap.h |  7 +++++
>  qapi/block-core.json         | 54 ++++++++++++++++++++++++++++++++++++++
>  qmp-commands.hx              | 33 +++++++++++++++++++++++
>  5 files changed, 211 insertions(+)

This API produces large replies and/or requires many calls to fetch all
bitmap data.  The worst case is a 101010... bitmap.

I consider the dirty bitmap to be data (vs control) and not something
that should be sent over a control channel like the QMP monitor.

How about writing the dirty bitmap to a file?  The new bitmap file
format that Fam is working on could be used.  That way the dirty bitmap
can be saved asynchronously without hogging the QMP monitor.