Re: [Qemu-devel] [PATCH 2/6] qmp: add query-block-dirty-bitmap-ranges

[John Snow]

On 02/10/2016 08:57 AM, Denis V. Lunev wrote:
> On 02/10/2016 01:08 PM, Stefan Hajnoczi wrote:
>> 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 <address@hidden>
>>> ---
>>>   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.
> Reasonable point.
> 
> May be it would be better to setup "special" NBD server inside
> QEMU which will allow to directly "read" bitmap data.
> 
> Any opinion?
> 
> Den

Or perhaps something like migration, where the client receiving the data
opens a socket of some sort, and QEMU connects to that socket to send
the data.