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From: "Denis V. Lunev" <den@virtuozzo.com>
Message-ID: <56BB58E7.7090307@virtuozzo.com>
Date: Wed, 10 Feb 2016 18:36:07 +0300
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Subject: Re: [Qemu-devel] [PATCH 2/6] qmp: add
	query-block-dirty-bitmap-ranges
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To: John Snow <jsnow@redhat.com>, Stefan Hajnoczi <stefanha@gmail.com>, Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Cc: kwolf@redhat.com, famz@redhat.com, qemu-devel@nongnu.org, armbru@redhat.com

On 02/10/2016 06:26 PM, John Snow wrote:
>
> 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 <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.
>> 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.
no. The point is that QEMU should be queried for data.
May be even via several sockets to provide it in
parallel.

Den