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Re: [Qemu-devel] [PATCH for-2.12 0/4] qmp dirty bitmap API

From: Denis V. Lunev
Subject: Re: [Qemu-devel] [PATCH for-2.12 0/4] qmp dirty bitmap API
Date: Mon, 20 Nov 2017 19:00:31 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 11/17/2017 06:10 AM, John Snow wrote:
> On 11/16/2017 03:17 AM, Vladimir Sementsov-Ogievskiy wrote:
>> 16.11.2017 00:20, John Snow wrote:
>>> On 11/13/2017 11:20 AM, Vladimir Sementsov-Ogievskiy wrote:
>>>> Hi all.
>>>> There are three qmp commands, needed to implement external backup API.
>>>> Using these three commands, client may do all needed bitmap
>>>> management by
>>>> hand:
>>>> on backup start we need to do a transaction:
>>>>   {disable old bitmap, create new bitmap}
>>>> on backup success:
>>>>   drop old bitmap
>>>> on backup fail:
>>>>   enable old bitmap
>>>>   merge new bitmap to old bitmap
>>>>   drop new bitmap
>>> Can you give me an example of how you expect these commands to be used,
>>> and why they're required?
>>> I'm a little weary about how error-prone these commands might be and the
>>> potential for incorrect usage seems... high. Why do we require them?
>> It is needed for incremental backup. It looks like bad idea to export
>> abdicate/reclaim functionality, it is simpler
>> and clearer to allow user to merge/enable/disable bitmaps by hand.
>> usage is like this:
>> 1. we have dirty bitmap bitmap0 for incremental backup.
>> 2. prepare image fleecing (create temporary image with backing=our_disk)
>> 3. in qmp transaction:
>>     - disable bitmap0
>>     - create bitmap1
>>     - start image fleecing (backup sync=none our_disk -> temp_disk)
> This could probably just be its own command, though:
> block-job-fleece node=foobar bitmap=bitmap0 etc=etera etc=etera
> Could handle forking the bitmap. I'm not sure what the arguments would
> look like, but we could name the NBD export here, too. (Assuming the
> server is already started and we just need to create the share.)
> Then, we can basically do what mirror does:
> (1) Cancel
> (2) Complete
> Cancel would instruct QEMU to keep the bitmap changes (i.e. roll back),
> and Complete would instruct QEMU to discard the changes.
> This way we don't need to expose commands like split or merge that will
> almost always be dangerous to use over QMP.
> In fact, a fleecing job would be really convenient even without a
> bitmap, because it'd still be nice to have a convenience command for it.
> Using an existing infrastructure and understood paradigm is just a bonus.
actually this is a very good question about safety/simplicity...

We actually have spent a bit of time on design and have not
come to a good solution. Yes, technically for now we can put
all into fleecing command and rely on its semantics. Though
I am not convinced with that approach. We can think that this
can be reused on snapshot operations (may be, may be not).
Also technically there are other cases.

This is actually a matter that QEMU should provide low level
API while management software should make decisions.
Providing merge etc operations is done using exactly that
approach. We can also consider this in a similar way we have
recently fixed "revert to snapshot" operation. Management
can make and should make a decision.


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