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

From: John Snow
Subject: Re: [Qemu-devel] [PATCH for-2.12 0/4] qmp dirty bitmap API
Date: Thu, 16 Nov 2017 22:10:48 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

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.

> 4. export temp_disk (it looks like a snapshot) and bitmap0 through NBD
> === external tool can get bitmap0 and then copy some clusters through
> NBD ===
> on successful backup external tool can drop bitmap0. or can save it and
> reuse somehow
> on failed backup external tool can do the following:
>     - enable bitmap0
>     - merge bitmap1 to bitmap0
>     - drop bitmap1

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