[libvirt] Offline manipulation of Dirty Bitmaps by qemu-img

[John Snow]

On 12/6/19 5:37 AM, Vladimir Sementsov-Ogievskiy wrote:
> 06.12.2019 1:37, John Snow wrote:
>> This has come up in the past, and I believe we discussed this at KVM
>> Forum, too:
>>
>> There have been requests from oVirt (via Nir Soffer) to add some offline
>> bitmap manipulation functionality. In the past, our stance has generally
>> been "Use QEMU without an accelerator, and use QMP to manipulate the
>> images."
>>
>> We like this for a few reasons:
>>
>> 1. It keeps bitmap management code tightly centralized
>> 2. It allows for the full suite of bitmap manipulations in either
>> offline or online mode with one tool
>> 3. We wouldn't have to write new code.
>> 4. Or design new CLIs and duplicate our existing work.
>> 5. Or write even more tests.
>>
>> However, tools like oVirt may or may not be fully equipped to launch
>> QEMU in this context, and there is always a desire for qemu-img to be
>> able to "do more", so existing management suites could extend
>> functionality more easily.
> 
> I think, all guys, who don't want to use Qemu directly for image manipulations,
> should hope for Kevin's "[RFC PATCH 00/18] Add qemu-storage-daemon", which is
> the correct solution of their problem. Still, I'm not one of these guys.
> 
>>
>> (Or so I am imagining.)
>>
>> I am still leaning heavily against adding any more CLI commands or
>> options to qemu-img right now. Even if we do add some of the fundamental
>> ones like "add" or "remove", it seems only a matter of time before we
>> have to add "clear", "merge", etc. Is this just a race to code duplication?
>>
>> On the other hand, one of the other suggestions is to have qemu-img
>> check --repair optionally delete corrupted bitmaps. I kind of like this
>> idea: it's a buyer beware operation that might make management layers
>> unhappy, but then again ... repair is always something that could make
>> things worse.
>>
>> Plus, if you manage to corrupt bitmaps badly enough that they can't even
>> be parsed, you might need a heavyweight repair operation.
>>
> 
> Improving "check" is a correct thing, because it's done inside qcow2 driver
> itself. We just don't have corresponding qmp command or command line option
> for Qemu to use this thing (or I missed it).
> 

OK, I agree. I made a redhat BZ to track that we want this: 1780416 -
RFE: qemu-img check --repair should optionally remove any
corrupted bitmaps

I'll work on a patch and we can debate the details there.

--js