Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command

[Wenchao Xia <xiawenc@linux.vnet.ibm.com>]

于 2013-5-17 17:14, Stefan Hajnoczi 写道:
> On Fri, May 17, 2013 at 02:58:57PM +0800, Wenchao Xia wrote:
>> 于 2013-5-16 15:47, Stefan Hajnoczi 写道:
>>> On Thu, May 16, 2013 at 02:16:20PM +0800, Wenchao Xia wrote:
>>>>    After checking the code, I found it possible to add delta data backup
>>>> support also, If an additional dirty bitmap was added.
>>>
>>> I've been thinking about this.  Incremental backups need to know which
>>> blocks have changed, but keeping a persistent dirty bitmap is expensive
>>> and unnecessary.
>>>
>>    Yes, it would be likely another block layer, so hope not do that.
>
> Not at all, persistent dirty bitmaps need to be part of the block layer
> since they need to support any image type - qcow2, Gluster, raw LVM,
> etc.
>
>>> I don't consider block jobs to be "qemu device" layer.  It sounds like
>>> you think the code should be in bdrv_co_do_writev()?
>>>
>>    I feel a trend of becoming fragility from different solutions,
>> and COW is a key feature that block layer provide, so I wonder if it
>> can be adjusted under block layer later
>
> The generic block layer includes more than just block.c.  It also
> includes block jobs and the qcow2 metadata cache that Dong Xu has
> extracted recently, for example.  Therefore you need to be more specific
> about "what" and "why".
>
> This copy-on-write backup approach is available as a block job which
> runs on top of any BlockDriverState.  What concrete change are you
> proposing?
>
   Since hard to hide it BlockDriverState now, suggest add some
document in qemu about the three snapshot types: qcow2 internal,
backing chain, drive-backup, which are all qemu software based snapshot
implemention, then user can know the difference with it eaiser.

   In long term, I hope to form a library expose those in a unified
format, perhaps it calls qmp_transaction internally, and make it
easier to be offloaded if possible, so hope a abstract-driver structure.
-- 
Best Regards

Wenchao Xia