Re: [Qemu-devel] [PATCH v5 03/11] block: add basic backup support to block driver

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

于 2013-6-7 15:18, Stefan Hajnoczi 写道:
> On Thu, Jun 06, 2013 at 04:56:49PM +0800, Fam Zheng wrote:
>> On Thu, 06/06 10:05, Stefan Hajnoczi wrote:
>>> On Thu, Jun 06, 2013 at 11:56:18AM +0800, Fam Zheng wrote:
>>>> On Thu, 05/30 14:34, Stefan Hajnoczi wrote:
>>>>> +
>>>>> +static int coroutine_fn backup_before_write_notify(
>>>>> +        NotifierWithReturn *notifier,
>>>>> +        void *opaque)
>>>>> +{
>>>>> +    BdrvTrackedRequest *req = opaque;
>>>>> +
>>>>> +    return backup_do_cow(req->bs, req->sector_num, req->nb_sectors, NULL);
>>>>> +}
>>>>
>>>> I'm wondering if we can see the logic here with a backing hd
>>>> relationship?  req->bs is a backing file of job->target, but guest is
>>>> going to write to it, so we need to COW down the data to job->target
>>>> before overwritting (i.e.  cluster is not allocated in child).
>>>>
>>>> I think if we do this in block layer, there's not much necessity for a
>>>> before-write notifier here (although it may be useful for other cases):
>>>>
>>>>      in bdrv_write:
>>>>      for child in req->bs->open_children
>>>>          if not child->is_allocated(req->sectors)
>>>>              do COW to child
>>>>
>>>> The advantage of this is that we won't need to start block-backup job in
>>>> sync mode "none" to do point-in-time snapshot (image fleecing), and we
>>>> get writable snapshot (possibility to open backing file writable and
>>>> write to it safely) as a by-product.
>>>>
>>>> But we will need to keep track of parent<->child of block states, and we
>>>> still need to take care of overlapping writing between block job and
>>>> guest request.
>>>
>>> There's one catch here: bs->target may not support backing files, it can
>>> be a raw file, for example.  We'll only use backing files for
>>> point-in-time snapshots but other use cases might not.  raw doesn't
>>> really implement is_allocated(), so the whole concept would have to
>>> change a little:
>>
>> Another use case may be parent modification. Suppose we have
>>
>>                      ,--- child1.qcow2
>>      parent.qcow2  <
>>                      `--- child2.qcow2
>>
>> We can use parent.qcow2 as block device in QEMU without breaking
>> child1.qcow2 or child2.qcow2 by telling QEMU who its children are:
>>
>>    $QEMU -drive file=parent.qcow2,children=child1.qcow2:child2.qcow2
>>
>> Then we open the three images and setup parent_bs->open_children, the
>> children are protected from being corrupted.
>>
>>>
>>> bs->open_children becomes independent of backing files - any
>>> BlockDriverState can be added to this list.  ->is_allocated() basically
>>> becomes the bitmap that we keep in the block job.
>>
>> Yes. But it is possible to keep a bitmap for raw (and those don't
>> implement is_allocated()) in block layer too, or in overlay: could
>> add-cow by Dongxu Wang help here?
>
> Yes absolutely.
>
> Stefan
>
   One advantage of external backup, or backing up chain, is that it
holds 'Delta' data only and is small enough. If it is changed toward a
'full' data writable snapshot, it become bigger. With backup chain
qemu-img can restore/clone a writable and usable one, So I don't
think adding that in qemu emulator helps much, and it will make things
more complicit.... user won't care who is doing the job, qemu or
qemu-img.

-- 
Best Regards

Wenchao Xia