Re: [Qemu-devel] [RFC PATCH v3 0/2] Point-in-time snapshot exporting over NBD

[Fam Zheng <famz@redhat.com>]

On 2013年11月20日 10:32, Ian Main wrote:
> On Thu, Oct 17, 2013 at 01:36:41PM +0800, Fam Zheng wrote:
>> This series adds for point-in-time snapshot NBD exporting based on
>> blockdev-backup (variant of drive-backup with existing device as target).
>
> In general this seems to work great.  I did a bunch of testing and was
> able to mount filesystems over NBD, do many writes (on backed up
> image)/reads (on target), intense IO etc and all held up fine.
>
> There are definitely some issues with some of the commands allowing you
> to blow things up.  I'm interested to hear opinions on whether this is a
> showstopper at this time or not.
>

Thanks very much for your testing, Ian. QEMU should report error instead 
of crashing with invalid operations. I added an "operation blocker" and 
incorporated into the next version.

In the future, we still want to review more on those commands and try to 
enable safe ones, and possibly also allow multiple block jobs on a BDS. 
For now it is good enough to only allow blockdev-backup on the source 
and NBD export on the target (which is safe, and all what we need for 
image fleecing, as this version shows). With that being a working 
implementation, I've posted v4. Please test and free to make comments.

>> We get a thin point-in-time snapshot by COW mechanism of drive-backup, and
>> export it through built in NBD server. The steps are as below:
>>
>>   1. (SHELL) qemu-img create -f qcow2 BACKUP.qcow2 <source size here>
>>
>>      (Alternatively we can use -o backing_file=RUNNING-VM.img to omit explicitly
>>      providing the size by ourselves, but it's risky because RUNNING-VM.qcow2 is
>>      used r/w by guest. Whether or not setting backing file in the image file
>>      doesn't matter, as we are going to override the backing hd in the next
>>      step)
>>
>>   2. (QMP) blockdev-add backing=source-drive file.driver=file file.filename=BACKUP.qcow2 id=target0 if=none driver=qcow2
>
> I had to create a custom python script to make these commands work as
> they require nested dicts.  Is that normal or did I miss something
> entirely?
>

Yes, I use a python script locally to test it too.

Fam