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

[Ian Main]

On Fri, Nov 22, 2013 at 01:47:26PM +0800, Fam Zheng wrote:
> 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.

That's great.  Interesting solution you have come up with.  I will give
it a try and let you know how it goes.
 
> >>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.

I like your notation.. I might try to make a patch for qmp/qmp-shell to
allow it to create nested dicts using the dot notation.

        Ian