[sheepdog-users] sheepdog about clone

Liu Yuan namei.unix at gmail.com
Fri May 16 05:20:07 CEST 2014

On Thu, May 15, 2014 at 04:08:08PM +0200, Valerio Pachera wrote:
> 2014-05-13 9:18 GMT+02:00 飞 <duron800 at qq.com>:
> > hello, I have a question for clone a sheepdong image.
> > my clone steps are:
> > 1. I create a image named myimg
> > 2. I write some files to  myimg
> > 3. I create a snapshot named 'newest', dog vdi snapshot -s newest myimg
> > 4. I ran 'dog vdi clone -s newest myimg myimg2'
> > 5. I ran 'dog vdi delete myimg'
> > 6. I attach the myimg2 to QEMU, but I can't found the files I wrote at
> > step 2.
> >
> I confirm such behaviour.
> (I'm running latest master branch 'Sheepdog daemon version
> 0.8.0_152_g65dde97').
> More precisely it happens if the snapshot is taken with the running guest.
> I created 250 files of 1M each inside the guest and run 'dog vdi snapshot'
> as soon as dd was done.
> I turned off the guest; cloned the snapshot and run the guest from the
> cloned vdi.
> The guest was showing all 250 file but many of them were empty!
> Also bash history was not saved.
> If I do the same thing but shutting down the guest before taking the
> snapshot, then all data is present on the cloned vdi (bash history
> included).
> Is it possible to force sheepdog to send a 'sync' request when taking the
> snapshot?

Linux kernel provide a mechanism for safe snapshot of file system, man fsfreeze(8).

So the right steps is:

1 guest kernel execute fsfreeze(8)
2 take a live snapshot, either by qemu-img or dog.
3 guest kernel thaw the file system by 'fsfreeze -u'

I think QEMU has some mechanisms to trigger the command execution inside the
guest from outside world.

It's welcome for someone can hook fsfreeze into 'dog vdi snapshot'.


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