[Sheepdog] Deleting snapshots

[MORITA Kazutaka]

At Sun, 3 Oct 2010 23:09:34 +0200,
Floris Bos wrote:
> 
> Hi,
> 
> On Sunday, October 03, 2010 05:55:02 pm you wrote:
> > Yes.  Sheepdog makes the current vdi read-only (snapshot vdi), and
> > creates a new vdi as a writable one (main vdi).
> 
> > > Is there any option I can use to tell sheepdog not to promote the new
> > > snapshot as main VDI?
> > 
> > Currently, no.  However, I think it would be better to reserve 0 as a
> > snap_id of the latest vdi (main vdi).  In this case, taking snapshots
> > doesn't change a snapshot id of the latest vdi.  The id is fixed to
> > zero always.  Is this the behavior you want?
> 
> That might work.
> 
> 
> Basically what I need is a new read-only snapshot for use by my client, and no 
> changes to the current VDI.
> After all, the current vdi may be in use by qemu, and qemu is totally unaware 
> of the snapshot I'm taking with my external program.
> 
> So the original VDI ID must stay writable, as there is no way to signal qemu 
> that it should start using another id.
> 

On second thought, we cannot avoid updating a vdi id when its snapshot
is created.  It is because a sheepdog client does copy-on-write based
on its vdi id.

So we need to use a savevm command from the qemu monitor to take a
snapshot of the running VM.  Currently, if you want to create a
snapshot from the external program, you need to get a lock of the vdi
to avoid corrupting running VMs, and if running VMs exist, you need to
give up taking a snapshot...

In future, I think we should implement a mechanism to notify the
running client that an external program creates a snapshot.

For example, if write accesses to snapshot objects return something
like SD_RES_READONLY_OBJ, we can tell the client that it should update
the vdi id.


Thanks,

Kazutaka