[Engine-devel] the future of template cloning

[Ewoud Kohl van Wijngaarden]

On Thu, Jan 12, 2012 at 05:50:56PM +0200, Itamar Heim wrote:
> On 01/12/2012 04:04 PM, Ewoud Kohl van Wijngaarden wrote:
> >On Thu, Jan 12, 2012 at 03:56:05PM +0200, Itamar Heim wrote:
> >>On 01/12/2012 03:50 PM, Jon Choate wrote:
> >>>>On Thu, Jan 12, 2012 at 8:44:56PM, Itamar Heim wrote:
> >>>>On 01/12/2012 02:19 PM, Jon Choate wrote:
> >>>>>We are going to be able to store the disks for a template on
> >>>>>different storage domains due to the multiple storage domain
> >>>>>feature.  Cloning a template will still be possible, but will it
> >>>>>provide any value?  Thoughts?
> >>>>
> >>>>you would want to clone the disks to the storage domains you would
> >>>>want to create thinly provisioned disks from for this template.
> >>>>
> >>>>so a template could have disk1 and disk2 on SD1, then disk2 cloned to
> >>>>SD2 to allow creating a VM with thin COW for disk2 on SD2 as well.
> >>>>
> >>>
> >>>But with the ability to create a template with disk 1 on SD1 and
> >>>disk2 on SD2 is that still a compelling feature?
> >>
> >>yes, since I may want to create virtual machines from this template
> >>where the 2nd disk is sometimes on SD1 and sometime on SD2 (just
> >>like today, where i can instantiate a VM from the template on
> >>multiple storage domains)
> >>The new feature will just allow me to not clone all the disks of the
> >>template to all storage domains, rather a subset.
> >But would this be interesting to the user or would it be better to do
> >this transparently so the user doesn't have to care? The user just wants
> >to use template X and store it on storage domain Y with COW. The system
> >could deduce it has to be cloned first and not bother the user.
>
> that means a user would be able to affect where the template
> resides. user may not have permissions to do so, would require to
> configure which quota this will happen from, etc.
> so I'm not sure doing it implicitly based on a user trying to create
> a disk on a domain.
For my point of view I assume templates are immutable so a copy only
wastes space but I don't see how that matters in this case. If the user
has no permission to copy a template, [s]he has to make full copy of the
template to create the VM anyway. Making a copy and then create a thin
COW VM consumes the same amount of space with as benefit that additional
VMs based on the template can also use COW.

Side note: I haven't at oVirt and my experience is based on RHEV 2.2, so
maybe I've overlooking several new features that would limit this.