[Kimchi-devel] Rethink the Relationship of Template and VM

[Mark Wu]

On 04/27/2014 10:38 PM, Zhou Zheng Sheng wrote:
> Hi all,
>
> I'm working on host device passthrough. A a problem troubles me: I see
> in current Kimchi code, when adding a specific SCSI or iSCSI LUN, it is
> added to the template but not the VM. This is counter-intuitive, because
> a template is to store the common information for lots of VMs, while a
> specific LUN should only be used by just one VM. It's OK for DIR, NFS
> and Logical storage pool being managed by the template, because actually
> the template allocates a new specific volume for a new VM, so the
> specific volume goes with the specific VM but not the template. Since we
> don't define a specific DIR storage volume in the template, why we
> define a concrete LUN in it?
>
> The first problem in the current design is that we can not attach a new
> LUN to a VM after the VM is created.
>
> Another problem is that we mix the "things in common" and "things
> special" in template, thus we make its responsibility complex. Static
> and dynamic validations are also mixed in template.
>
> The third problem is that if we get 10 LUNs for 10 VMs, the user has to
> create 10 templates at first, then create 10 VMs from the 10 templates
> respectively. It's a bit tedious.
>
> The fourth problem is that if a LUN is used by a VM, usually it should
> not be used by other VMs, so it implies that by specifying a LUN in a
> template, the template should only create just one VM. What's the point
> to make a template that only produces one VM?
>
> It seems a cleaner way is putting SCSI LUN handling code in VM. Firstly,
> we can assign a SCSI/iSCSI storage pool to the template. When it's time
> to create a VM from the template, Kimchi lists all the available LUNs in
> the storage pool and the user can just pick some LUNs. The VM override
> parameters are suitable to store the LUNs information. It also makes
> possible to implement some mechanisms to add/remove LUNs after a VM is
> created, which is a very typical user requirement.
>
> The same problem exists in host device passthrough. For a specific
> device, it should be passthrough to a specific VM but not a template.
> I'd like to collect some thoughts before my device passthrough patch V2.
> In which place you'd suggest me to put device passthrough information,
> template or VM?
Template should NOT include any vm specific information. As I mentioned
in the comments of your pci passthrough patch, we need allow user have
chance to customize the configuration when creating vm based on
template.
>
> Thanks for sharing your ideas!
>