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

[Sheldon]

On 04/29/2014 10:42 AM, Zhou Zheng Sheng wrote:
> Would anyone share some background information on why we implement LUN
> in template? I want to know the original author's considerations. I
> googled this topic in the mail list, didn't get much context information.
Aline, any comments?
>
> on 2014/04/28 21:08, Sheldon wrote:
>> On 04/28/2014 01:45 PM, Mark Wu wrote:
>>> 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.
>> ACK
>


-- 
Thanks and best regards!

Sheldon Feng(冯少合)<shaohef at linux.vnet.ibm.com>
IBM Linux Technology Center