[Engine-devel] SharedRawDisk feature detail

[Maor]

On 02/03/2012 04:59 PM, Itamar Heim wrote:
> On 02/02/2012 05:15 PM, Maor wrote:
>> Hello all,
>>
>> The shared raw disk feature description can be found under the following
>> links:
>>    http://www.ovirt.org/wiki/Features/DetailedSharedRawDisk
>>    http://www.ovirt.org/wiki/Features/SharedRawDisk
>>
>> Please feel free, to share your comments.
> 
> 1. Affected oVirt projects
> i'm pretty sure the history data warehouse will need to adapt to this.
> 
> 2. "The shared raw disk feature should provide the ability to attach
> disk to many VMs with safe concurrent access,"
> this could be read as if ovirt or vdsm somehow provides a mechanism for
> safe concurrent access.
> maybe something like "to multiple VMs that can handle concurrent access
> to a shared disk without risk of corruption".
> and having just written this - sounds like setting this flag at UI level
> should include a prompt to the user to make sure they understand that
> flagging the disk as shared *will* lead to corruption if it is attached
> to virtual machines which do not support and expect it to be shared with
> other virtual or physical machines[1]
I agree, I will change it.
> 
> 3. "The synchronization/clustering of shared raw disk between VMs will
> be managed in the file system. "
> 
> either i don't understand what this mean, or it could be read with a
> misleading meaning.
Maybe the following rephrase will be more accurate: "The
synchronization/clustering of shared raw disk between VMs should be
based on external independent application which will be synchronized
with the guest application."
> 
> 4. VM Pools
> VM Pools are always based (at least today) on templates, and templates
> have no shared disks.
> I'd just block attaching a shared disk to a VM which is part of a pool
> (unless there is a very interesting use case meriting this)
If there is no reason to attach shared disk to a VM from pool, maybe its
also not that relevant to attach shared disk to stateless VM.
Miki?

> 
> 5. "Quota has to be taken in consideration, for every new feature that
> will involve consumption of resources managed by it."
> 
> I thought quota is not relevant in this feature.
Why not?
Quota should be taken in consideration when adding new shared raw disk,
or moving a disk to other domains. We should also notice that shared raw
disk should not consume more quota when sharing the disk with different VMs.

> 
> 6. future work - Permissions should be added for disk entity
> so who can add a shared disk?
Data Center Administrator or System Administrator will be initialized
with permissions for creating shared raw disk, or changing shared disk
to be unshared.
Regarding attach/detach disks to/from VM, I was thinking that for phase
one we will count on the user VM permissions. If user will have
permissions to create new disks on the VM, he will also have permissions
to attach new shared raw disk to it.
> same as for floating disks, i find it hard to imagine a flow in which if
> someone flagged a disk as shared, suddenly everyone can have access to it.
> same as my statement of floating disks - I'll spend some more time to
> reflect on this specific part.
> 
> [1] an external LUN based disk could be shared with a physical server as
> well.