Re: [Xen-users] iscsi vs nfs for xen VMs

[Bart Coninckx <bart.coninckx@xxxxxxxxxx>]

On 01/30/11 00:50, Pasi Kärkkäinen wrote:
> On Sun, Jan 30, 2011 at 01:44:06AM +0200, Pasi Kärkkäinen wrote:
>   
>> On Sun, Jan 30, 2011 at 10:41:23AM +1100, James Harper wrote:
>>     
>>>> On Wed, Jan 26, 2011 at 11:38:52AM +0100, Christian Zoffoli wrote:
>>>>         
>>>>> Hi,
>>>>>
>>>>> if you want to push on performance the best one is:
>>>>>
>>>>> CLVM over iSCSI
>>>>>
>>>>>           
>>>> And if you need snapshots, just over normal LVM over shared iSCSI LUN.
>>>> (in this case your management toolstack needs to take care of locking LVM
>>>> control).
>>>>
>>>>         
>>> How sure are you of this? Every write to the original or the snapshot 
>>> involves a potential metadata update. If the original is mounted on one 
>>> node and the snapshot on another, I'm pretty sure that it would all go to 
>>> crap really really quickly.
>>>
>>> Have you actually tried this???
>>>
>>>       
>> It's widely used.
>>
>> Citrix XenServer (and XCP) do it like that, and also the 
>> Xen based VirtualIron did it like that.
>>
>>     
> Forgot to add this:
>
> In Citrix XenServer it's the XAPI toolstack that's 
> taking care of LVM locking, so only the "pool master" 
> is executing LVM commands.
>
> As long as you know you're executing LVM commands
> only from a single node, you're good. No need for CLVM.
>
> I guess you also need to refresh all the other nodes
> after executing LVM commands on the primary/master node.
>
> -- Pasi
>
>
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>   

I "fixed" this in a way I would like to have your idea on: what I did
was build the iSCSI LUNs on an IETD implementation. Each LUN points to a
LVM LV on this iSCSI server. To avoid worrying about which Hypervisor
will change the LUN, I save the DomU using the particular LUN over iSCSI
and then snapshot the LV on the iSCSI server. After I restore the guest
again and start dd-ing off the snapshot LV.


Rgds,

B.

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