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Re: RES: RES: [Xen-users] Shared volume: Software-ISCSI or GFS or OCFS2?

Bruno Bertechini wrote:
Well.. Let me use this thread and popup my environment and ask for

We have a EMC storage and 03 hosts (all with fibre channel adapters).

Choice 1 :

Regarding Xen backend : Files or LVM ? I'm thinking to use LVM to use online
resize/snapshots. Are there huge performance differences?

I can't define a infrastructure for EMC/NFS/OCFS/etc without choose before
select the appropriate backend.

What do you suggest guys?

Do you want (live) migration? If so then you'll need to be able to access the files/LUNs/logical volumes from each machine in the cluster.

That probably constrains your choices somewhat: there is cluster LVM (clvm) but I don't know enough to say anything for or against it; nfs and ocfs2 will both work of course; and then there's the third choice of LUNs in the EMC storage being assigned to more than one machine or migrated from machine to machine and I have no idea how that is done as I don't have that kind of hardware to hand.

I will say this something resizing and snapshots though.

You don't need LVM for resizing: you can grow a file by dd'ing empty space from /dev/zero on to the end of it. You'll need to restart the guest that's using the file to pick up the new file size (although I think that might have changed in 3.3, I'm not sure). Likewise you can grow LUNs defined in the array.

Snapshots are not the panacea you might think. If you just take a snapshot of a guest's virtual disk then what you've got is a corrupt virtual disk because you haven't got what was in the guest's memory and on the way to the disk. If you xm save the guest and copy the memory snapshot the disk then what you've done is basically a fork(2) of an OS -- when you restore the backed up copy it'll carry on doing exactly what it was doing at the time of the backup. My favourite example of the day is what happens when that guest is about to change a database over a network connection -- that change will happen twice. Let's hope it's $salary = $salary * 1.1.


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