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

[John Haxby <john.haxby@xxxxxxxxxx>]

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

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