Re: [Xen-users] Newbe wants to move block device virtual machine

["Daniel P. Berrange" <berrange@xxxxxxxxxx>]

On Tue, Nov 21, 2006 at 06:15:30PM +0100, Dick Kniep wrote:
> Hi list,
> 
> We are going to use Xen for our virtualization effort. Now I heard from some 
> friends who are experienced Xen users that performance of the block device 
> Xen machines is MUCH better than the performance of "file based" virtual 
> machines. (is that true?)

If you use blktap rather than loop devices for your file based virtual machines
there should be minimal difference, because blktap uses async/direct IO to 
bypass
the FS layer caching. You can switch to blktap by using 'tap:raw:' instead of
'file:' when configuring the disks.  The other benefit of using blktap for
file based disks is that it gives you much better data integrity upon crashes.

> Now if I use a block device as a basis for a virtual machine (on LVM), than 
> it 
> is not easy to move such a vm to another machine and make it accessible by 
> iScsi. (is that true?)

If you want to migrate VMs, then you need to use a network aware block device
such as iSCSI, or GNDB, or a shared SAN with Cluster-LVM. Of course same
applies for file  backed disks - you need to make sure the file is shared
across all hosts to which you migrate. So either way you need shared storage
at some point or another.

Regards,
Dan.
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