Xen koans

[CLIFFORD ILKAY]

William O'Higgins Witteman wrote:
> I have some basic questions about running a virtual machine inside of
> Xen.

I have been running it in a hosting environment since 2005. At the time, 
it was a bear to set up only because there were no official packages for 
the distro I was using but it is quite a bit simpler today. Lately, I 
have been experimenting with OpenVZ. It was considerably easier to 
install and configure but it is not the same thing as Xen or VMWare at 
all. OpenVZ is more like a fortified chroot.

> How do you direct network traffic to the virtual machine?

Bridging or NAT. I use bridging.

> Can the VM share data with the host OS?

When a VM is running, you have to use the usual methods for sharing 
files, no different than sharing files between physical machines. That 
includes the possibility of using a SAN or some distributed filesystem. 
Once a VM has been shut down, you have the option of mounting the 
partitions that are used by the VM somewhere and copying files to and 
from the filesystem of the VM.

> How does crash recovery work?

Very well. You can log in to dom0, the Xen hypervisor, and control the 
virtual machines quite easily. You could think of the dom0 as a poor 
man's KVM over IP for all the virtual machines. I can reboot the virtual 
machine and see boot time messages in the domU's.

> Is running virtual machines in Xen (or other) worth the hassle?

It depends on what you want. I think it is quite worthwhile because it 
enables me to virtualize the underlying hardware and migrate virtual 
servers from one physical server to another with very little fuss. Xen 
is very lightweight. I only have 64M allocated to the dom0 and have 
never had any problems with resource exhaustion in the dom0. I have 
found that even if one of the load average on one of the virtual 
machines is very high, the dom0 remains responsive so issuing an "xm 
shutdown somemachine" is perfectly feasible. I have not had to do that 
very often but the few times that I did, it proved to be invaluable.
-- 
Regards,

Clifford Ilkay
Dinamis Corporation
1419-3266 Yonge St.
Toronto, ON
Canada  M4N 3P6

<http://dinamis.com>
+1 416-410-3326
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