I had some trouble restarting a xen user domain tonight. The shutdown, destroy, console, and list xm commands were all simply hanging. I finally tried restarting the xend service which fixed the hanging problems. But when I tried to start the user domain that originally gave me problems I got this error message:

Error: Device 769 (vbd) could not be connected.
Device /dev/vg/lv is mounted in a guest domain,
and so cannot be mounted now.

I’ve been experiencing some issues with regard to backing up my Xen user domains. As I investigated the issues I once again found the lack of documentation to be frustrating, especially about such a fundamentally important issue as backups. There is simply no documentation on how to setup enterprise level backups for Xen. So here on some thoughts on what I see as the state of things, and where they could go form here to get better.

I noticed in testing some of the xm subcommands that I was getting errors. I found that these errors disappeared upon installing the python-xml package which provided the missing python modules needed by xm. I've reported the bug and hope to see it fixed promptly. In the meantime, make sure you install python-xml on any system running Xen. Who knows what other problems might result from this package being absent.

What’s the difference between the “pause”, “suspend”, and “save” commands in the Xen management software? I only had the vaguest notion. And while I don’t consider myself an expert I decided to put together a bit of a cheat sheet to keep it all straight, including what the commands did and what their proper matchings are.

So far my research has turned up a few solutions though none that I consider to be that great. I’ve also learned that both Xen and VMware seem to have this problem and it would not surprise if other virtualization solutions do as well. The root of the problem is actually that the Linux kernel relies heavily on interrupts for generating random numbers. In a virtualized environment a Linux kernel will be unable to generate random numbers due to being unable to access the hardware. The solution would probably be for the virtualization software to emulate a hardware random number generator that gets random data from the host system. That’s assuming that the Linux kernel isn’t adjusted in some way to account for this issue.

I’ve been using Xen for a little while now and while there are many aspects I like there are a few that are troubling. I’ve been wanting to write up some tutorials on using Xen but before that can happen I need to feel comfortable recommending Xen as a virtualization solution. At the moment I’m not sure I can do that, partially because of the issues I’ve encountered but mostly because those issues do not seem to be acknowledged or discussed in any meaningful way. That is perhaps the most troubling thing of all.

Virtualization is the future of computing. Virtualizing servers has many benefits which including increased manageability, reduced hardware costs, strengthened security, and others that are outside the scope of this article. Among the array of virtualization solutions Xen stands out as the one I am most interested in pursuing at this time as it is Free/Libre/Open Source Software (FLOSS) and provides the functionality and performance I desire.