I just read an article about Xen on Slashdot and it looks very cool. The comments indicate that it's faster and more secure than User Mode Linux, and it supports 'live migration', which allows running OS images to be moved between nodes in a cluster without having to stop them.

Caker, what are your thoughts? Has anyone in the UML hosting industry thought about moving from UML to Xen?

Maturin

Xen surely looks attracting (performance wise).

Moving running OS images between nodes requires a network filesystem though, may not work on NFS and need something like GFS (and most surely extensive testing). This would add to the current cost of a Linode...

I may be mistaken, but I think I remember seeing that the performance hit in UML can still be reduced.
For example IIRC when you call gettimeofday in your UML, the glibc calls the UML kernel which itself calls the host's glibc which calls the host's kernel. Removing the host's glibc call could help but I don't know the details.

A key to Xen adoption is toolkits too : things like umld need to be mature before a hoster can use Xen.

Xen is not as widely tested as UML too : hosters like rock-stable software so I imagine if it were to be used for Linodes we will have to wait for extensive tests and integration...