Migrating to Xen Cloud Platform (XCP Xen-API) on Debian wheezy

I recently decided to upgrade my server and host it in a new location (turns out that leaving the UK hosting was a smart move too, as new UK laws to rein in the press are more likely to punish and censor private web sites and bloggers)

As part of the upgrade, starting off with Debian 7 (wheezy) seemed to make a lot of sense. I've been using Xen as a standard server virtualization platform since Debian 5 (lenny), and upgrading through Debian 6 (squeeze) without any major problems. I had been managing it with the traditional xm toolstack without any major concerns.

Wheezy benefits from supporting Xen Cloud Platform with a standard set of packages, and it has been promoted heavily as a key technology in wheezy. XCP appealed to me for various reasons:

• xm toolstack is deprecated, so I had to change to xl or XCP anyway
• XCP seems to have the best range of features, such as hot migration
• XCP is the required Xen variant for anybody who wants to use Xen-based OpenStack

The gotchas

Reading through that list, it seems like there is no reason not to use XCP. In fact, there are major differences between the way xl/xm tools manage a host and the way XCP manages a host. While xl/xm offer the flexibility to manually configure networking and storage any way you like, XCP is more like a cookie-cutter virtualization system. It needs to be: without having low level control over networking and storage on the physical host, it could not offer things like hot-migration. Some people might get a shock, however, when moving to such a locked-down platform, as it takes away some of the things you would normally control manually on a Debian server:

• XCP takes over networking. Your /etc/network/interfaces file must be almost empty - you can still set up firewall rules from a separate script, but don't plan on having any customised IP address configuration or routing.
• XCP still has some bugs, such as requiring a quick config hack to use any subnet size other than /24, or another hack to make HVM networking function for any non-Linux VM. Hopefully these will be fixed before the official wheezy release.
• XCP management tools don't support IPv6 - although this doesn't prevent using IPv6 in a VM. If you were planning to use dom0 as an IPv6 gateway for the domUs, it won't work: however, you can set up a domU bridged to the physical ethernet so it can act as an IPv6 gateway for the other VMs.
• XCP takes over your storage and creates it's own PV, VG and LVs. This can inadvertently lead to the creation of a nested volume group if you are not aware of the exact way this works.

Making it work

Despite these gotchas, I have managed to get it working, doing all the things that were possible on regular Xen + Debian squeeze.

I would suggest that anybody attempting this read through all the bugs of severity important or serious and the updated README.Debian file (patch in bug 702337) to get off to a quick start without repeating some of the problems faced by early adopters like myself.