I’m currently working on a little Debian Xen server, and I encountered a few problems that aren’t documented.
The first problem I found was that serial ports don’t work with a default Xen setup (as documented in a previous blog entry). However the solution to this turns out to be putting xencons=off on the kernel command-line for the dom0 kernel. This allows the dom0 kernel to see all the serial hardware. If I had wanted to use the serial ports from a domU then something else would need to be done, but as I have no need for this I didn’t investigate the matter any further. Thanks to Brian May for discovering this for me.
Next in the early test relreses of etch the udev hotplug interface isn’t enabled. So I had to add kernel.hotplug=/sbin/udevsend to the /etc/sysctl.conf file. A newer version of udev appears to fix this without modifications to the kernel.hotplug setting though. The symptom of this was the error “Error: Device 768 (vbd) could not be connected. Hotplug scripts not working” where 768 is the number for /dev/hda (the same message would occur with number 769 if you use /dev/hda1). I choose to use unpartitioned virtual disks such as /dev/hda and /dev/hdb in Xen because there is no benefit in partitions (my Xen instances are not doing enough to need more filesystems than there are virtual IDE disks) and because I don’t desire the fake partition table thing that Xen apparently does.