First, populate a root filesystem in a directory on the server
machine. This can be on a distinct physical machine, or simply run
within a virtual machine on the same node.
Now configure the NFS server to export this filesystem over the
network by adding a line to /etc/exports, for instance:
/export/vm1root 126.96.36.199/24 (rw,sync,no_root_squash)
Finally, configure the domain to use NFS root. In addition to the
normal variables, you should make sure to set the following values in
the domain's configuration file:
root = '/dev/nfs'
nfs_server = '188.8.131.52' # substitute IP address of server
nfs_root = '/path/to/root' # path to root FS on the server
The domain will need network access at boot time, so either statically
configure an IP address using the config variables ip,
netmask, gateway, hostname; or enable DHCP
Note that the Linux NFS root implementation is known to have stability
problems under high load (this is not a Xen-specific problem), so this
configuration may not be appropriate for critical servers.