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An easy and simple way to manage VMs with Xen is to let linux boot off NFS, so you can serve these filesystems off a central storage machine and still have access to the complete directory hierarchy, enabling you to do fancy stuff like incremental backups. Sure, NFS is not the best way to do this, but in a low budget situation or with older hardware, this solution works quite well.

Creating a DomU for use with NFS isn’t such a hard thing, there are several possible pitfalls though. Here is a step-by-step guide:

Create empty directory on the NFS server and export it with no_root_squash for the VM itself and the xenhost or the host you want to use to bootstrap the VM

/some/dir/www.example.org www.example.org(rw,no_root_squash,no_subtree_check)
/some/dir/www.example.org xen.example.org(rw,no_root_squash,no_subtree_check)


Don’t forget to reload NFS exports

exportfs -r


Mount it on the xenhost (or the host you’re using for bootstrapping)

mkdir /tmp/www.example.org
mount -t nfs xen.example.org:/some/dir/www.example.org /tmp/www.example.org


Use debootstrap to bootstrap an initial Debian lenny install

debootstrap lenny /tmp/www.example.org/ http://mirror.switch.ch/ftp/mirror/debian


We need to do some extra things inside the VM before we can actually boot it up, so we chroot into it

chroot /tmp/www.example.org


Correctly set the hostname

echo "www" > /etc/hostname


And the hostsfile, at least localhost is required

echo "127.0.0.1 localhost" > /etc/hosts


Add and configure the loopback interface

echo "auto lo" > /etc/network/interfaces
echo "iface lo inet loopback" >> /etc/network/interfaces


Add all desired sources to /etc/apt/sources.list

nano /etc/apt/sources.list


Install the bigmem kernel for your architecture. If you do not want to do this because you already have a kernel and an initrd, at least install udev.

apt-get install linux-image-2.6-686-bigmem


Install locales to get rid of warnings regarding LC_*

apt-get install locales
dpkg-reconfigure locales


Adjust initramfs for NFS support (set BOOT=nfs instead of local)

nano /etc/initramfs-tools/initramfs.conf


Add the xen frontend drivers to the initramdisk (add xen-netfront and xen-blkfront)

nano /etc/initramfs-tools/modules


Regenerate initramdisk

update-initramfs -u all


Spawn a console on hvc0 (add the following to /etc/inittab)

0:2345:respawn:/sbin/getty 38400 hvc0


Leave chroot

[Control-D]


Copy the kernel and initramdisk to some location on the xenhost

cp /tmp/www.example.org/boot/* /vmkernel/


To be able to use NFS properly be sure to:

• DO NOT add the root to /etc/fstab as this will cause problems with the checkroot rcS script
• Add ASYNCMOUNTNFS=no to /etc/default/rcS to be able to mount additional nfs shares