One of the fantastic features about Xen is that when you build a new Xen virtual machine (VM), you can specify a file on domain0 as a physical device to the VM (domU). Here is an example from a Xen machine configuration file (typically found in /etc/xen):

disk = ['file:/xen_files/215_main_disk.img,hda1,w','file:/xen_files/215_swap.img,hda2,w']

The above example shows a correctly configured definition for device hda1 (which is mounted from the file /xen_files/215_main_disk.img) and hda2 (which is mounted from the file /xen_files/215_swap.img).

However, a frequent newbie error is to forget the “file:” tag. If you entered this, for example:

disk = ['file:/xen_files/215_main_disk.img,hda1,w','/xen_files/215_swap.img,hda2,w']

then when you run xm create for that machine, it will output the error “unpack list of wrong size” which isn't very helpful in telling your that you forgot the “file:” tag!

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