It is also possible to use a file in Domain 0 as the primary storage
for a virtual machine. As well as being convenient, this also has the
advantage that the virtual block device will be sparse --
space will only really be allocated as parts of the file are used. So
if a virtual machine uses only half of its disk space then the file
really takes up half of the size allocated.
For example, to create a 2GB sparse file-backed virtual block device
(actually only consumes 1KB of disk):
# dd if=/dev/zero of=vm1disk bs=1k seek=2048k count=1
Make a file system in the disk file:
# mkfs -t ext3 vm1disk
(when the tool asks for confirmation, answer `y')
Populate the file system e.g. by copying from the current root:
# mount -o loop vm1disk /mnt
# cp -ax /{root,dev,var,etc,usr,bin,sbin,lib} /mnt
# mkdir /mnt/{proc,sys,home,tmp}
Tailor the file system by editing /etc/fstab,
/etc/hostname, etc. Don't forget to edit the files in the
mounted file system, instead of your domain 0 filesystem, e.g. you
would edit /mnt/etc/fstab instead of /etc/fstab. For
this example put /dev/sda1 to root in fstab.
Now unmount (this is important!):
# umount /mnt
In the configuration file set:
disk = ['file:/full/path/to/vm1disk,sda1,w']
As the virtual machine writes to its `disk', the sparse file will be
filled in and consume more space up to the original 2GB.
Note that file-backed VBDs may not be appropriate for backing
I/O-intensive domains. File-backed VBDs are known to experience
substantial slowdowns under heavy I/O workloads, due to the I/O
handling by the loopback block device used to support file-backed VBDs
in dom0. Better I/O performance can be achieved by using either
LVM-backed VBDs (Section 6.3) or physical
devices as VBDs (Section 6.1).
Linux supports a maximum of eight file-backed VBDs across all domains
by default. This limit can be statically increased by using the
max_loop module parameter if CONFIG_BLK_DEV_LOOP is
compiled as a module in the dom0 kernel, or by using the
max_loop=n boot option if CONFIG_BLK_DEV_LOOP is compiled
directly into the dom0 kernel.
