Attach virtual disks to Xen virtual machines

The xen-disk tool allows a virtual disk to be attached dynamically to a running xen virtual machine. The tool is intended for testing new block protocol extensions, disk formats etc.

The virtual disk is represented by an OCaml module implementing a simple signature containing 'read block', 'write block' operations. There are several existing modules including

1. DISCARD: this is used if no disk image file is given on the command-line. It acknowledges all requests but doesn't do anything. It is used for checking the performance of the disk protocol code.

2. MMAP: this is used if a disk image file is given on the command-line with no format override. It calls mmap(2) on the disk image file and satisfies requests using memcpy()

3. VHD: this is used if a disk image file is given on the command-line with the "vhd" format specified. It calls mmap(2) on the disk image file and assumes the file is in vhd format.

Example: testing the performance of the disk protocol code

From domain 0 on a xen host running a VM named "debian", type:

xen-disk connect debian

A new virtual device will be created inside the VM as /dev/xvdX. Simple performance testing can be attempted via a command like:

# dd if=/dev/xvdb of=/dev/null bs=1M iflag=direct count=100
100+0 records in
100+0 records out
104857600 bytes (105 MB) copied, 0.125296 s, 837 MB/s

Example: attaching an existing file as a virtual disk

In domain 0 create a disk file using a process similar to:

dd if=/dev/zero of=disk.raw bs=1M count=1 seek=1K
losetup /dev/loop0 disk.raw
mkfs.ext3 /dev/loop0
losetup -d /dev/loop0

Next attach it to a running VM using a command like:

xen-disk connect debian --path disk.raw

A new virtual device will be created inside the VM as /dev/xvdX containing an ext3 filesystem, which you can mount.