I wanted to alter a file that was a disk image for a KVM virtual machine.  With a physical machine I use dd fairly often to save and alter the partition table and boot loader.  I wanted to do that to a KVM image. The problem being that when you use dd to write to a file, when dd is done it truncates the file. So I would lay down a new partition table and boot loader on my KVM image and find the image was now only a few kilobytes long. It used to be 20 gigabytes in size!

To do this we need to use the loop device:

# dd if=/dev/zero of=/var/lib/libvirt/images/foo.img bs=1024 count=20971520
# losetup /dev/loop0 /var/lib/libvirt/images/foo.img
# dd if=/tmp/bar.img of=/dev/loop0

Now you can use fdisk or other tools to examine the hard disk image on /dev/loop0. When you are done, tear down the loopback.

# losetup -d /dev/loop0

Now boot your KVM.

Why do I like to do this? Think about automated ways to install DBan. Or to laydown a gPXE bootloader to reinstall or reprovision the machine.  Fixing a corrupt partition table or MBR.  Doing things this way allows for a high level of automation.