====== Disks/Images layout in VMs ======

This is a layout I've starting using on my VMs. I use this scheme to waste as little space on the host as possible and to allow me easy "rollback" to a clean system.

The process I follow is:
  * Create initial set of images
  * Set them up in virt-manager, if the guest supports virtio, use it as device type for max performance
  * Install OS, do first boot, shutdown VM after first boot is completed
  * Snapshot clean installation.
  * Re-create system image (this allows it to be recompressed)

Writes should be kept at a minimum before compressing so that no //ghost// data needs to be recompressed.
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I install, exit the VM, snapshot, re-create the clean installation, compressed (compression requires re-creation/copy); create a //linked// image to be used instead of the clean image (this linked image will store the changes to the base image) and then start using the VM.
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This allows me to use the same base image for different set-ups since changes will be written to a separate image (maybe could run even simultaneously although I'm not sure about the performance penalty).

Additionally I use a separate image for swap so that swapped data doesn't get written to the other images (hindering compression).

Finally I use a third image for ''/home'' so that I can rollback the OS without affection homes. I set it up as an LVM volume to allow future expansion with even more images :-P

===== Initial image creation =====

  * First disk image: (System, base image) os.qcow2 <code>
  $ qemu-img create -f qcow2 os.base.qcow2 10G
  $ qemu-img snapshot -c empty os.base.qcow2
  $ ln -s os.base.qcow2 os.qcow2</code>
  * Second disk image: (Swap) swap.qcow2 <code>
  $ qemu-img create -f qcow2 swap.qcow2 2G
  $ qemu-img snapshot -c empty swap.qcow2</code> Optionally move to ''/dev/shm'' for faster guest swap.
  * Third disk image: (Home) home.qcow2 <code>
  $ qemu-img create -f qcow2 home.qcow2 8G
  $ qemu-img snapshot -c empty home.qcow2</code>

===== Post-installation =====

  * Snapshot ((Note snapshotting takes a *lot* of time))<code>
  $ qemu-img snapshot -c clean-install swap.qcow2
  $ qemu-img snapshot -c clean-install home.qcow2</code>
  * Layered/linked system image <code>
  $ qemu-img convert -c -O qcow2 os.base.qcow2 os.base.compressed.qcow2
  $ mv os.base.compressed.qcow2 os.base.qcow2
  $ qemu-img snapshot -c clean-install os.base.qcow2 # Optional
  $ qemu-img create -b os.base.qcow2 -f qcow2 os.delta.qcow2
  $ rm os.qcow2
  $ ln -s os.delta.qcow2 os.qcow2</code>