Starting with nbd-client 1:2.9.9-3, there's support for running root on
an NBD filesystem. This support works as follows:

- There's only support for initramfs for now. If you need something
  else, patches are welcome...
- To boot the system, add the kernel command line parameter
  "nbdroot=X.X.X.X,Y", with "X.X.X.X" being the IP address of the
  server, and "Y" the port on which the server is running. Starting with
  nbd-client 1:2.9.11-4, you can also add a third argument to the
  nbdroot parameter, to specify which NBD device to set up in case
  you're running off RAID-on-NBD. If you do not specify the third
  argument, then your root= parameter must contain the /dev/nbdX device
  node that you're trying to boot off of.
- You will want to set "KILLALL=false" in /etc/nbd-client; this to
  prevent the initscript from yanking the root filesystem from under
  your nose during shutdown or upgrade of the nbd package.
- DO NOT add configuration for the root filesystem to /etc/nbd-server.
  The initramfs will set it up, and the initscript should not know about
  it (otherwise the above KILLALL configuration has no effect).
- DO keep the initscript running; it will detect that you're running off
  an NBD device and add the nbd-client PID number to
  /lib/init/rw/sendsigs.omit.d, to prevent init from killing nbd-client
  prematurely at shutdown.