machine containing a replica (minus the state) of the system
configuration. This is mostly useful for testing configuration
changes prior to doing an actual "nixos-rebuild switch" (or even
"nixos-rebuild test"). The VM can be started as follows:
$ nixos-rebuild build-vm
$ ./result/bin/run-*-vm
which starts a KVM/QEMU instance. Additional QEMU options can be
passed through the QEMU_OPTS environment variable
(e.g. QEMU_OPTS="-redir tcp:8080::80" to forward a host port to the
guest). The fileSystem attribute of the regular system
configuration is ignored (using mkOverride), because obviously we
can't allow the VM to access the host's block devices. Instead, at
startup the VM creates an empty disk image in ./<hostname>.qcow2 to
store the VM's root filesystem.
Building a VM in this way is efficient because the VM shares its Nix
store with the host (through a CIFS mount). However, because the
Nix store of the host is mounted read-only in the guest, you cannot
run Nix build actions inside the VM. Therefore the VM can only be
reconfigured by re-running "nixos-rebuild build-vm" on the host and
restarting the VM.
svn path=/nixos/trunk/; revision=16662
89ef5c979b