Recently I mentioned in passing that I keep around spare virtual machines, and in comments Todd quite reasonably asked how one has such a thing (and sort of why one would bother). There are two parts to the answer, a general one and a libvirt one.

The general part is that one sort of my virtual machines are directly on the network, not NAT'd, using specifically assigned static IPs. In order to avoid ever having two VMs accidentally use the same IP, I pre-create a VM for each reserved IP with the (libvirt) name of the VM being its hostname. This still requires configuring each VM's OS with the right IP, but at least accidents are a lot less likely (and in my dominant use for the VMs, I do an initial install of an Ubuntu version with the right IP and then snapshot it).

The libvirt specific part is that I find it a pain in the rear to create a virtual machine, complete with creating and tracking a disk or disks for it, setting various bits and pieces up, and so on. Clever people who do this a lot could probably script it or build generic XML files or similar things, but instead I do it as little as possible, which means that I almost never delete virtual machines even if I'm not using them (although I shut them down). Right now my office desktop has ten VMs configured, none of which are normally running.

(I call this libvirt specific because it's fundamentally a user interface issue, since I could fix it with some sort of provisioning and de-provisioning script that automated all of the fiddly bits for me.)

The most important part of how I keep such VMs as 'spares' is that every time I set up a new VM, I snapshot its initial configuration, complete with a blank initial disk (under the imaginative snapshot name of 'empty-initial'). Then if I want to test something from complete scratch I don't have to go through the effort of making a new VM or erasing the disk of a currently unused one; I just find a currently unused VM, do 'virsh snapshot-revert cksvm5 empty-initial', connect the virtual DVD to an appropriate image (such as the latest FreeBSD or OpenBSD), and then run 'virsh start cksvm5'.

(My earlier entry on how I've set up my libvirt based virtual machines covers the somewhat different way I handle having spare customized Ubuntu VMs that I can use to test things in our standard Ubuntu server environment.)

Using snapshots instead of creating and deleting VMs is probably a bit less efficient at the system level, but not enough for me to notice and care. Having written this, it occurs to me that I could get much the same effect by attaching and detaching virtual disks to the VMs, but with current tooling that would take more work. Libvirt's virsh command line tools make snapshots the easiest approach.