A lot of my use of virtualization is to test and develop our Ubuntu systems and changes to them. If I have a significant configuration change for one of our Exim machines, for example, I will usually spin up a test VM, build a version of that machine, and test the configuration there before I risk deploying it onto the real machine. Similarly, I prototype a lot of the build and operating procedures for machines on VMs, because if something goes wrong I can pave and rebuild easily (or simply revert to a snapshot).

For a long time, I redid the Ubuntu install on these VMs from scratch every time I needed to change what I was doing with one of them, which was reasonably frequently (or at least I mostly did, because sometimes I was lazy and reused an existing Ubuntu install that wasn't too mutated from our baseline, and then sometimes I was wrong about how mutated it was). Years ago I toyed with the idea of preconfigured virtual machine templates, but I never went anywhere with it for various reasons, including how many potential image variants there were.

Recently I not so much figured something out as got lazy. Most of the VMs I build are of the latest version of Ubuntu and there's definitely one portion of our install system that changes extremely rarely, which is the initial install from our CD image. So at some point, I started just making a snapshot of that initial install in every test machine VM. This still means I have to go through our postinstall process, but that's mostly hands-off and especially I don't have to interact with the VM on the console; I can do it all through SSH.

(The CD stage of the install requires configuring networking and partitioning the local disk before Ubuntu can be installed and brought up on our network. After that it's deliberately accessible via SSH.)

This doesn't help if I need something other than Ubuntu 18.04, but in that case I do 'dd if=/dev/zero of=/dev/sda' and start from scratch with a CD install. I don't bother deleting the snapshot; I'll revert back to it when I go back to wanting this VM to be an 18.04 machine. Fortunately the CD install stage doesn't take too long.

(Most of our Exim machines are still running 16.04, so testing an Exim change is one recent reason I needed a 16.04 VM. I just recycled that one, in fact.)

PS: I can't keep a generic image and clone it to make new VMs, because our install CD sets the machine up with a specific IP address (whatever you enter). It's easier to keep frozen snapshots for several VMs, each with its own IP address in it, than to fix that somehow after I clone the image into a VM.

Sidebar: Something I should try out with VMWare Workstation

Normally I think of VMWare Workstation snapshots as happening in a straight line of time, but if I remember right you can actually fork them into a tree. If this works well, the obvious approach to dealing with different Ubuntu versions would be to keep forked snapshots of each. However, even if this is something you can do reasonably in the VMWare UI, I wonder what sort of performance impact it has on disk IO in the VM. I don't want to slow down my VMs too much for something that I only use occasionally.