Recursive Virtualization

What do you do if you have a lot of time on your hands? Well if you're a VM tester, you try to see how many levels of VM can you run. That is, running VM in a virtual machine on a VM that is running on a virtual machine, etc. etc.. The record that I've hear most often quoted is nine by an IBM system tester in the 1990s. Who's face I can picture, but for the life of me I am drawing a blank on a name. Perhaps someone out there will provide a name. He did this with a VM/ESA system. Nine isn't necessarily a limit from a technology perspective, but more from a sanity perspective. As I recall the ninth one wasn't setting any performance records. Also, it takes some patience dealing with keeping consoles and other things straight as to which goes to which system. I was amazed when I heard of this because there are various times I have struggled to just sort out 2nd and 3rd level systems. How many other people out there have formatted a 191 disk on the wrong level? Or issued Shutdown at wrong level? Ok, perhaps it's just me.

What's the point to running that many levels? From a business perspective, there really isn't one. It's a curious academic experiment. It may have turned up some unique bugs; but beyond that, it's just bragging rights. (Though, since I my poor brain can't even recall the chap's name, his name isn't even preserved in this Blog).

At smaller number of levels, there is business value. Whether it be for testing, training, or hosting purposes, second level systems can be very helpful and valuable. One of the things I often encourage new z/VM people to do is to build a second level system. It gives you an opportunity to try various commands and features in a worry free environment. And it's a very realistic model of a "real" z/VM system, with just a small number of LPAR or CEC features that are not virtualized up through a second level system.

I believe the ability to do recursive virtualization is another apsect of the purity of the z/VM virtualizaton model. It's not a be-all, end-all measure; but it is another view worth considering.