I've got a large web application – it runs about 150 stores – that right now is deployed in separate Ubuntu virtual machines. I notice that most of the machines are barely utilized, and I contemplate that this might be a good application for container technology.

I observe that there are "virtual machine" containers, such as LXD, and "application" containers like Docker. I think that I could well use either one – and paying an annual fee for Docker's full commercial version is of no concern at all if I should decide to go that way.

But – how should I decide, and what factors should enter into my decision-making? Can you please point out some good reading material here and elsewhere on the web?

I would go for load, so try to find a solution which will need less resources. But actually another aspect is the stability, so you need to take care about that too.
I have no idea if 150 docker/lxd/whatever containers can run parallel, personally I have no good experiences with docker.

I have read about people running hundreds of LXD containers.

If you go through this route, you might want to get a support contract at https://www.ubuntu.com/support

This application is "small" enough that it can run on a handful of VMs, but I ponder the wisdom of using things like "NFS" as I did.

We're about to move to RackSpace hosting, and to negotiate the contracts much more wisely than was originally done. My instincts tell me that a much smaller number of virtual machines ought to do the trick – if LXD (say ...) is used properly. I know that we are right now losing a lot of steam-pressure to unnecessary virtualization.

I know that RackSpace offers both "VMWare virtual machines," as we are using now, and other alternatives such as OpenStack. I'm pondering both, cautiously.

This is a "very legacy" PHP application that happens to book millions of dollars in gross sales every(!!) month. So, I have to be careful, both technically and politically. I'm not sure that I want to push the organization farther into "the new world" than they actually need to be. This isn't a service-architecture application and it never will be. Neither will it ever be run by an application-server / FastCGI setup.

Presuming, for now, that VMWare will still be the basic arrangement even given a brand-new hosting company, I need to consider how containers might enable me to make each of the VMs work harder than they presently do ... given that there is a standing need for all of them to have access to hundreds of thousands of individual files, which right now are supplied by an NFS virtual server ... and also(!) given that they'd be "running in a virtual environment, despite their own virtualization."

(P.S.: I've also been pondering NFS "file-content caching" on the various VMs that we right-now have. But, if anyone has anything to say about that ... and I'm all ears(!) ... please open a new thread, say, "here (why not?)," to discuss that possibility and your experiences with it.)