Just notes on Virtualization.
VMware ESxi & Vsphere- Thousands of instances no issues until there is a massive update from Vmware. ESXi has a free version, I highly recommend this if you want to do what the “Enterprise Gold Standard” kids are doing.
Xen(Citrix or Opensource) - No issues until a server or cluster loses power, then minor issues with powering up some instances running older OS’s. Both versions of free and paid run as fast as the hardware you can slap them on. Not as efficient at over provisioning as Vmware nor handles large memory configurations as well as Vmware or Hyperv. Large memory being over 2TB or more.
Hyper-V - Great for Microsoft Windows Instances, limited support for most NIX systems. Works well with all things “Microsoft”, is free to OMG I PAID WHAT? - Runs better on newer hardware, slow IOV compared to VMware. Networking wise not as many “good” options as Vmware, still a great Free server OS and Free virtualization platform for developers as Hyper-V ships with Pro & Enterprise versions of Windows OS.
VirtualBox - Runs on nearly every platform, much akin to what Vmware GSX used to be (If your not sure, its ok) Simplified Virtualization that is cross platform and enterprise ready. This product does not get the attention it deserves. In benchmarks it greatly depends upon which OS you are installing it upon. Top speed ranks on OpenSuse & Windows 2019 pre November patch
Quem & KVM - If you are a NIX ninja and like command line operations, spending nonsensical hours configuring every little damn chunk of memory, or need to intercept memory calls real time, both are good provided you LOVE C and the command line shell of NIX.
Docker - Once, not long ago it was good for a free product. You could write chef and puppet scripts to spin up and down thousands of instances real time and do it all for inexpensive to free, then Docker became greedy. The problem is the short comings with port issues, network performance issues, and portability issues. Sure, its still “free” for individuals but for the enterprise, its now a cost center and there are better options that just work better, including Hyper-V and Xen.
Many things break on docker as its not a dedicated hypervisor, its a drop in add on top product that isnt keeping up with the changes at the OS level. Other products handle portable virtualization better.
From a Developer Standpoint, run Virtualbox or Hyper-V, you can run both on the same machine. You will find you can export your image to nearly anything.
From a Sys Admin standpoint, Vmware is hard to beat if money is no object. If budget is a concern, then it becomes which is more important, clustering or performance, you can do your own ROI and figure it out.