[Info-vax] OpenVMS on x86 and Virtual Machines -- An Observation

[terry-groups at glaver.org]

On Wednesday, January 30, 2019 at 10:09:25 AM UTC-5, geze... at rlgsc.com wrote:
> Traditionally, OpenVMS has been run on dedicated hardware. In the past two decades, an initially small but increasing number of systems have been, and are, running on one or another emulator (e.g., simh, Charon, AVT, etc.). With the advent of OpenVMS on x86, there is an increasing discussion of running OpenVMS x86 on various virtual machine hypervisors (e.g., xen, VirtualBox, Hyper-V).
> 
> Questions ensue along the lines of "What if my (fill in your supported VM) infrastructure is using enterprise-class storage facility that is not supported by OpenVMS?" 
> 
> What matters in a hypervisor-based environment is not the underlying storage or network device used by the hypervisor. What does matter is the simulated device presented to the client virtual machine.

I discovered this (with an emulator, not a hypervisor) when I found that the AlphaVM emulator could attach SAS and SATA drives directly to the emulated VMS instance. I expected SAS would work (the most common use seems to be directly connecting a physical tape drive to the emulated VMS system) but was rather surprised that SATA worked - apparently the SAT layer handles the command differences. This is slower than providing a virtual drive to the emulated VMS system, since the attached device no longer benefits from optimizations / caching performed by the host operating system. In the case of AlphaVM, it provides some assists (6/10 byte command conversion, SCSI ID and LUN translation, etc.)