I have limited experience with mainframe emulation, but from what I've seen, trying to do it in a live business environment is challenging at best because all o

I have limited experience with mainframe emulation, but from what I've seen, trying to do it in a live business environment is challenging at best because all of that old mainframe software was written with a lot of assumptions that don't apply to non-mainframe environments. Things like blazing-fast I/O, virtually guaranteed uptime of the mainframe, etc. As one example, when I was a systems engineer, I got a call from a team that was trying to run a business application from z/OS in emulation on an x86 server. It was running incredibly slowly, even on a fairly beefy server. I looked in its data directory, and there were hundreds of thousands of individual files in a single directory. Not a tree. One directory. That might work fine on a mainframe, but just doing a dir or ls (depending on the host OS) took something like ten minutes. Similarly, I remember some folks being dumbfounded at the idea that their batch processes needed error-handling when they were doing things that involved talking over the network, because when everything was running within that one giant IBM system, failures were pretty rare. Those both sound like they should be easy to tack on when running in an emulator, but they're just the tip of the iceberg. When you have code that's 30-50 years old, written for a vastly different platform, there are going to be a lot of gotchas like that.