> DB2 runs on Linux The DBA's I interned with would yell at you for this claim. They'd say UDB runs on Linux, but DB2 only runs on z/OS.

I worked with both DB2 & UDB during my internship 2 decades ago, and while largely compatible with core SQL functionality, that changed quickly after you got off the "normal" path. UDB was released at a much faster cadence, and if you developed to the latest features of UDB, you'd often find yourself unable to deploy against DB2.

A few of my last projects as an intern: 1) storing BLOB objects in DB2 from a .Net app and 2) porting an "interactive" batch-processed COBOL app to ASP.Net form 3) getting DB2 Connect clustering on windows working (I never did get this working despite spending an entire summer on the phone with IBM support).

I've some fond memories of working with the mainframe. Like the time I managed to crash the entire development LPAR with a specific SQL query that broke the DB2 query optimizer... The phone call from the NOC was immediate and went something along the lines of "I don't know what you did, but don't do it again, you just took down all of development". Development was something like +10k users...

What replaces JCL?

Worth noting, in the 90's general computing was still increasing at a very rapid pace, where you could replace a late 80's mainframe with an early 90's x86 or PPC server. You won't have the built in layers of redundancy, but that isn't always necessary for any given environment. The cases of many mainframes still in use today are on much faster/newer systems where that critical uptime is much more important.

I'm not really a fan of mainframes, only pointing out that the companies still on them are generally on them as a direct replacement to general purpose servers isn't purpose fit, and conversion is excessively costly, even compared to IBM mainframe contracts.