I only ever worked with the Linux/Windows variant. I can’t believe I am saying this about an IBM product, but I found it to be actually rather pleasant to work with.
As an IBM hobbyist user, picture something worse than VMS in 'hackerdom'. IBM's mainframe OSes are like NT/OS2 taken to the total extreme with objects, because by default you don't see files but objects which might have files... or not.
Imagine the antithesis of Emacs. That's an IBM environment with 3270 terminals and obtuse commands to learn.
As somebody who administers several large DB2 clusters all linked together with multiple replication modes (HADR, SQLREP) for an emergency services communication platform, I can confirm this. It's pretty damn rock solid even on Linux these days.
Kind of, but there are some subtle differences in my opinion. Oracle is top-to-bottom evil, whose business model basically boils down to screwing over their clients and everyone else at every possible opportunity, comparable to the likes of McKinsey or Accenture.
IBM is a bit more nuanced. My wife grew up in an IBM town and a lot of her family and her friends’ families used to work there in the 70s and 80s. People, especially the engineers, used to take pride in their work there.
I believe the mainframe version was first.
There is a version baked into the os/400 operating system (i series).
Then unix/windows Db2 came last, if memory serves.
https://en.wikipedia.org/wiki/IBM_Db2