From my experience working with it: in short, it is just a fancy name for shared memory between machines, with software libraries on top to manage synchronization, distributed transactions, et cetera.
From a sysadmin perspective it behaves less like a "single system image" and more like multiple images with shared resources.
From an app dev perspective, sysplex is very similar to a single system image, especially when they are using the Db2 database.
WRKSYSSTS
That’s IBM i, this is IBM Z
People say IBM mainframes are so reliable that they never fail, yet if a customer wants 5 9s IBM sells them a mainframe cluster.
My understanding that this clustering is more to handle external factors. It allows mainframes to be placed in different buildings anywhere from the same campus to up to 200kms away ideally providing enough distance to deal with natural disasters.
Mainframe itself (the "fridge") is a distributed system, as well as human brain is. It's all the management tooling that makes it appear as a single system.
One possible reason why: customers might want to be able to upgrade their hardware/OS/whatever and retain their 5 9s.
Good luck trying to get five 9s when you're limited to a single DC. Cross-DC SYSPLEXing (GDPS) got us to five 9s and beyond without breaking a sweat. Our systems can never go down, ever.
From VISA with love.
Still remember those days that these are for those transaction processor marketing against mainframe we used. Not in the field. Where they go to ?