Yeah people don’t realize the IO bandwidth (off the central CPUs mostly) is the differentiator. And writing SQL for DB2 is easier than writing efficient spark jobs (with minimal shuffles across those cheap small machines).

How is that level of IO bandwidth achieved exactly? Or how is it different from the x86 architecture?

There is a ton of technical info out there, but in a nutshell, the hardware is extremely optimized for I/O bandwidth, cache's are able to be accessed by more cores at a time (I think the L3 cache is 32-way accessible for example). The goal is to have nearly everything in at least RAM (up to 40TB of it) or better yet in a cache closer to a CPU so there is no latency.

And when it comes to reaching things outside of the mainframe itself, generally anything you can do to make accessing the data wider is better... Why use a single 16GB/sec HBA when you can interleave 8 or more FCAL paths to the same disk. Why use FCAL when you can use Infniband, etc etc.. They have a LOT of PCIe lanes available, and enough chips to keep most of them busy most of the time.

TL;DR if it's expensive and offers more paths between things, it's probably an option for a mainframe.

People often forget how much computer science and engineering is in those machines that allow them to fully saturate two and a half full height racks of high performance IO while running COBOL programs that are blissfully unaware of all that goes behind their ABI and just see a ridiculously fast computer.

> I have heard stories (from actual mainframers) about the sql-performance and key-value performance (read like mongodb) on these boxes that are eye-watering

Does that mean great or awful?

I assume he means that DB perf on mainframes are insane. That would make sense, given their low-latency architecture and I/O throughput capabilities.

I would love to see an article that breaks down clustering v. supercomputers v. mainframes in tangible use-cases. It is all a bit opaque to me where one officially starts and the other one(s) begin. As well as where (and to what degree) the value prop is of one versus the others.

I've heard folks say mainframes are just legacy stuff carried forward - I've heard others say they have specific use-cases that are not well solved by other current technologies (usually a cost of refactoring). That said, are there any current use-cases, where if I were to write code from ground zero, that are best solved on mainframe, hands down? That is my root question.

Great... order of magnitude-ish higher than a really high end x86 box with pure flash storage...

It was so beautiful it made him cry. although eye-watering usually means something negative or stinky (like onions or a a particularly putrid fart)