The Future of Big Iron: An Interview with IBM’s Christian Jacobi

at the heart of the Mainframe success is this:

> I’d say high-availability and resiliency means many things, but in particular, two things. It means you have to catch any error that happens in the system - either because a transistor breaks down due to wear over the lifetime, or you get particle injections, or whatever can happen. You detect the stuff and then you have mechanisms to recover. You can't just add this on top after the design is done, you have to be really thinking about it from the get-go.

and then he goes into details how that is achieved. the article nicely goes into some details.

oh and combine the 99.9999999% availability "nine nines" with insane throughput. as in real time phone wiretapping throughput, or real time mass financial transactions, of course.

or a web server for an online image service.

or "your personal web server in a mouse click", sharing 10.000 such virtual machines on a single physical machine. which has a shared read only /ist partition mounted into all guests. not containers, no, virtual machines, in ca 2006...

"don't trust a computer you can lift"

I used to work for Citrix, which is "software that turns Windows into a mainframe OS". Basically, you get remote thin terminals the same as you would with an IBM mainframe, but instead of showing you green text you get a Windows desktop.

Citrix used to sell this as a "cost saving" solution that inevitably would cost 2-3x the same as traditional desktops.

The real benefit for both IBM mainframes and Citrix is: latency.

You can't avoid the speed of light, but centralising data and compute into "one box" or as close as you can get it (one rack, one data centre, etc...) provides enormous benefits to most kinds of applications.

If you have some complex business workflow that needs to talk to dozens of tables in multiple logical databases, then having all of that unfold in a single mainframe will be faster than if it has to bounce around a network in a "modern" architecture.

In real enterprise environments (i.e.: not a FAANG) any traffic that has to traverse between servers will typically use 10 Gbps NICs at best (not 100 Gbps!), have no topology optimisation of any kind, and flow through at a minimum one load balancer, one firewall, one router, and multiple switches.

Within a mainframe you might have low double-digit microsecond latencies between processes or LPARs, across an enterprise network between services and independent servers its not unusual to get well over one millisecond -- one hundred times slower.

This is why mainframes are still king for many orgs: They're the ultimate solution for dealing with speed-of-light delays.

PS: I've seen multiple attempts to convert mainframe solutions to modern "racks of boxes" and it was hilarious to watch the architects be totally mystified as to why everything was running like slow treacle when on paper the total compute throughput was an order of magnitude higher than the original mainframe had. They neglected latency in their performance modelling, that's why!

Note that Citrix or any similar Windows "terminal services" or "virtual desktop" product fills the same niche as ordinary web applications, except that Win32 GUI apps are supported instead of requiring a rewrite to HTML. The entire point is that existing apps can be hosted with the same kind of properties as a web app, minus the rewrite.