The big advantage should be reliability, reliability and more reliability.  And smaller floorspace/power/cooling needs compared to a large number of racks of lesser machines, which over time can (I am told) be a significant benefit.

For development, and maybe for rapidly adding new 'servers' if you havs a 'one server per customer' environent, creating extra virtual linux machines would also be very handy, I suspect.

You'll quite probably be the first person in the universe to port AOLserver to Linux/S390, as well as trying out OpenACS there.  Sounds like a lot of fun   Make sure you post the results of your experiment here!

The only possible 'problem' I can think of would be that you're running a newer Linux port, will need to recompile some software on that new platform, and there are less people with experience of Linux/S390 than Linux/x86 you will be able to rely on for help.

You might be in for a milk run. Postgres 7.1, and its associated support packages, are already ported; tcl8.3 is ported; perl is ported; and I'm quite certain I saw a port of aolserver3.2 a few week back -- but I can't find it now. Regardless, the port may be fairly simple. AOLserver already runs on Alphas, so it's 64-bit clean, and the S/390 groups are fairly active so I'm guessing help could be forthcoming.

In re: to your questions, there are a number of power & cost reasons to use a mainframe rather than a PC farm, but if you're dependent on some i386 closed-source stuff, you don't have that option. And of course, the hardware is absurdly stable (even after 20 years, I should point out. Though by that time you'll wish it wasn't.).

I've been trying to get some time on that shared IBM machine for a couple months now. I have an unhealthy interest in what AOLserver, on a mainframe, could do on the SpecWeb99. If you get the chance, show us some numbers!

Estimates are that about 75 percent of all business data worldwide still resides on mainframes and 85 percent of all business transactions are carried out on mainframes.
The mainframe world is driven by increased usage of digital services that constitute more and more of the world economy. These services are more often than not run by mainframes in a central data center setting. These demands are subject to the fluctuations of web-driven demand and therefore need the strengths that only the mainframe can deliver: reliability, availability and scalability all wrapped in the most secure computing environment.

The mainframe is a self-contained processing center, powerful enough to process the largest and most diverse workloads in one secure “footprint.” The mainframe is also just as effective when implemented as the primary server in a corporation’s distributed server farm.