[reacharavindh]

I’ve had a few good early years working at the system level z/OS, z/VM and the mainframe hardware from z10 era.

The reliability is indeed legendary for the usecases it is originally designed for (running Z/OS or TPF, IBM DB2 on Z/OS, CICS and COBOL batch jobs). However, IBM marketing folks will try to sell you on specialty processors that can run Java applications, Linux VMs(s390 arch) etc - that’s where the reliability rails come off.

Most serious mainframe users I worked with had their legacy applications which did one of the original usecases I mentioned, and it runs reliably. At the hardware level, redundancies are engineered at every level. You can hit plug CPUs like blades on a running system for maintenance, and replace them. Same with memory modules, storage devices etc. Upgrades to z/OS are also so thoroughly documented that you can avoid downtimes or plan for minimal ones..

[nuc1e0n]

Using Xen (https://xenproject.org/), you can live migrate running Virtual Machine containers from one physical computer to another on commodity PC hardware while continuing to serve requests (no down time). You can then turn off the computer the VM was originally running on, upgrade it then migrate the VM back to it again. I did this once while pinging the VM from another machine. It didn't even drop any packets. My jaw dropped though.

[jacooper]

Proxmox also does this

[PaulHoule]

The "specialized processor" is a different microcode for the same CPU. My understanding is that this microcode has disabled a few instruction so z/OS won't run but doesn't really do anything special for Java like

https://en.wikipedia.org/wiki/Jazelle

The point is that Java or Linux workloads could be run on some other CPU and face competition but this is not the case for z/OS. Thus there is a reason to lower the price for workloads that could be easily migrated but keep it high for captives.