I'm completely fascinated by the diagram. In a four rack system, 2.5 rack is dedicated to I/O, half a rack is just empty and the remaining is the actual processing and memory.
The I/O probably isn't endless networking adaptors, so what is it?
Probably channels. In an IBM mainframe, each I/O device is connected on its own channel, which is actually a separate computer that handles the transfer to/from the main CPU. This has been the case going back to the System/360, which is why mainframes are legendary for their transaction throughput and reliability. There's probably a lot of redundancy in the I/O hardware, as they have to be rock solid and hot swappable while the system is running.
“The IBM z17 supports a PCIe I/O infrastructure. PCIe features are installed in PCIe+ I/O drawers. Up to 12 I/O drawers per IBM z17 can be ordered, which allows for up to 192 PCIe I/O and special purpose features.
For a four CPC drawer system, up to 48 PCIe+ fan-out slots can be populated with fan-out cards for data communications between the CPC drawers and the I/O infrastructure, and for coupling. The multiple channel subsystem (CSS) architecture allows up to six CSSs, each with 256 channels.
The IBM z17 implements PCIe Generation 5 (PCIe+ Gen5), which is used to connect the PCIe Generation 4 (PCIe+ Gen4) dual port fan-out features in the CPC drawers. The I/O infrastructure is designed to reduce processor usage and I/O latency, and provide increased throughput and availability.”
There's also the problem in that they need to take into account floor loading. They're not going to tell a customer upgrading from an older machine to a new one that, "oh, by the way, the rack weighs twice what it used to, so you'll need to upgrade your floor while you're at it." Especially important for raised floors.
The I/O probably isn't endless networking adaptors, so what is it?