optics

Modules do not fail randomly at three times a published rate, so the excess has a cause and it is usually thermal or a batch. The cohort cuts that localize it, the counters that separate a dying module from a dirty connector, and the environmental cause that looks like a hardware defect.

Port counts decide everything: a two-tier Clos of 64-port switches tops out at exactly 4,096 endpoints at full bisection, which is why that number appears in so many cluster designs. The switch and optics count both ways, what oversubscription actually slows down, and where the money goes.

Past the two-tier limit there are two ways forward and they are not close. One triples the switch count and adds fifty percent more optics for two extra hops of latency; the other is the same shape with bigger switches. The counts for both, and the reason the industry follows switch radix.