Is there any good use case of having ssds in a data center, if you did not care about cost?

Input/Output rate, bandwidth and IO roundtrip delay.

* even the slowest SSDs have significantly higher I/O rates than the best mechanical drives, and the comparison between best-in-class mechanical and enterprise-class PCIe SSDs is just ridiculous: a 15K SAS drive will do 200 IOPS, a high end SSD will do a million

* 15K SAS drives will top out around 250MB/s on bulk sequential reads (that's a best-case scenario), high-end PCIe SSD are in the 2.5GB/s range

* HDDs have a latency of 10~20ms, SSDs have a latency of 100~200µs (RAM has a latency of ~100ns)

They're used in DCs plenty when speed is required

Yep, I work for a very large government department here in AU and we have a tiny bit of SSD in our DC for the stuff that really needs it.

It's probably not 5% of our total storage, though.

IOPs

They had some in some of their other reports but too small a population it would seem...