I suspect that was a battery-backed RAID controller cache, which is pretty common in all modern servers or SAN disk arrays.
Compared to that, the future looks amazing: Intel Launches Optane DIMMs Up To 512GB: Apache Pass Is Here!
This is just the beginning! Pretty soon you'll be seeing practically everyone running databases with 100% of the I/O coming directly from these. Compared to that, NVMe flash storage at a "mere" 3500 MB/s looks glacially slow!!
Somewhat ironically, the great work that has been done by the various C#, Java, JavaScript, and Rust core developers bringing asynchronous I/O programming to the masses is now hampering performance in this shiny new world of non-volatile memory. The overhead of even the most efficient futures-based async I/O is simply massive in comparison to the latency of NVDIMMs, which are byte-addressible and are only 2-3x slower than normal DIMMs. When your storage latency is measured in nanoseconds, individual instructions matter and user-to-kernel transitions are murder...