The Register have put together a nice graphic and table displaying current storage technologies and how they relate to each other.  They constructed the graph to demonstrate the major boundaries in storage, between cache/memory, local storage and external storage and how these are going to move thanks to new technology.  NVMe-over-fabric will enable companies to utilize external storage at latencies lower than internal storage that still uses SATA or SAS, with only pure PCIe local storage outpacing its potential.  X-Point, assuming it lives up to the hype, will blur the line between local storage and memory/cache storage, offering latency previously only seen in system memory or on-die cache.

They also provide a table to give you some rough ideas how this translates between storage media, normalizing it a theoretical task which would take L1 cache 1 second to access, this can make it somewhat easier to comprehend for some than nanoseconds.

"Two technology changes are starting to be applied and both could have massive latency reduction effects at the two main storage boundary points: between memory and storage on the one hand, and between internal and external, networked storage on the other."

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