Phase-change prototype claimed to be world's fastest SSD

We've been hearing about phase-change memory (PCM) for years. The non-volatile storage technology seems to be inching closer to prime time, though, and the latest implementation looks very impressive indeed.

The research division behind Hitachi Global Storage Technologies (HGST) is demoing a PCM-based solid-state drive at the Flash Memory Summit in Santa Clara, California this week. The phase-change SSD is claimed to be "orders of magnitude" faster than NAND-based drives, and HGST has the numbers to prove it. The SSD reportedly crunches three million random read IOps in a "queued environment." When requests are coming one at a time, the random read latency is said to be less than 1.5 microseconds. To put those figures in perspective, consider that Intel's datacenter-oriented DC P3700 SSD is rated for 450k random read IOps and a random read latency of 115 µs.

So, yeah. Phase-change memory is fast.

Details about the PCM drive's write performance are tellingly absent from HGST's press release. However, in a paper published earlier this year (PDF), HGST researchers pegged the write latency at "about fifty times longer than reads at current lithography limits." Write performance is "already comparable with NAND flash," the paper said, and it's "expected to improve further with advances in lithography."

The phase-change memory used in HGST's demo drive is built on a relatively coarse 45-nm process. Each die has only 1Gb (128MB) of storage, which is a fraction of the capacity of modern NAND. There's no word on the origin of the chips, but they probably come from Micron. The research paper mentions Micron memory specifically.

Although the PCM prototype is built on a Gen2 PCIe x4 interface, it doesn't rely on the NVM Express protocol. NVMe can't keep up with phase-change memory's wicked-fast read latencies, according to HGST, so the firm designed its own "low-latency interface protocol." Dubbed DC Express, this protocol is the subject of the aforementioned research paper.

HGST's technology demonstration shouldn't be confused with a product announcement. However, the fact that phase-change memory is fit for demos—and is this fast—certainly bodes well for the future.

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