NVDIMM: Nonvolatile... Not NVIDIA

Subject: General Tech, Memory, Systems | February 10, 2013 - 03:44 AM | Scott Michaud
Tagged: NVDIMM, micron, IMFT NAND, imft

So a RAM chip, a NAND module, and an “ultracapacitor” walk into stick...

This week Micron released a press blast for technology called, “NVDIMM”. The goal is to create memory modules which perform as quickly as DRAM but can persist without power. At this point you could probably guess the acronym: Nonvolatile Dual In-line Memory Module. It has been around for a few years now, but it is in the news now so let's chat about it.

I often like to play the game, “Was this named by an engineer or a marketer?” You can typically tell who was responsible for naming something by gauging how literally it breaks down into a simple meaning versus not having any apparent meaning at all. A good example of an engineer name is UHF, which breaks down into ultra-high frequency because it's higher than VHF, very-high frequency. A good example of a marketing name would be something like “Centrino”, which sounds like the biggest little penny-slot machine in the world. I would quite comfortable guessing that NVDIMM was named by an engineer.

This is AgigA Tech's module, who provides the capacitors for Micron and their NVDIMMs.

The actual makeup of NVDIMMs is quite sensible: DIMMs are fast but die when the power goes out. You could prevent the power from going out but it takes quite a lot of battery life to keep a computer online for extended periods of time. NAND Flash is quite slow, relative to DIMMs, in normal operation but can persist without power for very long periods of time. Also, modern-day capacitors are efficient and durable enough to keep DIMMs powered for long enough to be copied to flash memory.

As such, if the power goes out: memory is dumped to flash on the same chip. When power is restored, DIMMs get reloaded and continue on their merry way.

According to the Micron press release, the first NVDIMM was demonstrated last November at SC12. That module contained twice as much NAND as it did DIMM memory: 8GB of Flash for 4GB of RAM. Micron did not specify why they required having that much extra Flash memory although my gut instinct is to compensate for write wearing problems. A two-fold increase to offset NAND that had just one too many write operations seems like quite a lot compared to consumer drives. That said, SSDs do not have to weather half of their whole capacity being written to each time the computer shuts down.

Who knows, double-provisioning might even be too little in practice.