What eMLC and eSLC do for SSD longevity

Enterprise NAND from Micron.com (c) 2010 Micron Technology, Inc.

Enterprise NAND from Micron.com (c) 2010 Micron Technology, Inc.

I talked last week with some folks from Nimbus Data who were discussing their new storage subsystem.  Apparently it uses eMLC (enterprise Multi-Level Cell) NAND SSDs for its storage and has no SLC (Single Level Cell) NAND at all.

Nimbus believes with eMLC they can keep the price/GB down and still supply the reliability required for data center storage applications.  I had never heard of eMLC before but later that week I was scheduled to meet with Texas Memory Systems and Micron Technologies that helped get me up to speed on this new technology.

eMLC/eSLC defined

eMLC and its cousin, eSLC are high durability NAND parts which supply more erase/program cycles than generally available from MLC and SLC respectively.  If today’s NAND technology can supply 10K erase/program cycles for MLC and similarly, 100K erase/program cycles for SLC then, eMLC can supply 30K.  Never heard a quote for eSLC but 300K erase/program cycles before failure might be a good working assumption.

The problem is that NAND wears out, and can only sustain so many erase/program cycles before it fails.  By having more durable parts, one can either take the same technology parts (from MLC to eMLC) to use them longer or move to cheaper parts (from SLC to eMLC) to use them in new applications.

This is what Nimbus Data has done with eMLC.  Most data center class SSD or cache NAND storage these days are based on SLC. But SLC, with only on bit per cell, is very expensive storage.  MLC has two (or three) bits per cell and can easily halve the cost of SLC NAND storage.

Moreover, the consumer market which currently drives NAND manufacturing depends on MLC technology for cameras, video recorders, USB sticks, etc.  As such, MLC volumes are significantly higher than SLC and hence, the cost of manufacturing MLC parts is considerably cheaper.

But the historic problem with MLC NAND is the reduction in durability.  eMLC addresses that problem by lengthening the page programming (tProg) cycle which creates a better, more lasting data write, but slows write performance.

The fact that NAND technology already has ~5X faster random write performance than rotating media (hard disk drives) makes this slightly slower write rate less of an issue. If eMLC took this to only ~2.5X disk writes it still would be significantly faster.  Also, there are a number of architectural techniques that can be used to speed up drive write speeds easily incorporated into any eMLC SSD.

How long will SLC be around?

The industry view is that SLC will go away eventually and be replaced with some form of MLC technology because the consumer market uses MLC and drives NAND manufacturing.  The volumes for SLC technology will just be too low to entice manufacturers to support it, driving the price up and volumes even lower – creating a vicious cycle which kills off SLC technology.  Not sure how much I believe this, but that’s conventional wisdom.

The problem with this prognosis is that by all accounts the next generation MLC will be even less durable than today’s generation (not sure I understand why but as feature geometry shrinks, they don’t hold charge as well).  So if today’s generation (25nm) MLC supports 10K erase/program cycles, most assume the next generation (~18nm) will only support 3K erase/program cycles. If eMLC then can still support 30K or even 10K erase/program cycles that will be a significant differentiator.

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Technology marches on.  Something will replace hard disk drives over the next quarter century or so and that something is bound to be based on transistorized logic of some kind, not the magnetized media used in disks today. Given todays technology trends, it’s unlikely that this will continue to be NAND but something else will most certainly crop up – stay tuned.

Anything I missed in this analysis?

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