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resetting the assumptions about flash endurance

by Zsolt Kerekes, editor - July 25, 2011

Recently I had a long discussion with STEC about how they manage MLC flash inside their enterprise and industrial SSDs and what's different about what they do.

To be honest I wasn't expecting to hear anything new. It's over 30 years since I came across the concept of wear-out in non volatile semiconductor memories in my previous career as an electronics engineer.

And it's over 6 years since published the first white paper devoted exclusively to the subject wear leveling (written by SiliconSystems - now part of WD).

Nowadays rarely a week goes by without some mention of subtle factors in dealing with MLC speed and reliability from memory chip makers, SSD controller companies and even systems companies.

But STEC is a significant SSD company so I thought I'd be polite and listen.

My first impression during STEC's presentation was - this is just a comforting sounding bunch of new brand names for techniques which are already well established and which other controller companies have already been doing for years.

But I was at first surprised and then delighted to see I was wrong.

STEC has discovered that MLC flash endurance is a more elastic concept than most experts believe.

By using back doors into the flash chips and studying this problem for many years STEC has discovered that some ways of managing industry standard MLC result in much better endurance, sustained performance and data reliability than simply relying on wear leveling protocols. It's the how you write and not just how much you write that makes the new difference.

I find it hard staying quiet long when people are talking about SSDs - so pretty soon I was asking lots of questions. The main one being - why have you been keeping this stuff secret? Everyone should know about this....

Soon everyone will.

STEC has lifted the NDA on the presentation they gave me - from which the images and highlights below are taken.

notes adapted from an STEC presentation about true enterprise MLC SSD technology
  • A growing number of enterprise applications require highly reliable, high-performance, long-lasting solid-state drives at a lower cost than today's SLC flash-based drives.
  • Deployment of standard consumer-grade or so-called enterprise MLC (eMLC) in SSDs for enterprise applications do not meet requirementsfor performance, endurance, and reliability required.
STEC's CellCare™ Technology Fills the Gap Between eMLC and SLC Flash

STEC says that so-called "enterprise grade" MLC has weaknesses when used in enterprise apps.
  • can't guarantee 10x full disk writes per day for 5 years - which is a rule of thumb requirement for typical enterprise SSD storage. (Some SSD storage in cache apps need much more endurance than this figure - but this can be accomodated using overprovisioning, RAM cache or SLC.)
  • long term availability - eMLC is currently a marketing / process tweak / selection from memory suppliers which provides endurance at an intermediate between SLC and consumer MLC. These tweaks may not work or deliver usable enterprise MLC in future product generations - as geometries move from 3x to 1x nanometer.
  • data integrity in eMLC worsens with wear - which can result in dramatic read performance degradation after several years use in customer systems
  • eMLC costs more than consumer grade MLC. A technology like CellCare can potentially deliver lower cost enterprise SSD capacity - particulalry when it's packaged in an ASIC rather than FPGA.

STEC has been collecting data about flash cell data integrity, performance and endurance for several years.

This led the company to conclude that conventional wear leveling schemes don't work as well as its own technologies.

For more info about STEC's CellCare™ Technology - take a look at STEC's whitepaper (pdf)

To see what other leading enterprise SSD makers have to say about the issue of SLC vs MLC vs eMLC take a look at these articles.
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