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MLC Flash Versus SLC Flash

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EMC’s recent announcement at EMC World of Project Lightning documents a program to increase the use of flash devices in enterprise storage. The project includes increased use of flash storage in EMC arrays, all-flash storage configurations, and support for Multi-layer Cell (MLC) flash. This last subject–MLC flash and its difference from SLC flash–piqued my curiosity.

Many years ago I studied electrical engineering. I was an awful at it. Analog was never my thing. I much prefer ones and zeroes. But I challenge myself to think about electronics once every blue moon. So I decided to delve into SLC and MLC flash technologies to understand how they differ and why we should care.  The content below summarizes my online research and the little bit I remember from school. If you can add, correct, or update this article I would be happy to have your comments.

What is the Difference Between MLC and SLC Flash?

MLC flash uses many discrete voltage levels to store multiple values, or bits, per cell [1].  Single-layer Cell (SLC) technology uses fewer voltage levels to program a single bit of information to the cell.  MLC technology obviously produces greater density which means it stores more data cheaper.  But the higher density comes with a cost: MLC produces storage that is more sensitive to temperature changes, slightly slower, and more likely to fail than SLC flash.

SLC flash is ten times the endurance for write/erase operations [2].  At an average of 10,000 write/erase cycles an MLC flash cell will die.  SLC flash cells can sustain an average of 100,000 write/erase cycles.  But why do MLC flash cells fail more than SLC?

Why Do MLC Cells Fail More Than SLC Cells?

I am unable to find an answer to this anywhere on the web.  If you see one, I would love to read it.  But in the dark, dusty corners of my memory I remember enough about electronics to hazard a guess at this.  As I see it, there are two reasons why MCL flash should fail more than SLC: one reason is a statistical and the other is electronic.

The statistical argument is that MLC cells are being written two 50% more than SLC cells.  They will simply wear out sooner. An SLC cell is storing the value of zero or one.  When an application writes to the data being held by that cell there is a 50% chance that the cell’s value has changed and it requires reprogramming.  Because MLC flash stores two bits, there is a 75% chance that the new two-bit data differs from the existing value.  This means an MLC is written to 0.75 times for each 0.5 times an SLC cell is written. That’s a 50% increase.

The electronic argument is based on MLC flash programming requiring a wider range of voltages [3].  Higher voltages produce greater amperage.  This exacerbates electromagnetic migration.  And the higher voltage on the transistor’s gate will increase erosion of the polysilicon that separates the gate and the channel.  Both of these will result in circuit failure.

How Is MLC Being Made More Reliable?

Because MLC is so much more cost effective than SLC, industry innovation is improving MLC reliability.  Here are a few techniques I found online [4]:

  • Hardware can level writes, which distributes writes throughout the device to avoid balance cell overuse and avoid hotspots.  This means an entire flash drive will tend to fail at once after a long time.  This as opposed to a small number overworked hotspots failing quickly.
  • Hardware can include DRAM cache which can be used to coalesce writes, which decreases cell write count.
  • Flash devices can be over-provisioned for error detection, correction, and dynamic bad cell replacement.
  • There are also a variety of proprietary techniques from flash manufacturers.

One challenge with flash today is the lack of consistent and objective endurance measurements.  It is difficult for storage vendors to publish availability guarantees when the reliability of the underlying media is uncertain.  This means to support flash devices in its VMAX arrays–which are rated at six nines (99.9999%) availability–EMC has to do a tremendous amount of qualification of the devices. This qualification process should always mean that flash support in enterprise storage should consistently lag its support in consumer devices, where availability requirements are much lower.

Summary

No one denies that SSD storage is becoming more common in the enterprise. EMC’s support of MLC devices is only one of the items introduced by Project Lighting that will increase flash presence, producing better performing and more efficient storage. If you are interesting in learning more on the subject, follow the links below to the sources for this article. Also considering Googling “tlc flash” to see the higher density, less reliable Triple-layer Cell (TLC) that will certainly find its way to the enterprise after more innovation.

References

My information came from documents I found as a result of Google searches. Here are my recommendations for further reading.

  1. http://www.smxrtos.com/articles/mlcslc.htm
  2. http://www2.electronicproducts.com/Choosing_flash_memory-article-toshiba-apr2004-html.aspx
  3. http://www.supertalent.com/datasheets/SLC_vs_MLC%20whitepaper.pdf
  4. http://www.infostor.com/index/articles/display/1169849064/articles/infostor/disk-arrays/disk-drives/2010/july-2010/mlc-vs__slc_flash.html

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