Make Mine a Single

by Ralph Green, Jr

USB Pen drives are widely used. I carry one or two of them with me most of the time. There has been a change in the drives in the last couple of years and that may affect how you should select and use these drives. It is something called Multi Level Cell or MLC.

When these drives were introduced, I remember seeing statements from the manufacturers that data on the drives was guaranteed for up to 10 years. I don't see those notices any more and there is a reason. Early drives stored one bit of data in each cell on the chip of the flash drive. The older flash technology is called SLC or Single Level Cell. A 1 gigabyte drive would have 8589934592 bits. For my examples, I will use numbers based on a 1 gigabyte drive. There are also error correction bits stored and you don't really get 1 gigabyte of storage on 1 gigabyte drive, but that should be close enough for this article.

For all flash devices, when the cell is written, what happens is that a certain voltage value is put in a cell to encode its value. Nominally, this would be 5 volts for a 0 and 0 volts for a 1. On an SLC drive, you would need 8589934592 cells to store the data. When it comes time to read back the data you wrote, the circuit reads the voltage value in each cell and determines if it is a 0 or a 1. The exact threshold varies, but generally if the voltage is over about 4 volts, it is deemed to be a 0 and if it is that voltage or lower, it is deemed to be a 1. You may ask why a value of 5 volts is not used, since that is what we stored. The voltage of the cell starts to drop as soon as it is stored. This is a fairly slow process, which is what makes flash memory practical at all. But, it does happen and it is the root of my concern about MLC.

On MLC memory, multiple bits are encoded into a single cell. I'll use a 2 bit cell because those are most common as I write this, but I read about 3 bit cells. For a flash drive with 2 bit cells, you would need only 4294967296 cells for a gigabyte. 2 bits would be encoded using something close to the following table.

  Bit value    Voltage
    00          Greater than 5.5 V
    01          4 V to 5.5 V
    10          3.5 V to 4 V
    11          Less than 3.5 V

You see there is less wiggle room on this circuit. As the voltage drops over time, the bit values will change. That happens on SLC, too. But, MLC will be worse because the voltages are closer together. It seems likely to me that the time I can keep a USB drive in a drawer with an archive of my family photos is going to be a lot less if the drive uses MLC cells. I have not found a good longevity study, but I am looking. This technology is too new for any such studies to be real, anyway. They will have to model the decay in some way and make projections.

There are other downsides to MLC cells in flash memory. It is slower than SLC and it is less durable. The speed advantage of SLC has kept some SLC in production. In applications where that speed matters, SLC is used. One of the declining operating systems supports something they call ReadyBoost. This uses a USB drive plugged into the computer as a form of cache to help make it tolerable in performance(It is intolerable is many other ways, but that is another article). ReadyBoost needs fast drives. At one time, all USB flash drives that were marked ReadyBoost were also SLC. That does not seem to be the case any more. When I say durable, I am referring to how many times data can be written to the drive. MLC memory generally can handle about 10000 write cycles and SLC can handle 100000 write cycles.

I am not suggesting that MLC technology is bad. I think you just need to understand its parameters and use it where it is appropriate. Toshiba estimated that 10000 cycles was enough for many years of usages for SD cards in digital cameras. They are probably right and I don't hesitate to use MLC based SD cards in my camera. I don't plan to leave my only copy of pictures in the camera for years, and so this seems like a good application for MLC to me. The lower cost of MLC based cards means my camera can hold more pictures than I can take on any trip and a spare card is just a few dollars.

If you are looking for a drive with SLC memory cells, a 2 GB drive marked ReadyBoost seems like a good candidate. I have only seen one drive promote itself as using SLC. That is the IronKey drive. Their product has several advantages over the cheap drives most people sell, but they are too expensive for me, at this time. Be careful what you buy, if this matters to you. I have seen reports of a Transcend drive that used to advertise as SLC. The same model and capacity now comes with MLC memory. I recently ordered 5 Memorex M-Flyer drives that are listed as being SLC memory. I plan to take one apart and see if I can tell if that is true.

Bibliography

1. SLC vs. MLC: Whitepaper

 http://74.125.95.104/search?q=cache:FZ-5UyRXm8AJ:www.supertalent.com/datasheets/SLC_vs_MLC%2520whitepaper.pdf+MLC+SLC+flash&hl=en&ct=clnk&cd=3&gl=us&lr=lang_en(approve sites)

2. SLC vs. MLC: An Analysis of Flash Memory

 http://www.oempcworld.com/support/SLC_vs_MLC.htm

3. Choosing flash memory, by Brian Kumagai and Scott Beekman of Toshiba America Electronic Components

  http://www2.electronicproducts.com/Choosing_flash_memory-article-TOSHIBA-apr2004-HTML.aspx