Understanding SLC and MLC

What does make NAND tick?

The landscape of solid-state drives is growing. Many companies, Intel included, have a growing portfolio of drives for sale. Various people much smarter than I have talked about how "Not all SSDs are created equal," but there's one fundamental difference at play that most technologists understand, and not all consumers do. In fact, it's a difference that's pivotal to the difference in the Intel X25-E and X25-M product differentiations: the underlying flash technology. Hopefully, for those who don't know the difference between SLC and MLC flash, this can help explain the difference between the two, and why a drive with less than half the density can cost twice as much.

Imagine yourself with a glass of water... or even better, a shotglass. Now, draw a line right in the middle of the shotglass. If you fill the glass past that line, it's "full," if the water level is below the line, it's "empty." That, in essence, is how flash memory works. The glass is just a transistor, and the water, electrons. Now how much time do you think it would take to fill up that shotglass? Not much at all; you don't have to be too accurate if you're just filling it up or emptying it, and looking to see if it's full or empty is pretty easy, too.

Now let's take that same shotglass and draw three lines up the side. The glass now can be empty, a third full, two-thirds full, or completely full. Now try filling the glass between lines two and three. It isn't that hard, but I bet it would take you a little longer, and you'd be a bit more precise when filling it up.

These two analogies are exactly what's different between the two types of flash. The same shotglass (or cell), is holding either a 1 or a 0 in SLC, or 11, 10, 01, 00 in MLC. It takes a bit more work to fill that second glass. It's not that hard to read between the lines, but filling... that's a harder story.

So why not always use MLC if it costs half as much? Well, besides taking a little more care filling, what about reliability? Let's take charge-loss as one event that's easy for me to illustrate. If we take the same glasses of water, and let time take its toll, we'll have some evaporation happening. Back to the full/empty glass, to get evaporation to empty the glass will take some time, but on the second glass, just a little bit of evaporation can move the glass from full to two-thirds full. The same thing can happen in a flash cell, where escaping electrons can move the bit from one state to another. There's more headroom in the SLC, but in the MLC, to move from 00 to 01 is more possible. Thankfully for us, a lot of engineers spend their time protecting the products from these occurrences and designing reliability into a drive to compensate for this, but the inherent design of SLC is more reliable.

So now what? Do you not buy an MLC drive? Trust me, it's good enough for what 99% of us do, and I personally would rather have the extra gigabytes, since my iTunes playlist doesn't demand the extra fortification SLC brings. But if I were building a database server, it'd be a different story.

Couldn't put it better myself. There's a slight problem with the "MLC is good enough for 99% of us do".
While MLC is likely to be the type of technology that will be mainstream for the, yet to come, golden age of flash memory, the reductions in process technology causes the number of erase cycles on NAND flash to go down to a rather dramatic level, as density continues to increase. An OCZ drive Solid drive has Samsung NAND chips that are from a newer revision that saw it's number of erase cycles reduced from the regular 10.000 to 5.000 cycles, a worrisome drop in lifespan.
While most current NAND chips have extra cells for when the other ones get bad, there's still a whole lot of technological evolution to go through before we actually can assert if MLC is here to stay or if it's just a stop gap while SLC chips don't have the right amount of capacity for most storage purposes and price.

Source: Intel