However, there is an alternative type of flash memory, called Multiple Level Cell (MLC), that uses the same basic transistor but the number of electrons on the floating gate, and thus the threshold voltage, can be controlled more accurately. By cutting up the voltage range into more slices, more bits can be stored in each cell. In some cases this can be as many as 3 bits (8 voltage levels) but the most common is the 2 bit (four level) cell.
Thus, MLC technology is able to store more information per cell than SLC, resulting in a lower cost per megabyte and generally happy faces all round.
Unfortunately, there are a number of drawbacks to MLC memory.
First, the floating gate’s ability to store electrons deteriorates each time the transistor is charged and discharged. Eventually this will lead to the threshold voltage no longer reaching its trigger point so the cell will never register as being charged and the cell will not store data properly.
For SLCs it takes around 100,000 charge/discharge cycles before errors can occur but because the gap in threshold voltages between each state in an MLC is much smaller, they have considerably less tolerance to deterioration. The result is MLCs can only last around 10,000 charge/discharge cycles before data corrupts.
Also, because of the more delicate way in which charging must be controlled, it takes longer to program an MLC, resulting in slow write speeds for MLC memory devices. It’s worth noting, then, that Intel’s E drives will use SLC memory and its M drives, MLC memory, which explains the performance figures stated on the previous page.
Having seen that each cell can potentially last only 10,000 cycles, I’m sure alarm bells are ringing for a few of you. However, you shouldn’t be worried because by using some sophisticated data distribution and error checking algorithms, Intel ensures its drives can write 100GB of data everyday for five years before errors will occur, though they only warrant the drives for three years.
So, that’s the basic SSD theory and it’s largely the same for any of the SSDs out there but what’s really interesting is what extra expertise Intel has brought to the party. Whereas most SSD manufacturers just make the memory and buy in a controller chip (or in the case of companies like OCZ, they buy in the whole lot, give it a tweak, and rebrand it), Intel has the expertise to design and make every aspect of its SSDs from the ground up.
The result is the near elimination of one of the most common problems with existing SSD solutions – the controller just can’t keep up with the read and write commands it is dealing with. This scenario manifests itself as random pauses when doing seemingly innocuous tasks, which can make the experience of using an SSD little better than a conventional hard drive.
By using its own super-fast controller, Intel ensures that its SSDs will always exhibit that snappiness that is the biggest boon of SSDs in the first place.