What’s In My SSD? SLC, MLC, or TLC?

Someone recently asked The SSD Guy if there is a way to determine whether an SSD is SLC, MLC, eMLC or TLC.

I found it a little odd to be asked this, since most vendors tell what kind of flash they use in an SSD’s specifications, especially if it’s SLC.

Not finding it there then the next thing I would look at is the price.  Raw SLC NAND flash now sells for about 6-10 times as much as its MLC counterpart, so an SSD with a price of around $1/GB is likely to be MLC and one that sells for around $10/GB is probably SLC.

TLC SSDs are really rare.  There is the 840 from Samsung and the Ultra II and X300 from SanDisk.  It’s also pretty rare to find an SSD that is based on eMLC, because eMLC achieves its higher endurance by slowing the part down.  There are other, better ways to extend endurance.

Now that you know all this, please note that the kind of flash used to produce the SSD is probably unimportant.  Certain MLC SSDs outperform some SLC SSDs, both in speed and in endurance – it all has to do with the quality of the controller.  Some useful comparisons of SLC and MLC SSD performance are in the post: “Not All SSDs are Created Equal“.  There’s a thorough explanation of controller techniques in The SSD Guy’s series on SSD controllers.

Furthermore, NAND chips allow controllers to change flash pages inside the chip from TLC to MLC to SLC as needed, so some SSDs actually internally manage hot data to SLC and cooler data to MLC or TLC.

For those that have read this far, I would recommend changing your focus away from questions of SLC vs. MLC vs. TLC and focus instead upon the SSD’s published specifications (like speed and endurance) and on its SMART attributes.  The SMART attributes will always tell you how your SSD is wearing, and the specifications will tell you how it is expected to perform.

If you don’t trust the manufacturer’s performance specifications, then I highly recommend using the SNIA performance test specification, which gives unbiased performance results for any SSD.