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TLC NAND SSDs: The crippling problem storage makers don't advertise

Jon L. Jacobi | Nov. 10, 2015
What you need to know about the new breed of SSDs.

Credit: Gordon Mah Ung

With last week’s release of Crucial’s BX200 SSD, a drive that features TLC (triple-level cell) NAND, it’s time to shine a light on this burgeoning segment of the SSD market—especially as vendors happily quote numbers that would have you believe that these SSDs perform just like any other.

Most of the time TLC SSDs perform quite well. But copy a large amount of data to a TLC drive, and part way through the operation you’ll see something discomforting—a startling drop in write speed. With some drives it’s relatively mild, but in the case of many recent TLC drives, the drop is so drastic you’ll wonder if the SSD is dying. It’s not, but you may wish it was.  

The cache catch

Over the last few years, SSDs have gained a reputation as a universal kick in the pants for system performance, providing super-fast random access times and three times or more the sustained write speed of a hard drive. The latter makes dealing with large files and backups far less tedious.

But that assumes that the SSD uses SLC (single-level cell/one-bit) or more commonly, MLC (multi-level cell/two-bit) NAND, and not the relatively new kid on the block, triple-level cell or TLC NAND. TLC can store three bits per cell for greater data density and more capacious SSDs in the same form factor, but its sustained write performance is comparatively poor—in some cases exceedingly.

To overcome TLC’s slothful write performance, vendors use a healthy dose of SLC—or TLC treated as SLC in Samsung’s case as it’s the writing three bits that’s slow—and possibly some DRAM as a far faster cache. As long as the amount of data being written to the drive fits in the cache, all is well, and you’ll get the performance vendors claim.

But the second the data overflows the cache, you’re writing directly to the TLC, and performance suffers accordingly. In some cases, such as with Samsung’s 840 (depending on capacity) and 850 EVO, the drops are comparatively mild—from 400MBps to 300MBps in our 20GB copy tests.

In other cases the drop is astonishing (and painful). As you can see below, the Crucial BX200’s sequential write performance skids from 400MBps to 80MBps when the cache is overwhelmed. Gulp. 

bx200 480a and 960
Performance hits the skids when the amount of data exceeds the size of the Crucial BX200’s cache. 6GB for the 480GB model and 12GB for the 960GB models. Click on image to enlarge.

Additionally, smaller capacity drives sport smaller caches and you’re writing to the TLC that much sooner. In the BX200’s case, the 960GB model has a 12GB cache, the 480GB model has 6GB, and the 240GB model only 3GB. As you can see above, performance tanks right on cue at the 6GB mark for the 480GB BX200 and the 12GB mark for the 960GB BX200. The same holds true for all TLC drives.


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