Big study: SLC and MLC NAND flash found to be equally reliable

A new paper about the reliability of NAND flash memory technology found that high-end SLC memory is not more reliable than the more affordable, and more widely used, MLC NAND flash memory. The study used data from millions of drive days over six years of solid state disks used in Google data centers. It included 10 different drive models, three flash types (MLC, eMLC and SLC) and enterprise as well as consumer disks.

There are several interesting findings in the paper, including hard data that shows MLC-based flash drive are just as reliable as the more expensive enterprise-class SLC disks. Another big conclusion is that age and not use correlates with increasing error rates. This suggests over-provisioning of SSDs may be unnecessary as flash wear-out didn't seem to be a problem. The study also backs up the claim that SSDs have lower failure rates than hard disk drives, but the bad news is they're more likely to lose data during normal life.

Key conclusions:

Ignore Uncorrectable Bit Error Rate (UBER) specs. A meaningless number.

Good news: Raw Bit Error Rate (RBER) increases slower than expected from wearout and is not correlated with UBER or other failures.

High-end SLC drives are no more reliable that MLC drives.

Bad news: SSDs fail at a lower rate than disks, but UBER rate is higher (see below for what this means).

SSD age, not usage, affects reliability.

Bad blocks in new SSDs are common, and drives with a large number of bad blocks are much more likely to lose hundreds of other blocks, most likely due to die or chip failure.

30-80 percent of SSDs develop at least one bad block and 2-7 percent develop at least one bad chip in the first four years of deployment.

The full paper can be viewed over here (PDF).

Via: ZD Net