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How do 3D NAND flash vendors ensure bit rot doesn't occur?

Marc Staimer takes a closer look in this Expert Answer at how 3D NAND flash vendors keep bit rot from taking place, a vexing challenge given 3D NAND flash technology

Bit rot for flash SSDs is quite a bit -- pun intended -- different than bit rot on hard disk drives.

Bit rot for HDDs occurs when the magnetic polarity of a bit spontaneously flips from electromagnetic radiation in the surroundings. Flash SSD bit rot occurs when the state of an NAND cell changes from electron leakage.

As the number of states within a cell increases, so does the potential for electron leakage. SLC has two states, 0,1; MLC has four states, 00, 01, 11, 10; and TLC has eight states, 000, 001, 010, 011, 100, 110, 101, 111. That means bit rot is most likely to occur with TLC NAND flash drives.

The way manufacturers are handling the increased probabilities of bit rot is through the extensive use of error correcting codes (ECC). Obviously, the ECC for TLC must be considerably more sensitive than ECC for SLC or MLC. And the 3D NAND TLC drive vendors know this and have incorporated much more sensitive ECC.

Determining how effective a 3D NAND TLC drive is at combating bit rot comes down to the unrecoverable bit error rate (UBER) as rated by the 3D NAND flash vendor. Keep in mind that 3D NAND TLC drives are best suited for read- not write-optimized applications. This is most similar to the application fit for nearline or "fat" HDDs. The UBER rate for a SATA HDD is 10-15. The UBER rate for nearline SAS HDDs is 10-16. The UBER ratings for 3D NAND TLC drives have not been released as of this writing; however, they are expected to be at least the same or higher as SATA or SAS HDDs.

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