Toshiba’s New MLC NAND Flash SSDs

Toshiba has recently announced a new series of SSD’s based on MLC NAND (Yahoo Biz story). This is only the latest in a series of MLC SSDs which Toshiba has released.

Historically, MLC (multi-level cell) NAND has supported higher capacity but has been slower and less reliable than SLC (single-level cell) NAND. The capacity points supplied for the new drive (64, 128, 256, & 512GB) reflect the higher density NAND. Toshiba’s performance numbers for new drives also look appealing but are probably overkill for most desktop/notebook/netbook users

Toshiba’s reliability specifications were not listed in the Yahoo story and probably would be hard to find elsewhere (I looked on the Toshiba America website and couldn’t locate any). However the duty cycle for a desktop/notebook data drive are not that severe. So the fact that MLC can only endure ~1/10th the writes that SLC can endure is probably not much of an issue.

SNIA is working on SSD (or SSS as SNIA calls it, see SNIA SSSI forum website) reliability but have yet to publish anything externally. Unsure whether they will break out MLC vs SLC drives but it’s certainly worthy of discussion.

But the advantage of MLC NAND SSDs is that they should be 2 to 4X cheaper than SLC SSDs, depending on the number (2, 3 or 4) of bits/cell, and as such, more affordable. This advantage can be reduced by the need to over-provision the device and add more parallelism in order to improve MLC reliability and performance. But both of these facilities are becoming more commonplace and so should be relatively straight forward to support in an SSD.

The question remains, given the reliability differences, when and if MLC NAND will ever become reliable enough for enterprise class SSDs. Although many vendors make MLC NAND SSDs for the notebook/desktop market (Intel, SanDISK, Samsung, etc.), FusionIO is probably one of the few using a combination of SLC and MLC NAND for enterprise class storage (see FusionIO press release). Although calling the FusionIO device an SSD is probably a misnomer. And what FusionIO does to moderate MLC endurance issues is not clear but buffering write data to SLC NAND must certainly play some part.