How to Build a Solid 25nm SSD(and how not to)

- 2011-02-18

Some of the early attempts at 25nm NAND SSDs have created some negative perceptions about the quality the NAND itself, which is really misplaced. It is possible to build great SSDs with 25nm NAND; but you do have to be deliberate about your process and objectives (it also helps to have inside knowledge about how that NAND works). I want to use today’s blog to lay out some of the key principles that guided the development of the RealSSD™ C400—our own 25nm SSD (the retail version will be branded as the Crucial M4 SSD):

Label Capacity Must Equal User Capacity
There’s nothing wrong with over-provisioning (reserving some of the NAND capacity for better performance and durability), but the drive label must state the capacity the user has access to. This is basic marketing honesty. The entire hard-drive industry got sued for this years ago and established standards for user capacity as a result. Micron follows these standards—we always market our drives at the true user capacity. In the case of the C300 and C400, the user capacities are identical—64, 128, and 256 GB (the C400 also offers a 512 GB).

Performance Must Not Degrade Sequential Performance
We’ve taken a long-term view of the market; each new generation of drives must haveequal or better performance than the last. New NAND designs do present challenges, but because Micron leads NAND development, our SSD team has early insight into new products, and we start work early to make sure our SSDs make the best use of that NAND. The C400 is proof of that. As shown in the chart, it’s noticeably faster than the C300.

Keep Endurance High
SSD Enthusiasts are aware that new NAND designs start out at lower endurance cycle counts than the previous generation, and are sometimes wary of next-generation SSDs as a result. But cycle counts don’t necessarily translate 1:1 to drive endurance specs—good NAND management (via the firmware and controller) is the key. We specify SSD endurance in total bytes written (TBW). The 25nm C400 offers the same endurance as the C300 for the 128, 256 and 512 GB models—72 TB TBW. This is equivalent to 40GB per day every day for 5 years, and far exceeds the patterns of any PC user. The 64 GB drive endurance is rated at 36 TB TBW—that’s 20 GB per day over the same time period, which still exceeds typical consumer use patterns. We take the reliability of the C400 very seriously and have gone to great lengths to develop advanced firmware algorithms that manage the NAND. Again, being NAND developers gives us the unique ability to design end-to-end SSD quality as a complete system, alongside our NAND design team. I hope you don’t let these early attempts at 25nm SSDs dampen your enthusiasm about this new technology. The SSD market is going to change dramatically in the next few years, and leading-edge NAND (and SSDs from the companies that make that NAND) is what will make it possible. Crucial’s M4 SSD will hit the shelves in mid-March. There’s a lot to be excited about; you’ll see proof in the C400/M4 reviews in a just a few weeks.