Micron's New M600 SSD Offers 16nm NAND, Dynamic Write Acceleration
The new M600 drive family will launch at capacities from 128GB to 1TB across multiple form factors. mSATA, 2.5-inch conventional mobile drives, and the PCIe-capable M.2 platform are all supported with multiple drive sizes and form factors, as shown below.
The M600 uses Micron's 16nm NAND, which will allow the drive to hit a better cost-per-GB than previous products. One of the downsides to smaller NAND geometries, however, is that unlike other products, smaller NAND tends to actually be slower NAND. The reason manufacturers continue pushing smaller geometries is that the vast increases in density have more than offset the modestly lower performance. Better SSD drive controllers have also hidden the losses, as have faster interfaces.
The evolution of drive caching and write acceleration can be thought of as a method of extending drive performance through more sophisticated management even as the underlying NAND actually becomes slower. NAND is classified into three major types -- SLC NAND contains one bit of information per cell, MLC NAND contains two bits, and TLC NAND contains three bits. In the past, entire drives were made of a single type of NAND, with SLC NAND being the fastest and most expensive, and most durable, while TLC NAND was significantly slower, cheaper, and less robust.
It's important to understand that these distinctions happen within the controller chip. TLC NAND can still be treated like SLC, with just one bit of data written to each cell -- and if the manufacturer does that, the NAND will be much faster. Samsung's 840 EVO was the first drive to experiment with defining a section of an SSD as SLC NAND while the bulk of the drive used TLC NAND. Now Micron is adapting a similar strategy but expanding on the amount of available fast storage and allocating it more flexibly.
Micron's Dynamic Write Acceleration:
Micron's program differs from Samsung's caching strategy for the 840 EVO in several ways. First, it's not available on every drive, and there's no explanation for why -- the M600 2.5-inch 512GB and 1TB drives don't offer Dynamic write acceleration.
Second, Micron the amount of space Micron allocates to DWA changes on the fly depending on controller load, workload, and available free space. Under certain circumstances this can apparently offer a much larger boost than a conventional static cache like the 840 EVO uses, though of course these figures are from the manufacturer and should be taken with a grain of salt:
According to Micron, accelerating writes in this fashion can lead to dramatic improvements in total power consumption because, while the drive consumes slightly more power in some cases, it still slashes the amount of time required to perform the write. There are a few caveats with Dynamic Write Acceleration. It cannot accelerate sustained random write workloads indefinitely -- eventually the drive will begin performing garbage collection, and write acceleration doesn't function when it's taking out the trash.
Micron also notes that Dynamic Write Acceleration can increase write amplification as drive wear, since data is being written to the drive twice -- once as SLC, once as MLC. The company compensates for this by using "proprietary NAND trims to optimize for 50% more NAND endurance" than SSDs that do not use DWA. This explains why these drives are rated for up to 400TB of writes -- absolutely huge for an SSD -- and may also be the reason why the 2.5-inch 512GB and 1TB flavors don't feature the technology. Micron can charge more for M.2 and mSATA variants and absorb some of the cost of binning high-quality NAND that way, whereas 2.5-inch drives are subject to more robust price competition.
We'll have to wait and see how the new caching technology performs on the bench before rendering final judgment, but Micron's approach may have advantages over Samsung's static allocation method. We'd also like to see this technology come to all of Micron's M600 product lines rather than being reserved for certain lower-density SKUs. Still, performance gains should impressive on the M.2 and mSATA drives.