Micron's M600 SSD accelerates writes with dynamic SLC cache

In some ways, Micron's M600 SSD is exactly what you'd expect. It combines the company's own 16-nm NAND with a marginally updated version of the usual Marvell controller. This pairing is familiar from the Crucial MX100, which is sold through Micron's consumer brand, but the M600 is quite different. Instead of treating the flash as two-bit MLC NAND, the M600 dynamically switches cells into single-bit SLC mode to improve write performance.

The M600 family. Source: Micron

Dubbed dynamic write acceleration (DWA), this switching occurs at the block level. Incoming writes are written in SLC mode before being moved to MLC storage during lulls in activity. All unused NAND is available to this effective write cache, which stretches across both user-accessible storage and overprovisioned area.

SLC caches are found in several Serial ATA SSDs on the market right now. However, those implementations rely on relatively small, static caches. The M600's write cache can be much larger—basically the entire drive—and its size changes on the fly. Just keep in mind that the SLC storage capacity is 50% lower than what the NAND can hold in MLC mode.

DWA is designed to accelerate the bursts of small writes typical of client systems. Performance improvements can be felt with the drive up to 99% full, Micron claims, but there are some caveats associated with sustained workloads. SLC caching can't improve steady-state performance with prolonged random writes, for example. Also, only so much data can be written in SLC mode before the drive has to shift back to MLC. The following graph from Micron's DWA whitepaper illustrates the change in speed as a secure-erased M600 128GB is filled to capacity with sustained sequential writes.

Source: Micron

The M600 enjoys speedy SLC write speeds until its logical saturation—the percentage of LBAs consumed—reaches 46%. After that, data is committed to the flash with much slower MLC writes. The speed drops again at 58%, when the drive starts transferring cached SLC data to MLC blocks in order to make room for the unabated stream of incoming writes.

The benefits of DWA are most pronounced in lower-capacity SSDs that have less NAND-level parallelism than their larger siblings. In fact, among 2.5" versions of the M600, only the 128GB and 256GB have the feature enabled. Micron says the 512GB and 1TB are fast enough to "saturate the bus" without it, at least for that form factor. M.2 and mSATA variants of the M600 scale up to 512GB, and they're all infused with SLC mojo. Those mini 512GB drives have fewer components and evidently less parallelism than their 2.5" counterparts.

Dynamic write acceleration also promises power-efficiency benefits. Less energy is consumed when writing the same amount of data, according to Micron, and faster transfers allow the drive to spend more time in a low-power state.

Unfortunately, this kind of caching inevitably increases write amplification. Micron estimates additional amplification up to 2X, depending on the application. You'd hardly know it from the endurance ratings, though. Unlike typical client SSDs, which are specced for ~72TB of total writes, the M600 is good for up to 400TB in its top capacity.

Capacity Die config Max sequential (MB/s) Max 4KB random (IOps) Endurance
Read Write Read Write
128GB 8 x 16GB 560 400 90,000 88,000 100
256GB 16 x 16GB 560 510 100,000 88,000 200
512GB 32 x 16GB 560 510 100,000 88,000 300
1TB 64 x 16GB 560 510 100,000 88,000 400

Despite its considerable endurance, the M600 isn't based on a higher-grade sorting of Micron's flash. The chips are pulled from the same stock as those in the MX100, but "proprietary NAND trims" are adjusted "to optimize for 50% more NAND endurance than SSDs that do not feature dynamic write acceleration." It's also worth noting that SLC writes are less damaging to the flash than MLC ones—and that some cached data will be erased before it's even been transferred to MLC storage.

Once you get past all the caching wizardry, the M600 is pretty conventional. You get a three-year warranty, support for the ultra-low-power DevSleep state, and the TCG Opal 2.0 and IEEE 1667 certification required for encryption management software and Microsoft's eDrive specification.

There's always a catch, and in this case, it's the M600 will only be available to PC makers, system builders, and businesses. Unlike with some previous Micron SSDs, a Crucial-branded consumer version isn't in the cards. Micron wants to cut down on the crossover and confusion between its products and retail-oriented Crucial units. The Crucial team is free to use dynamic write acceleration in a separate product, though, and I suspect we'll see one before too long. It just won't be a re-stickered version of the M600.

Since the M600 isn't a retail product, we don't have an official price list. Micron tells us the 1TB version will sell for around ~$450, which seems reasonable to me. Drives are shipping to OEMs now, and we have a few of 'em in our labs for testing. We can't say more until the curtain lifts on reviews, though. Stay tuned.

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