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Micron's M600 solid-state drive reviewed

A truly dynamic SLC/MLC hybrid

Flash memory is simultaneously shrinking and expanding. The actual memory cells are getting smaller thanks to advancements in fabrication technology, fueling an increase in density that enables higher-capacity chips. For the most part, that's just great. Higher bit densities reduce the cost per gigabyte, while higher-capacity chips facilitate SSDs with more total storage.

Unfortunately, stuffing more gigabytes into each chip also presents a problem. SSDs now require fewer chips to hit the same capacity, and in some cases, they don't have enough NAND components to fully exploit the parallelism in modern controllers. That's why smaller SSDs typically have lower write performance ratings than larger ones.

Multiple SSDs use high-speed caches to make up for slower write performance, but none are quite as ambitious as Micron's new M600. The memory giant's latest contender can toggle any part of its NAND between SLC and MLC modes, allowing it to tap unused area as a dynamic SLC cache. Micron claims this cache delivers a nice performance boost at lower capacities, and we've tested the 256GB version against the 1TB flagship and dozens of other SSDs to see how well the scheme works.

We've also taken a mini M.2 variant for a spin to see how it stacks up. There's much to discuss, so let's get started.

A familiar cocktail with a twist
The M600's hardware components are similar to those in the Crucial MX100, which is sold by Micron's consumer brand. Both drives combine the company's homegrown 16-nm flash with an off-the-shelf Marvell controller.

Although Crucial used to mirror Micron's offerings, the MX100 marked the beginning of a divergence. Having identical lineups created too much confusion and pricing overlap, Micron told us, so the two brands will do their own thing moving forward. They'll continue to pull from the same internal technology portfolio—and Crucial will still target consumers while Micron focuses on businesses, system builders, and PC makers—but their drives will be different.

Forgive the depth of field; I'm running out of ways to photograph SSDs.

Micron's 16-nm NAND can switch between SLC and MLC modes on the fly. The MX100 doesn't take advantage of this capability, but in the M600, custom firmware manages the transitions at the block level. The caching scheme, dubbed Dynamic Write Acceleration, houses incoming data in SLC blocks before moving the data to MLC storage during idle periods.

SLC NAND has higher writes speeds because the programming process is much simpler with only one bit per cell. However, SLC also has less storage capacity than two-bit MLC NAND, reducing the M600's effective cache size to half its MLC capacity. This discrepancy poses a problem for sustained writes that leave no idle time to transfer data from SLC to MLC blocks. The M600's solution is to switch to MLC writes as the NAND approaches saturation. If writes continue unabated, the M600 eventually starts transferring previously cached data to main storage while continuing to write fresh data from the host.

As one might expect, this juggling act has performance implications. The graph below shows how the M600 256GB's sequential write speed changes as the drive fills up in an HD Tune test.

The M600 enjoys the higher write speed of SLC NAND up to 46% of its total capacity. Performance drops considerably in the stretch from 46-58%, when writes continue in MLC mode. After that, the drive has to perform SLC-to-MLC transfers while continuing to write incoming data. This multitasking plunges the sequential write speed below 100MB/s and into mechanical storage territory.

Obviously, Dynamic Write Acceleration isn't ideal for sustained workloads. The scheme is meant to accelerate the short bursts of write activity found in typical client workloads.

There's another potential fly in the ointment, though. Any cached data that makes its way to MLC storage will be written twice, eating into the NAND's limited endurance. Fortunately, SLC writes are less damaging than MLC ones. Micron has also adjusted "proprietary NAND trims" to offset the endurance penalty associated with write caching. These top-secret settings are "configured during the NAND fabrication process," and they can be "tuned for different performance and endurance characteristics." Despite the name, they're unrelated to the TRIM command used by the operating system to identify deleted data.

Tweaking NAND trims is apparently quite effective, because the M600 has the most impressive endurance ratings we've seen in a client-oriented SSD. Micron's other drives in this category are specced for 72TB of total writes, and so are their Crucial cousins. But the M600 starts at 100TB for the 128GB version, and the endurance rating scales up with the capacity, taking advantage of the fact that larger drives have more flash to burn. The 1TB monster is good for 400TB of total writes, which works out to a staggering 374GB per day for the length of the three-year warranty.

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

Surprisingly, the 1TB and 512GB variants don't have Dynamic Write Acceleration. Those drives are already fast enough for the controller, according to Micron, and the math works out. The Marvell chip can address up to four chips on each of its eight memory channels, making 32-die configurations ideal for peak performance. At 16GB per die, the cut-off point is 512GB.

Despite falling short of that threshold, the 256GB variant matches the performance specifications of its larger siblings. The 128GB version is a little slower, according to the official figures, but not nearly as much as one would expect given the die count.

The 1TB version (left) has four times the flash of the 256GB (right), whose back is completely bare.

In a moment, we'll see how the M600 256GB and 1TB compare in a wide variety of tests. There are a few loose ends to tie up before that, including encryption support, which is particularly important to Micron's corporate clients. The M600 supports all the right standards: TCG Opal 2.0, IEEE 1667, and Microsoft eDrive. Even if the 256-bit AES encryption doesn't keep the NSA out of your business, it should at least prevent thieves and opportunists from snooping.

To protect against data loss due to flash failures, the M600 employs a RAID-like redundancy scheme called RAIN. It also has a measure of power-loss protection. These aren't must-have features for a lot of enthusiasts, but they provide some peace of mind, and they should appeal to business customers. Noticing a theme yet?

The M600's high endurance ratings are likely to attract folks who write a lot of data, making health monitoring especially important. Unfortunately, Micron doesn't offer an easy-to-use Windows app with health information and other drive-related functionality. At least the SMART attributes are loaded with tickers for lifetime remaining, host writes, reserve blocks, reallocated sectors, and various error types. Anyone who wants to monitor drive health with third-party disk utilities can easily tap into those attributes.

And then there are the miniature M600s...