When Micron introduced its M600 SSD in September, it was only a matter of time before the other shoe dropped. The memory giant fabs its own NAND and sells SSDs to PC makers and other corporate customers, but it doesn't make Micron-branded drives available to everyday consumers. Instead, that market is covered by its Crucial division, which draws from the same pool of in-house technologies. Sooner or later, the M600's funky SLC/MLC caching scheme was destined pop up in an equivalent Crucial product.
That product is the MX200, which Crucial announced at CES last month. The drive has the same DNA as the M600, and it's already available for as little as $0.46/GB. But it's not the whole story.
The MX200 fell into our laps alongside the BX100, an unexpected second shoe more akin to one of those minimalist, barefoot designs that's basically just a glove for your foot. This trimmed-down alternative uses a completely different controller, and it has an even lower price tag. BX100 SSDs are currently selling for as little as $0.36/GB, making them among the cheapest around.
We've been testing the new drives to see how they compare to the current crop of contenders, and the results might surprise you. Turns out there's something to those funky toe shoes, after all.
Same NAND, different controllers
The MX200 treads familiar territory, so that's the best place to start. Like its M600 predecessor, the drive combines an eight-channel Marvell 88SS9189 controller with Micron's own 16-nm NAND.
Each flash die packs 16GB of storage, making it easier to hit higher capacities. The higher-density dies create problems for lower-capacity drives, though. SSD controllers achieve peak performance by addressing multiple dies simultaneously, but smaller drives don't have enough individual NAND chips to fully exploit that controller-level parallelism. Smaller SSDs typically have slower write performance ratings than their larger counterparts as a result.
To counteract this problem, the MX200 takes advantage of a dynamic switching capability built into Micron's 16-nm NAND. The flash can switch entire blocks between a single-bit SLC config, whose simpler programming mechanism enables higher write speeds, and a two-bit MLC config, whose greater bit density enables higher capacities. The MX200 writes incoming data to SLC blocks before moving it to MLC zones during idle downtime. The drive essentially treats its entire unused capacity as a dynamic write cache.
Appropriately dubbed Dynamic Write Acceleration, this caching scheme is a clever way to speed up the short, bursty writes typical of client workloads. However, the fact that SLC blocks have half the storage capacity of MLC ones creates problems for sustained transfers. Left unchecked, SLC writes could saturate the drive's unused flash before its MLC capacity is exhausted. Dynamic Write Acceleration avoids this problem by shifting writes into MLC mode before the flash is completely consumed by SLC blocks. If writes remain relentless, the MX200 eventually starts moving previously cached data into MLC blocks while simultaneously handling incoming writes.
We can see the impact of this three-tiered approach by continuously writing data. This sequential write speed plot from my M600 review nicely illustrates how transfer rates change as the drive fills up:
The higher speeds up to the 46% mark illustrate the performance benefits of writing in SLC mode. The middle portion shows the hit associated with MLC writes, while everything past 58% highlights the severe penalty incurred when the drive is shuffling cached data alongside incoming writes. Yikes.
The 500GB and 1TB versions of the MX200 have a sufficient number of NAND chips for peak performance, so they don't require Dynamic Write Acceleration. The feature is only enabled on the 250GB drive. It's also active in the mSATA and M.2 versions of the MX200, both of which are available in 250GB and 500GB capacities. For those drives, write caching is enabled in part to improve power efficiency. Completing writes faster allows the MX200 to return quickly to a low-power state.
Apart from its caching setup, the MX200 mirrors the laundry list of features attached to other recent Crucial SSDs. It has a thermal throttling mechanism that reduces performance when the drive is in danger of overheating, hardware-accelerated AES encryption with support for the requisite IEEE and TCG Opal standards, and an extra layer of RAID-like protection to compensate for physical flash failures. Crucial also throws in a free copy of Acronis True Image HD to ease upgrades from existing systems.
The more budget-minded BX100 has none of that. Under "advanced features," its spec sheet lists well-worn bullet points like TRIM support and active garbage collection—capabilities pretty much every SSD has had for years. Although the drive can monitor its internal temperature, thermal throttling isn't part of the package. There's no support for SLC caching, hardware-based encryption, or RAID-like internal protection. And no mini versions, either. The BX100 is only available in a 2.5" form factor.
As one might expect, homegrown flash is still part of the equation. The BX100 uses the same 16-nm NAND as the MX200, though the chips are addressed exclusively in MLC mode.
Instead of tapping yet another Marvell controller, the BX100 employs Silicon Motion's SM2246EN. This four-channel chip has half as many parallel NAND channels, but each one supports transfer rates up to 400MB/s, so internal bandwidth remains plentiful. The most critical bottleneck is the Serial ATA interface, which tops out at less than 600MB/s for the drive as a whole.
As we saw in our review of the Silicon Motion-based Adata Premier SP610, modern four-bangers can mostly keep up with their eight-channel counterparts. Crucial's performance specifications largely concur. Here are the essential stats for the BX100 and MX200 families. Note that the MX200 starts at 250GB, while the BX100 reaches down to 120GB.
|Capacity||Max sequential (MB/s)||Max 4KB random (IOps)||Endurance
The BX100's specs largely shadow those of the MX200, though the budget drive's write speeds are only competitive at higher capacities. The lack of SLC caching really hurts the BX100 120GB and 250GB. Compare those to the MX200 250GB, which enjoys the same performance ratings as its larger siblings. In a moment, we'll see how the 250GB and 500GB versions of each drive compare in a broad range of tests.
Crucial's three-year warranty is the same for both drives, but the MX200 has a much higher endurance specification. The 1TB incarnation is rated to withstand 320TB of total writes, a limit that scales down predictably with the total capacity. Meanwhile, the BX100 is rated for 72TB of writes across the board.
Even 72TB is a heck of a lot of writes for a consumer-grade drive. The SSDs in my main desktop are only subjected to a few terabytes of writes per year, and most folks seem to be in the same boat. At that pace, it would take more than a decade just to burn through the BX100. Also, keep in mind that our ongoing SSD Endurance Experiment has demonstrated that SSDs can endure substantially more writes than their official specifications promise.
Curious users can monitor SSD health using Crucial's new Storage Executive software. This utility displays SMART attributes that log host writes, bad blocks, important errors, and other statistics. The main monitoring pane also shows the MX200's host writes, but that field is conspicuously absent for the BX100. Weirdly, the required data is still accessible via the SMART attributes.
Storage Executive is a surprisingly bulky 150MB download for something that runs in a web browser. At least the utility has some other functions, including a firmware updater and a secure-erase tool. It doesn't run automatically when the system is booted, either, making it less obtrusive than some other SSD software.
As usual, we've tested Crucial's new hotness against a staggeringly deep field of SSDs from the past few years. The benchmarking bonanza begins on the next page.