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Crucial's m4 solid-state drive

The RealSSD rides again

Back in the early days of SSDs, the flash controller was king. Marvell's first effort on that front was a bit of a disappointment. That controller, code-named Da Vinci, crucially lacked TRIM support at a time when it was widely offered by the competition. Forced to contend with flash-based storage's pesky block-rewrite penalty, the one Da Vinci-based drive we reviewed was much slower than its rivals and inexplicably expensive given the comparatively sluggish performance.

Our faith in Marvell's controller chops was restored last summer when Crucial's RealSSD C300 burst onto the scene sporting shiny new silicon. With a little help from some blazing-fast flash memory and a 6Gbps Serial ATA interface, the controller otherwise known as Van Gogh set a new standard for all-around SSD performance. Marvell's second-generation design was further validated when Intel drafted the chip to anchor its latest high-end SSD, the 510 Series.

The Intel 510 Series is a perfect example of how other factors play a role in defining SSD performance today. Despite having the same controller and similar flash chips to the C300, the Intel 510 Series boasts much higher sequential throughput at the expense of performance with highly random I/O loads. The two drives present a very different set of trade-offs, and Crucial is about to add one more twist with a successor to the C300 based on the very same flash controller, this time paired with next-generation NAND built on a 25-nm fabrication process.

If you're a big system builder like Dell or HP, this C300 sequel will be known as the Micron RealSSD C400. For regular folks, Micron will sell the drive through its consumer brand as the Crucial m4. The lower-case "m" is presumably there to avoid confusion with the M4 carbine—the last thing Crucial wants is you picturing the drive mowing down its competition with a rapid-fire hail of IOps. Or something.

The RealSSD C400 and m4 are the same hardware with different names. Crucial tells us that both will receive the identical firmware updates down the line, so they're pretty much interchangeable. Amusingly, our review sample came in a box labeled Crucial m4, but the drive itself bears the RealSSD C400 name.

As we've noted, the m4 uses Marvell's second-generation SSD controller. The chip you see above is a slightly newer variant than the one found inside the C300. However, Crucial says this new revision doesn't bring any major changes. That mirrors what Marvell told us about the BKK2 version of the chip in Intel's 510 Series, which it said was "comparable" to the original Van Gogh silicon used in the C300.

Since it's been around for more than nine months now, the 88SS9174's 6Gbps SATA support is old hat. The m4 looks set to make better use of the additional bandwidth than its predecessor, though. While the RealSSD C300 carries sequential read and write ratings of 355 and 215MB/s, respectively, the m4's spec sheet advertises 415MB/s reads and write speeds as high as 260MB/s. Those sequential numbers don't quite match the Intel 510 Series' 500MB/s reads and 315MB/s writes, but the Intel drive has been optimized specifically with sequential throughput in mind.

Crucial has gone with a more balanced approach for the m4, and it shows when we look at the drive's IOps ratings. With random 4KB reads, the m4 is capable of crunching 40,000 IOps. Switch to writes, and that number increases to 50,000 IOps. The Intel 510 Series is limited to just 20,000 random-read IOps and only 8,000 random writes.

Flash controller Marvell 88SS9174-BLD2
Interface 6Gbps
Flash type Micron 25-nm MLC NAND
Available capacities 64, 128, 256, 512GB
Cache size 256MB
Sequential reads 415MB/s
Sequential writes 95MB/s (64GB)
175MB/s (128GB)
260MB/s (256, 512GB)
Random 4KB reads 40,000 IOps
Random 4KB writes 20,000 IOps (64GB)
35,000 IOps (128GB)
50,000 IOps (256, 512GB)
Warranty length Three years

Like just about every other SSD, the m4's performance ratings fall with the drive's total capacity. Reads aren't affected, but writes take a hit when you drop down to the 128 and 64GB models. This isn't some insidious attempt at artificial segmentation—because lower-capacity drives employ fewer memory chips, they can't exploit all of the parallelism built into the controller.

Speaking of parallelism, the Marvell chip has eight memory channels compatible with second-generation ONFI NAND. The 256GB m4 we'll be looking at today spreads its flash over 16 chips split evenly between both sides of the drive's circuit board.

Micron builds these NAND chips using a cutting-edge 25-nm fabrication process, creating a clear distinction between the m4 and both the RealSSD C300 and the Intel 510 Series, which use 34-nano flash. This distinction is important because the move to finer fabrication techniques is a key step in reducing SSD prices—the more dies you can cram onto a wafer, the cheaper your per-gigabyte cost, at least in theory.

25-nm NAND brings its own challenges, specifically in the realm of endurance. Flash chips can only withstand a limited number of write-erase cycles, and everything we've seen points to that figure being lower with 2x-nm NAND than it is with the 3x-nm stuff. The Micron 29F128G08CFAAB flash chips found on the m4 appear to be identical to the ones OCZ is using in its Vertex 3 SSD. According to OCZ, the chips can endure 3,000 write-erase cycles, which is less than the 5,000 cycles typical of 34-nano flash.

Crucial wouldn't confirm the write-erase limit of the m4's flash chips, but it does publish endurance specifications for the drive as a whole. According to the company, the m4 can write 72 terabytes of data over its lifetime. Amortize that over a five-year span, and you're looking at 40GB per day. Which is a lot. 72TB is also the same Total Bytes Written (TBW) rating that Crucial slaps on the C300. All flavors of the old RealSSD share this rating, but 64GB variants of the m4 do not. The new drive's smallest capacity point is limited to 36TB of writes, which still works out to 20GB a day for five years.

The little chip you see over to the right in the picture above is the m4's 256MB DRAM cache. Intel's 510 Series gets by with only 128MB, but Crucial used 256MB on the C300, so the larger cache isn't unexpected. Neither is the drive's 7% overprovisioning percentage, which is common for consumer-grade SSDs. Of the drive's 256GB of flash capacity, 238GB is available to end users. The rest is dedicated to "spare area" used as temporary storage by the controller.