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Crucial's M550 solid-state drive reviewed

A bigger, faster M500

When someone asks me which SSD to buy, I typically recommend getting the highest capacity they can afford from a reputable brand. Some budget drives should be avoided, especially at lower capacities, but most decent SSDs offer similar all-around performance. For the majority of consumers, the differences in pricing are more important than the differences in performance.

Right now, the Crucial M500 is one of the most affordable contenders. Crucial is the consumer brand of memory giant Micron, so it gets a family discount—and first dibs—on the latest and greatest flash memory. That's a good recipe for success in the increasingly commoditized world of consumer SSDs, especially when one adds the strong reliability reputation Crucial has built over multiple generations of solid, er, drives.

We like the M500 480GB and 960GB a lot. Both offer decent all-around performance, and they're very cheap per gig. We're not as crazy about the 240GB variant, though. The M500 uses higher-density flash chips than most of its peers, and the 240GB model doesn't have enough of them to saturate the controller's NAND interface. The drive is notably slower in certain scenarios as a result, though it's still much faster than mechanical storage.

Keenly aware of the M500 240GB's handicap, Crucial has developed an updated family of SSDs that seems custom tailored to address it. The new M550 uses flash from the same 20-nm generation as its predecessor. However, the smaller capacities in the lineup employ lower-density NAND chips than the rest of family, so they should be able to wring more performance out of the controller.

In addition to tweaking the die configuration for lower capacities, the M550 is supposed to be faster than the M500 across the board. The new lineup's 128GB-to-1TB spread offers a little more storage than the M500's 120-to-960GB options, too. Let's take a closer look.

A new spin on an old favorite
Instead of supplanting its predecessor, the M550 complements it. The old M500 has become noticeably cheaper in recent months, and the M550 slots in above it as a more premium solution.

Crucial's latest hotness comes in the same form factors as its elder sibling: 2.5", mSATA, and M.2. The top 1TB capacity is only available in 2.5" form, though. The M.2 and mSATA versions are meant for notebooks and limited to 512GB and smaller capacities. We're focusing our attention on the 2.5" SATA family, which includes the following models:

Capacity Die config Max sequential (MB/s) Max 4KB random (IOps) Endurance Price $/GB
Read Write Read Write
128GB 16 x 8GB 550 350 90,000 75,000 72TB $99.99 $0.78
256GB 32 x 8GB 550 500 90,000 80,000 72TB $168.99 $0.66
512GB 32 x 16GB 550 500 95,000 85,000 72TB $336.99 $0.65
1TB 64 x 16GB 550 500 95,000 85,000 72TB $530.99 $0.51

All the drives are built on 20-nm MLC NAND that rolls off of Micron's production line. These chips store two bits per cell, just like the flash used on most modern SSDs.

Unlike the M500, which has 16GB (128Gb) flash chips throughout, the M550 series reserves those high-density dies for the 512GB and 1TB units. The 128GB and 256GB drives have smaller 8GB (64Gb) NAND to provide more independent chips for the controller to address simultaneously. Thus, the M550 256GB has the same amount of I/O parallelism as the 512GB version—and nearly identical performance specifications.

The 1TB variant has the same performance specs as the 512GB drive, suggesting that there's little to be gained by adding more than 32 NAND dies. The write speed ratings for the 128GB model drop off substantially, though. That drive's 16 dies just aren't enough to saturate the controller.

Each M550 capacity offers a little more storage than the equivalent M500, but the amount of onboard flash hasn't changed. It's just partitioned differently, with more available to the user, less to the controller and its associated firmware.

Crucial expanded the amount of user storage by makin' it rain... slightly less. Ahem. RAIN is a RAID-like redundancy scheme that uses parity to protect against data loss due to physical flash failures. In the M500, the RAIN stripe is set at 15:1, which monopolizes 17GB of flash on the 240GB version. That ratio has been scaled back to just 127:1 in the M550. If my math is correct, only 2GB of the M550 256GB's total flash capacity is dedicated to parity data. Crucial passes the savings on to the user: the M500 reports 223GB of user-accessible storage in Windows, while the M550 256GB weighs in at 238GB.

SSDs with RAID-like data protection typically have slightly lower capacities, like the M500, or extra NAND dedicated to storing redundancy data, like the Intel 730 Series. The M550's slimmed-down stripe is an interesting twist, and it was motivated by the lessons Crucial learned with the M500. The NAND platform is now more mature, Crucial says, and the company is confident it can meet reliability expectations with a scaled back RAIN implementation.

The M550 doesn't just guard against data loss due to flash failures. It also features a layer of power-loss protection powered by onboard capacitors. If the power is cut unexpectedly, these caps should supply enough juice for the drive to complete pending writes and to shut down gracefully, without corrupting user data.

Much of the M550's drive-level intelligence is part of custom firmware running on an off-the-shelf Marvell 88SS9189 controller chip. Crucial says there are "no substantial differences" between this chip and the 88SS9187 silicon in the M500. Like pretty much every other SSD controller, the Marvell solution has eight memory channels and a 6Gbps Serial ATA interface. 256-bit AES encryption is handled in hardware, and the firmware has the necessary hooks for Microsoft's eDrive standard, in addition to the TCG Opal 2.0 and IEEE 1667 specs.

Despite the fact that the M550 uses largely identical controller and NAND technology to the old M500, the new drive is supposed to be faster. Crucial credits the M550's "native write acceleration," which doesn't involve compression, NAND buffering, or DRAM-based caching. Unfortunately, the company doesn't say what this native acceleration does entail.

In a moment, we'll look how the M550's performance compares to that of the M500 and a range of other competitors. There are a few more details to cover before we get to those results, including a couple of features targeted primarily at mobile users.

The M550 supports the DevSleep low-power state used by Windows 8's connected standby mode. Crucial claims the drive pulls less than three milliwatts in this state, and it says active power consumption is less than 150 mW. Interestingly, the 2.5", mSATA, and M.2 versions of the M550 all have similar power consumption ratings.

Adaptive thermal protection is the other feature aimed at mobile devices. If the M550 gets too hot, it throttles performance until thermals return to optimal levels. This capability is most important for notebooks and tablets, but it could also benefit cramped small-form-factor and all-in-one rigs. Desktop users with adequate system airflow probably don't have to worry about their SSDs overheating.