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Corsair's Force F120 solid-state drive


SandForce sheds overprovisioning to boost capacity
— 12:55 PM on June 21, 2010

SandForce's SF-1200 is easily one of the most intriguing solid-state disk controllers on the market. Not only is SandForce a new player in the field, but it also has a unique special sauce dubbed DuraClass. This secret blend of herbs and spices includes elements of compression, encryption, and even RAID-like redundancy, but the company has so far been reticent to reveal the recipe. My inner nerd is desperately curious about how all the elements intermingle, especially since the SF-1200 has shown so much promise.

Although seemingly dependent on command queuing to achieve optimal performance, the SF-1200 boasts competitive sequential transfer rates, truly stunning random-write throughput, and the promise of greater flash longevity than other MLC-based designs. Unfortunately, the first SF-1200-based SSDs were also burdened with a rather high cost per gigabyte, straining their value proposition.

Drive makers weren't jacking up prices to cash in on early adopters, though. The SF-1200 comes from a controller family that was architected with enterprise-class environments in mind. To deal with the high volume of random writes typical of high-performance servers and workstations, enterprise-oriented SSDs generally reserve a much greater portion of their flash capacity as free area that's inaccessible to the user. This practice, called overprovisioning, can improve write performance and enhance a drive's longevity. The initial SF-1200 SSDs used an overprovisioning percentage of 28%, causing drives with 128GB worth of flash to offer only 100GB of useful storage. Typical desktop SSDs only use 7% overprovisioning, so they can wring 120GB of storage space from the same amount of flash.

To keep SF-1200-based drives from having to fight with extra capacity tied behind their backs, SandForce developed a new firmware revision for the SF-1200 that scales overprovisioning down to 7%. Solid-state drives sporting this firmware have already started popping up online, and Corsair's Force F120 is the first one to arrive in the Benchmarking Sweatshop. The F120 comes on the heels of the 100GB Force F100, predictably offering a more conventional 120GB of usable capacity.

Since nothing has changed on the controller front, we won't go into too much detail there. I suggest reading our three-way SandForce showdown for the skinny on the SF-1200 and what we know about the chip's architecture. The design's defining feature is DuraWrite, a term SandForce uses to describe a mix of compression and other techniques that conspire to achieve a write amplification factor of just 0.5. SSDs typically have write amplification factors of greater than one, but the SF-1200 actually writes less to the flash than the operating system thinks has been written to the disk, which should conserve precious write-erase cycles. The SF-1200's on-the-fly encryption engine is no doubt tied closely to DuraWrite, and both are likely intertwined with the controller's flash-level RAISE redundancy scheme.

Most solid-state drives spread their flash chips across multi-channel arrays in a bid to improve performance. Think of a striped RAID 0 array, but with flash chips instead of hard drives. RAISE, which stands for Redundant Array of Independent Silicon Elements, looks more like a parity-infused RAID 5 array. To protect against data loss due to the failure of a flash die, RAISE reserves an area equal to the capacity of one die to store redundancy data. This redundancy data is likely a parity/hash hybrid, and it's spread across the entirety of the drive rather than being sequestered on a single die.

I'm dwelling on RAISE a little because it complicates the overprovisioning picture. The pseudo-parity redundancy data used by RAISE is stored in an SSD's spare area so that it doesn't reduce the amount of storage capacity available to the user. Just how much of that free area does RAISE consume on the F120? To answer that question, we need to take a closer look at the drive's flash chips.

The F120 has 16 Intel NAND chips that pack 8GB apiece. According to Intel, there are two dies per chip, which means 4GB is dedicated to RAISE redundancy data. That leaves the F120 without about half as much usable spare area as other drives with 7% overprovisioning.

Solid-state drives use their free area as reserve capacity to negate bad blocks that might crop up in the portion of flash available to the user. With less free area at its disposal, the F120 will have to make do with fewer backup blocks in reserve. SandForce has confirmed that a lower overprovisioning percentage can also increase the SF-1200's write amplification factor in "worst case corners," but it claims there's no impact with typical usage scenarios, such as installing an operating system. In desktop environments, I suspect the F120's life expectancy will be longer than that of typical SSDs due to the SF-1200's low write amplification factor. Of course, if you're going to be using an SSD as an OS and applications drive and aren't writing tens of gigabytes of data per day, longevity shouldn't be an issue for any mainstream SSD.

In addition to serving as a pool of backup blocks, an SSD's spare area can be used to accelerate write performance by providing incoming writes with fresh flash pages. Less overprovisioning means fewer flash pages available for inbound writes, but SandForce says the SF-1200's peak throughput for random writes is unaffected by a lower overprovisioning percentage.

Interestingly, Corsair's web site lists a random-write capacity of just 15,000 IOps for the F120, F60, and F240, all of which use 7% overprovisioning. The F100 uses SandForce's "Max IOps" firmware to hit 30,000 random-write IOps, neatly doubling the F120's peak theoretical throughput. SF-1200-based drives that don't make use of SandForce's juiced firmware have thus far been capped at 10k random-write IOps, suggesting that the IOps ceiling may have been raised for drives with 7% overprovisioning. We'll get a better sense of things as we dig through the results of our extensive performance testing.

According to the specifications on Corsair's web site, overprovisioning should have no impact on the SF-1200's sustained read or write speeds. The Force F100 and F120 both boast 285MB/s read- and 275MB/s write-speed ratings. Corsair doesn't list the random-read throughput of either drive, but SandForce says the SF-1200 is capable of crunching 30,000 4KB random-read IOps.

By now you must be wondering whether the F120 is just an F100 running different firmware. After all, changing the SF-1200's overprovisioning doesn't require new hardware. SandForce even allows SSD makers to release multiple firmware revisions for each drive, enabling end users to choose between overprovisioning percentages. As it turns out, though, the F120's circuit board differs from what you'll find under the skin of our F100. I've posed the two side-by-side in the picture below. The F100 is on the left, while the F120 sits to the right.

Both drives may use the same SF-1200 controller and basic layout, but Corsair has completely revamped the mix and position of other surface-mounted components. Micron flash memory chips are used on the F100, while Intel ones appear on the F120. The flash chips on each drive are fabricated using 34-nm process technology, and I suspect they're very similar, if not identical apart from what's silk-screened on the surface. After all, Intel does have a flash joint venture with Micron.

As with other SandForce-based solid-state drives, you won't find a DRAM cache memory chip anywhere on the F120. The SF-1000 family is designed to be used without a traditional DRAM cache.

We appreciate good warranty coverage here at TR, so we're pleased to note that Corsair recently bumped up the coverage for all its SSDs up to three years. That matches the three-year warranties offered by most other SSD makers and the vast majority of mechanical hard drives. However, it's worth noting that flagship hard drives like Western Digital's Black series and Seagate's XTs come with five years of coverage.