Although SandForce was formed in 2006, the company didn't infiltrate PC enthusiast circles until last year, when its SF-1500 controller broke cover in OCZ's Vertex 2 solid-state drive. SandForce entered a market populated by big names like Intel and Samsung, and the upstart managed to impress with a unique controller design laced with an intriguing black box of compression and encryption technologies known as DuraWrite. Before long, controllers from the SF-1000 family could be spotted in a wide range of SSDs from OCZ, Corsair, Mushkin, Patriot, and others.
Now, more than a year old and restricted by 3Gbps Serial ATA connectivity, the SF-1000 family appears to be on its way out. In its place, SandForce has developed an updated line of 2000-series controllers destined for next-generation SSDs. Fittingly, the family is making its desktop debut inside OCZ's Vertex 3.
The Vertex 3 isn't due to hit store shelves for another couple of weeks, but it's arguably the most anticipated SSD of the year. Indeed, we've been so curious about the drive's performance that we've been testing a pre-production sample with firmware that hasn't yet been finalized. OCZ expects the firmware only to get faster from here on out, so we've decided to take a closer look to see where it's at now.
Before dipping into our benchmark results, let's unravel the drive to see what makes it tick. The Vertex 3 features an SF-2281 controller drawn from SandForce's SF-2000 family of "client" SSDs. SandForce also makes enterprise-oriented controllers based on the same architecture, but they're targeted at servers rather than notebooks and desktop PCs. This isn't a case where power users need to be dipping into the enterprise pool, either; SandForce names enthusiasts as a specific target market for its client family.
Perhaps the most notable feature of the SF-2281 is 6Gbps Serial ATA connectivity. The faster interface nicely matches what's being offered by the latest SSDs from Crucial and Intel, but it's more than just a checkbox feature to keep up with the Joneses. According to SandForce, the SF-2281 offers sequential read and write speeds up to 500MB/s—more than enough to saturate a 3Gbps SATA pipe. OCZ is even more optimistic about the Vertex 3, which it says can push 550MB/s in sequential reads and 525MB/s in writes.
|Flash controller||SandForce SF-2281|
|Flash type||Micron 25-nm MLC NAND|
|Available capacities||120, 240GB|
|Sequential writes||500MB/s (120GB)
|Random 4KB reads||60,000 IOps|
|Random 4KB writes||60,000 IOps (burst)
20,000* IOps (sustained)
|Warranty length||Three years|
For a system drive, the ability to handle a lot of random read and write requests is arguably more important than blazing sequential throughput. According to the SF-2281's spec sheet, the controller is capable of chewing through 60,000 IOps with random 4KB reads. The same IOps rating applies to 4KB random writes, but only in short bursts. With a sustained random-write load, the controller's performance rating drops to 20,000 IOps, at least according to SandForce. OCZ instead prefers to say that the Vertex 3 can sustain 60k random-write IOps until the drive gets "dirty." After that, garbage collection will kick in and random-write performance will drop—how much it drops depends on a variety of factors, including how the drive has been used up to that point.
Interestingly, OCZ tells us the 120 and 240GB flavors of the Vertex 3 should perform almost identically. The 120GB variant is limited to 500MB/s sequential writes, the company says, but all other performance specifications are the same. 120GB may be as small as the third-gen Vertex gets; there's no indication 60GB or lower-capacity variants are in the works.
Like the SF-1000 series, SandForce's new SSD controllers are designed to operate without the aid of dedicated DRAM cache memory. Internal buffers within the controllers provide a measure of built-in cache capacity, but I suspect they're not nearly as large as the 128-256MB of cache available on competing SSDs. The cacheless SF-1000 family was pretty darned fast without, so I wouldn't worry too much about the lack of DRAM on the Vertex 3.
Besides, the drive's NAND flash memory chips are much more interesting. The SF-2000 family is compatible with a wide range of flash technologies, including ONFI 2.0 and Toggle DDR specifications. SandForce says per-die transfers top out at 166MT/s, a figure that becomes considerably more impressive when you factor in the controller's eight memory channels.
On the Vertex 3, OCZ pairs the SF-2281 with MLC NAND courtesy of Micron. Most of the flash found inside contemporary SSDs is built using 34-nano fabrication technology. The Vertex 3's chips are fabbed on an advanced 25-nm process that squeezes more dies into every square inch of wafer area. Provided there are no problems with yields, these chips should be cheaper to purchase than their 34-nano counterparts, leading to lower SSD prices. There is a catch, though. NAND chips fabbed on finer process technologies tend to withstand fewer write-erase cycles than those fabbed with older process tech. OCZ says that each of the Vertex 3's flash cells is good for 3,000 write-erase cycles.
SandForce controllers are a shining example of why a given flash technology's write-erase endurance shouldn't be viewed in a vacuum. The number of flash cycles that each cell can survive is but one part of a larger equation that defines how much data can be written to a drive over its usable life. Another important variable is a controller's write-amplification factor, which refers to how much data is actually written to the flash for each write request that comes from the host. SSDs typically have a write-amplification factor that's greater than one but very close to it. Thanks to a suite of compression techniques, SandForce controllers can write less to the flash than is asked by the host, resulting in a write-amplification factor that's less than one.
DuraWrite is the name SandForce gives this black box of compression mojo, and the company has been reluctant to give us a closer look inside. We suspect there's an element of encryption involved, since SandForce controllers offer on-the-fly AES encryption that can't be disabled. The latest SF-2000 series support 256-bit AES encryption, which is a nice upgrade over the 128 bits of protection offered by the SF-1000 family.
DuraWrite's ability to lower a drive's write-amplification factor is especially valuable as the market transitions to flash chips that tolerate fewer write-erase cycles. Another unique SandForce technology that's well-suited to potentially flaky flash is RAISE, which stands for Redundant Array of Independent Silicon Elements. Similar to RAID 5, this redundancy scheme dedicates the capacity of one flash die to storing parity-like information. You lose a little capacity in the process, which is why RAISE is optional for the SF-2000 family. With RAISE disabled, the SandForce controllers rely on ECC error correction capable of recovering up to 55 bits per 512-byte sector.
RAISE is enabled on the Vertex 3, which uses a typical 7% overprovisioning percentage to offer 224GB of Windows storage capacity on a drive with 256GB worth of NAND. The Vertex 2 debuted with a much higher 28% overprovisioning percentage familiar from enterprise-class SSDs. Having more "spare area" at the controller's disposal can improve performance somewhat, but it can also inflate drive's cost per gigabyte—arguably a more important consideration for folks shopping for consumer-grade SSDs.
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