With 8-10 memory channels each, the controllers behind our stack of SSDs have plenty of internal parallelism. Saturating those parallel data pathways is the key to exploiting each controller's performance potential. We couldn't get SSD makers to go into too many specifics about what's required to keep each controller's memory channels at full utilization, but the number of NAND dies is an integral component of the equation.
Solid-state drives split their NAND dies between multiple physical packages. The size of the NAND dies can vary, as can the number of dies per package. To help you get a sense of how the various SSDs and capacity points stack up, we've whipped up a handy chart detailing each model's die configuration.
|Corsair Force Series 3||60GB||8 x 64Gb||1||$95|
|120GB||16 x 64Gb||1||$170|
|240GB||32 x 64Gb||1 or 2||$315|
|Corsair Force Series GT||60GB||8 x 64Gb||1||$110|
|120GB||16 x 64Gb||1||$190|
|240GB||32 x 64Gb||1 or 2||$355|
|Crucial m4||64GB||16 x 32Gb||2||$105|
|128GB||32 x 32Gb||2||$180|
|256GB||32 x 64Gb||2||$345|
|Intel 320 Series||40GB||6 x 64Gb||1||$93|
|120GB||16 x 64Gb||1 or 2||$200|
|300GB||40 x 64Gb||2||$530|
Let's start with the easy ones: the Corsair Force 3 and Force GT, which use the same die configurations at each capacity point. All of the dies weigh in at 64Gb, so the number of them doubles with each step up the ladder. We'll be making stops at 60GB, 120GB, and 240GB. Both of these 240GB drives come in two configurations: one with 32 dies spread across the same number of physical packages, and another with two dies per package. Corsair assures us the performance of these die configs is identical. For what it's worth, our Force 3 240GB sample has one die per package, while the Force GT we tested has two.
Despite slight differences in packaging, the Corsair Force SSDs should give us a good sense of how the SandForce controller's performance scales up with the number of NAND dies. Clearly, there's something to be gained from having more than one die per memory channel. The 60GB Force SSD has enough NAND dies to match the eight channels in the SandForce controller, but it's tagged with lower performance ratings than the 120GB and 240GB drives.
The scaling picture will be a little more complicated with the Crucial m4. This drive uses 32Gb NAND dies to serve the 64GB and 128GB capacity points, but the 256GB unit is equipped with 64Gb dies. As a result, the 64GB drive has 16 dies, while its higher-capacity brothers have 32 dies each. Any performance deltas between the 128GB and 256GB versions of the m4 will be due to differences in the NAND dies themselves rather than their number. All of the m4s squeeze two dies per package, so those higher-capacity models also have the same package counts.
Admittedly, our selection of Intel 320 Series SSDs doesn't map perfectly to the capacity points we've collected for the others. We've reached all the way down to a 40GB model at the low end and up to a 300GB monster at the high end. The 40GB drive is only marginally cheaper than its 60GB and 64GB competition, though. While the 300GB model costs considerably more than our 240GB and 256GB examples, it's the only 320 Series north of 180GB.
Like the Force drives, the Intel 320 Series uses 64Gb NAND dies throughout. The fact that the 40GB model sports four fewer NAND dies than the controller has memory channels probably won't help performance. The 120GB version has an additional 10 NAND dies and a slightly unconventional configuration. There are 10 NAND packages on the chip but 16 flash dies, so some of the packages have one die, while others pack two.
We'd feel worse about throwing the Intel 320 Series into the cage with a bunch of 6Gbps rivals if Intel were offering its drives at substantial discounts. Despite being based on an older controller architecture that uses a dated SATA interface, the 320 Series 120GB costs more than the competition.
Because of the capacity differences involved, it's better to look at each drive's cost per gigabyte. In the chart below, we've combined Newegg prices with the amount of storage capacity available to end users. We've included Western Digital's Caviar Black 1TB, one of our favorite 7,200-RPM desktop drives, for reference.
From a cost-per-gigabyte perspective, the Intel 320 Series is a pretty lousy deal. The 40GB drive is the most expensive of the bunch by a fair margin over the next-closest alternative, a 60GB Force GT that should be quite a bit faster. The higher-capacity flavors of the Intel 320 Series don't look all that good on this scale, either.
Surprisingly, the highest-capacity Force 3 and Crucial m4 models offer the most storage per dollar. The Corsair SSD is just 31 cents shy of the elusive dollar-per-gigabyte threshold, while the Crucial m4 runs four cents more. In both cases, the 120-128GB variants will set you back about $1.40 per gigabyte.
Although the Intel 320 Series and the Force GT don't follow the same behavior, they aren't excepted from the other trend our cost-per-gigabyte analysis reveals. Across the board, the lowest-capacity SSDs cost more per gigabyte than their higher-capacity counterparts. Budget SSDs may offer lower costs of entry, but their value proposition isn't quite as strong, at least from a capacity perspective. To see how the cheaper drives shake out overall, we'll now move on to performance.
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