Results: 128 KB Sequential Performance
Looking at the various SSD 525 capacities helps us predict what's going to happen as Intel adds more die. For reference, the configuration breaks down as follows:
|Intel SSD 525 mSATA||Total Flash||Packages||Die Count|
|30 GB||32 GB||4||4|
|60 GB||64 GB||4||8|
|120 GB||128 GB||4||16|
|180 GB||192 GB||3||24|
|240 GB||256 GB||4||32|
Each 25 nm IMFT die is 64 Gb, or 8 GB. SandForce's second-gen storage controllers enable up to eight channels, but simply populating each channel isn't enough to maximize performance. For that, you need to interleave four die per channel.
As we just discussed, the 240 GB SSD 525 is the only model boasting our ideal configuration. Although it's true that speed scales with capacity, that's only because larger drives require more die. If your drive happens to employ 32 Gb die, you can build a 120 GB model that's just as fast. This is why SSDs leveraging Toggle-mode memory used to be so fast compared to other interfaces at lower capacities. Shoot, a 120 GB SF-2281-based drive armed with Toggle-mode NAND was nearly as fast as a 240 GB ONFi-equipped SSD. Most LSI SandForce partners continue to use ONFi flash, though enterprise-oriented SF products tend to rely on Toggle-mode memory.
Sequential Read Performance
Intel's quintet of SSD 525s easily devour compressible data patterns. From the lowly 30 GB model up to the 240 GB flagship, each capacity turns in almost identical numbers. Really though, for the past two years, almost every new drive has managed to saturate the SATA 6Gb/s interface with sequential reads. With enough speedy NAND, many SSDs would exceed 700 MB/s if the interface allowed it.
Switch to nearly incompressible data, though, and the picture begins to change. The 120, 180, and 240 GB drives offer up to 500 MB/s, but the 30 and 60 GB models start to choke (relatively, of course). We see the 60 GB SSD 525 peak at 400 MB/s, which is still pretty good. However, the 30 GB drive doesn't scale at all. From queue depths one through 16, the little SSD 525 flatlines near the 200 MB/s mark. Clearly, the smaller models run out of juice.
Now is a good time to point out that SandForce-based SSDs equipped with slower asynchronous NAND demonstrate the same slow-down when they're presented with incompressible data, even at higher capacities. A 30 GB SSD 525 armed with asynchronous flash would be in a world of hurt. But mercifully, faster ONFi 2 NAND helps stop the bleeding. Newer 20 nm flash will eventually ship in 128 Gb die; a 30 GB using that memory would only populate two of eight channels, putting it somewhere in USB flash drive territory.
Sequential Write Performance
Similar results present themselves when we switch to writes. The 30 GB model continues to pull up the rear, though the other four SSDs top 500 MB/s committing easily-compressible writes to NAND. SandForce's controller is able to compress simple data patterns with surprising alacrity, but the SATA 6Gb/s limit keeps the four larger models dabbling around the 500 MB/s mark. It is true that much of the data touching a normal operating system is compressible to one degree or another. However, seldom is it as compressible as Iometer's repeating data buffer.
Writing 128 KB sequential blocks of incompressible data (that is, almost completely random information) is where the rubber meets the road as far as die count goes. The above chart speaks for itself. If the legend wasn't included, you'd still be able to figure out which line belongs to each drive. They're organized perfectly by capacity and die count.
Most consumer drives don't scale based on queue depth when they're hit with 128 KB sequential writes. Usually, the peak at a queue depth of one or thereabouts; stacking more commands doesn't accomplish much. That's unfortunate because the 30 GB could really use a boost. Consistently, the small drive can't break above 50 MB/s. The 60 GB model looks like a rock star in comparison, though 100 MB/s isn't stellar either.
The three larger drives perform much better, though. The 180 and 240 GB SSDs manage 260 and 320 MB/s, respectively. Increasing die count clearly corresponds to higher performance. But each time capacity goes up, performance jumps by a smaller percentage. Moving from 30 to 60 GB yields a 100% speed-up. From 180 to 240 GB, performance is only up by ~25%.