We are usually not very thrilled about reviewing SSDs with 120 or 128 gigabytes storage capacity. This capacity is just not good for showing what a particular SSD design can do. Today’s SSDs use eight-channel controllers and 64-gigabit flash memory chips, so the controller can only use 2-way interleave in a 120/128GB SSD. The controller’s bandwidth is thus not utilized fully, which explains the difference in practical performance between SSDs of different capacities. The gap may vary depending on the specific hardware platform, but there is a gap nevertheless. The SandForce platform is especially susceptible to this effect since it doesn’t incorporate a DDR SDRAM buffer. As a result, the difference in speed between 120 and 240GB models, for example, is quite conspicuous.
In fact, we don’t have to look far for examples. We only have to compare the specified sequential write speeds of different-capacity SSDs from the same series. Here’s what we have with the Corsair Force GT series which we often use in our tests:
The diagram doesn’t begin at 0 on the Y axis, yet you should keep it in mind that we compare the officially specified speeds which are measured with compressible data. In real-life applications the difference in performance is going to be much bigger. We can illustrate this by benchmarking 120GB and 240GB Corsair Force GT drives in CrystalDiskMark 3.0.1 using incompressible data:
The picture says a lot, we guess. The SSDs differ twofold in terms of sequential writing. Their performance with 4KB data blocks differs considerably, too. It is certainly going to show up in practical applications.
That said, we still can’t help testing 120 and 128GB SSDs. They are just very popular. Their capacity is optimal for a system disk, for example, and they will anyway be much faster than any conventional hard disk. But again, if you need maximum performance, such drives won’t be the best option.
As you may have already guessed, we’ll test 120 and 128GB SSDs today. These are products from Silicon Power and Transcend. Both represent somewhat unusual implementations of the second-generation SandForce platform. Their flash memory type isn’t typical and the Transcend also has a nonstandard capacity of 128 rather than 120 GB. It will give us a chance to check out the influence of the 13% reserved space on the performance of SandForce-based SSDs.