Introduction And Specifications
Advances in controller error correction technology allow SSD makers to utilize low-cost flash in value-oriented solid-state drives. As manufacturing lithography shrinks, the number of rated program-erase (P/E) cycles decreases. Available P/E cycles take another hit moving from two- to three-bit-per-cell flash.
For several years, we had NAND rated for 5000 cycles. Then it was 3000 cycles. Now, that stuff is starting to trail off, displaced by 1000-cycle flash that is already appearing in today's low-cost consumer SSDs. This will soon give way to 500-cycle flash, which is what you'd find in thumb drives.
As far as we can tell, every SSD controller maker is working towards extending the life of low-endurance flash. They all have a "special sauce" and a trademarked marketing name. The proprietary software accompanies Low-Density Parity-Check code (LDPC), which uses a very small amount of space to build error correction data, leaving more capacity for users. Silicon Motion's next-generation ECC code for solid-state storage products just so happens to be called NANDXtend.
Most of today's SSDs employ BCH code to correct bit errors. The controller vendors add their own spin to enhance the technology as well. When a bit fails, the controller does what is called a read retry. This adds latency and consumes more power compared to a successful read.
Next-gen error correction technology is progressive. Not every bit needs to be dug out with a big shovel. If the deepest method was used for all correction, latency would be higher across the board and power consumption wouldn't look so pretty. Silicon Motion's NANDXtend is a three-level algorithm that progresses through the ECC as needed until your data is error-free.
Silicon Motion also incorporates another layer of data protection, RAID Data Recovery, which doesn't require excessive over-provisioning to implement RDR. The custom version reportedly takes just .1% of the flash capacity.
Silicon Motion's internal testing with a 120-degree bake to accelerate the degradation process shows a 3x endurance increase.
The end result is that SSD vendors can use lower-cost, lower-endurance flash and still guarantee the same number of writes per day that we have now.
The Silicon Motion SM2256 Four-Channel SSD Controller
It's important to understand that Silicon Motion doesn't make SSDs you can buy. The company builds controllers, firmware and reference drives so companies like Intel, Crucial and Corsair can create the final product to sell. Silicon Motion's SM2246EN was the company's break-out controller, and it has several design wins. Crucial recently released its BX100 series with the SM2246EN, and SanDisk has a model of its own. That's a big change from Marvell, which both companies featured in the past.
The specifications gleaned from Silicon Motion's marketing material cites performance with Toshiba A19 TLC flash. But today we have a special treat on our test bench: the SM2256 controller with Samsung's 19nm TLC flash. This is the same NAND that shipped in Samsung's 840 EVO drives.