Articles: Storage

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One Controller - Many SSDs

Each SandForce controller implements an original concept of compressing data “on the fly” prior to writing it into flash memory. Although this makes the controller itself somewhat more complex, the benefits are indisputable. The compression of data helps reduce the number of flash memory accesses to increase performance. Another consequence of compression is that the service life of an SSD is increased since there are fewer rewrite cycles. SandForce controllers also feature RAISE technology which uses some of the available flash memory for storing checksums which help recover data in case of read errors.

In other words, the SandForce architecture is well-suited for inexpensive NAND flash of inferior quality and performance. Even if the SSD uses asynchronous chips with ONFI 1.0 interface and a bandwidth of 50 MB/s, the eight-channel SF-2281 controller helps achieve acceptable speed: up to 400 MB/s with non-compressible data. And if data can be compressed, which is typical of desktop applications, the data-transfer rate may get much higher and even exceed the bandwidth of the SATA 6 Gbit/s interface. Thus, the SSD manufacturer can look for the optimal balance between performance and price, delivering very different products.

Taking the products we’re going to test today as an example, let’s see how many SSD modifications can be built on the basis of one and the same SandForce SF-2281 controller.

So we can see that these SSDs use very different NAND flash types, some of which are 3 or 4 times as fast as others. However, the manufacturers are not eager to specify real-life characteristics for their products. Thanks to the SF-2281’s ability to compress data written into memory, they prefer to talk about peak performance which depends on the controller only.

We’ve got a paradoxical situation as the consequence: SSDs with almost identical official specs may differ by 50% in price due to differences in their internal design. Obviously, they can’t deliver identical performance. The benefits of faster flash memory should show up somehow.

Indeed, the choice of flash memory affects the internal speed of an SSD. Synchronous flash memory not uses a stobe signal to transfer data, but is at least twice as fast as asynchronous flash memory thanks to DDR mode support. Therefore synchronous-flash SSDs have fewer bottlenecks and can deliver high performance irrespective of whether data can be compressed or not (archives, JPEG images, H.264 videos and other such files are almost completely incompressible). The downside is that such SSDs are more expensive.

The cheaper asynchronous variety of flash memory limits the data transfer speed between the controller and flash memory chips even if they are connected through as many as eight channels. When there is a high incoming data stream that can't be reduced by compression, the overall performance of the SSD degrades.

In other words, expensive synchronous NAND flash with a bandwidth of 166 or 200 MB/s has practical benefits. This is the reason why SSDs with ONFI 2.0 or Toggle Mode NAND flash are positioned as premium products. Asynchronous-flash SSDs are more affordable.

Now that we’ve found out the reasons for the different positioning, let’s take a look at each of the products we are about to test.

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