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Analysis: Where We’ll Use PCI Express 3.0

PCI Express 3.0: On Motherboards By This Time Next Year?
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Storage

AMD is already integrating support for SATA 6Gb/s into its 8-series chipsets, and third-party motherboard vendors are adding USB 3.0 controllers. Intel is lagging behind in this area, with no chipset support yet for either USB 3.0 or SATA 6Gb/s (Ed.: Note that pre-production P67-based motherboards we've seen here in the lab do incorporate SATA 6 Gb/s support, but lack USB 3.0). However, as we’ve often seen in the AMD versus Intel saga, innovation at AMD usually inspires Intel. Given the data rates for both next-generation storage and peripheral interconnects, it's clearly not necessary to drop either technology onto PCI Express 3.0. Rather, a single lane of second-generation PCI Express is ample for both USB 3.0 (at 5 Gb/s) and SATA 6 Gb/s (which no storage device can even come close to saturating).

Of course, when it comes to storage, the interaction between drives and controllers is only part of the equation. Consider that dropping multiple SSDs on a SATA 6 Gb/s chipset and creating a RAID 0 array does actually have the potential to saturate the single lane of second-gen PCI Express that most motherboard vendors are using for their implementations. Deciding whether USB 3.0 and SATA 6 Gb/s can truly utilize PCI Express 3.0 support requires a closer look at the math.

As mentioned, USB 3.0 runs at 5 Gb/s. But as with PCI Express 2.1, USB 3.0 employs 8b/10b encoding, which lowers the actual peak speed to 4 Gb/s. Divide bits by eight to convert to bytes, and you get a peak throughput of 500 MB/s, which is the exact same speed as a modern PCI Express 2.1 lane. SATA 6Gb/s runs at 6 Gb/s of course, but its own 8b/10b encoding scheme drops the peak rate from a theoretical 6 Gb/s to an actual speed of 4.8 Gb/s. Again, convert that to bytes and you get 600 MB/s, or 20% more than the peak speed of a PCI Express 2.0 lane.

The problem here is that even the fastest SSDs cannot fully saturate a SATA 3 Gb/s connection. Nothing comes close to saturating a USB 3.0 connection, and the same holds true for the latest iteration of SATA 6Gb/s. At least as far as we're concerned today, PCI Express 3.0 isn't really a necessity for driving the biggest buzzwords in the platform space. Hopefully, as Intel shifts into its third generation of NAND flash manufacturing, however, speeds increase and we start to see devices capable of pushing beyond what a 3 Gb/s SATA port could have sustained in the past.

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