Today, the bandwidth issue isn't particularly pressing. After all, there still aren't any SATA-based drives able to move more than 300 MB/s. But it's important to take note of your motherboard's concessions, if there are any. For those who've purchased (or are looking to purchase) boards that divide PCI Express connectivity between graphics cards and storage controllers, you could be hurting performance in a very tangible way, even today. Moreover, as you start attaching next-gen devices to those controllers, you don't want to be dismayed by performance below what you were expecting.
Existing mainstream chipsets from do not provide sufficient PCI Express bandwidth for USB 3.0 or SATA 6Gb/s controllers because, while PCH-based PCI Express lanes supposedly offer a second-gen interface, they run at first-gen transfer rates (250 MB/s instead of 500 MB/s). Motherboard manufacturers can work around this by routing add-on components through PCIe switching logic or by physically wiring these controllers to PCI Express 2.0 lanes, which typically drive your graphics cards. AMD chipsets (starting with the 700-series) are fully PCI Express 2.0-compliant and consequently don’t exhibit such a limitation.
Motherboard solutions that reroute USB 3.0 and SATA 6Gb/s logic through the chipset’s PCI Express 1.1 links will lead to bottlenecked bandwidth. This also applies if you decide to install a x1 PCI Express USB 3.0 or third-gen SATA add-on card into any Intel platform or an older AMD-based machine. The 250 MB/s of PCIe 1.1 is the most you’ll get. Our benchmark results show that effective bandwidth may even be much lower.
Let’s focus on AMD for a moment. The company beat Intel to the inclusion of SATA 6Gb/s support in its latest southbridge revision, which complements the 890GX platform. The chipset serves up six SATA 6Gb/s ports natively, requiring no add-on controller at all. USB 3.0 is not yet supported by any chipset, but hooking up a discrete USB 3.0 controller to a single 500 MB/s PCI Express 2.0 link is a common and definitely workable approach.
For Intel systems, we have to recommend that you pay much closer attention when it comes to motherboard selection. Since 16 PCI Express 2.0 lanes are more than enough for a single graphics card, PCIe switches like the PLX device used on Gigabyte’s P55A-UD7 and MSI's P55-GD85 can accommodate the bandwidth requirements of additional USB 3.0 and SATA 3.0 controllers by dynamically allocating bandwidth from the P55 PCH when a pair of graphics cards is already monopolizing the processor's available second-gen PCI Express. These two boards show that this flexible bandwidth allocation remains a good solution, even if two graphics cards on x8 PCI Express 2.0 connects tax the PCIe bandwidth in CrossFire mode. Any motherboard that comes with added USB 3.0 and SATA 3.0 hardware should be running a PCI Express switch to administer the available bandwidth efficiently.
Bandwidth limitations on Intel's side will require a chipset refresh. It's possible to manage existing bandwidth more efficiently through PCI Express switching, though, which leads us to our main recommendation. If you're truly concerned with buying a board now that includes USB 3.0 and and SATA 6Gb/s support, it makes sense to purchase a higher-end platform in order to make sure you get a configuration with switching capabilities, at least (assuming, of course, that you're in the market for an H55-, H57-, or P55-based setup). Should you instead opt for an X58-based machine, these potential bottlenecks won't be an issue. Of course, that's a viable option as well and, frankly, more attractive to performance enthusiasts.
Though the promise of SATA 6Gb/s is exciting for enthusiasts eager to adopt early, the performance benefits are still limited, and we think it'd be best to wait. The market simply lacks drives faster than 300 MB/s.
With USB 3.0, though, the situation is different. Even if you plug a USB 2.0 x1 PCIe add-on card into a PCIe 1.1 slot, you'll still receive up to 250 MB/s of bandwidth, or a minimum of about 160 MB/s, as you can see in our benchmark results. Compared to USB 2.0 performance (at an effective 30-35 MB/s), this is already well worth the upgrade.