What's Best? U.2, M.2, SATA Express, & SATA
Below is the transcript for the video, which reads as a complete article for folks who prefer the written form:
PAX East heralded a strong re-emergence of another new storage interface – this time, it's the U.2 interface that we saw on Gigabyte's unreleased Broadwell-E motherboards. This TLDR video recaps the differences between U.2 and M.2 storage devices as quickly as we can, with some additional information on SATA Express – like where it's gone.
First up, an extremely abbreviated recap of current chipsets: Intel's 100-series chipsets have high-speed IO lanes that are almost entirely addressable by the motherboard vendor, allowing for more differentiation between products. These are called HSIO lanes. Z170 has 26 HSIO lanes that can be assigned to GbE, SATA, PCI-e, or PCI-e enabled devices – like U.2 and M.2.
What is U.2? The U.2 interface was originally called SFF-8639, but has been renamed. The U.2 interface connects directly to PCI-e lanes on the motherboard, rather than going through the SATA interface, and that makes U.2 an expansion on SATA Express. U.2's pin-out allows use of 4 total PCI-e lanes. As such, its maximum theoretical throughput on Gen3 is 4GB/s. The U.2 pin-out resembles the SAS connector, but with way more pins for the lanes. Several of the pins are reserved for the refclock, lanes 0-3, the SMBus, and Dual Port. The remainder of the pins are used for signaling, power and control, and the other refclock.
(Above: Can't fit many of these on a motherboard).
(Above: A U.2 SSD)
On the motherboard, U.2 is a double-decker connector that receives a similarly double-decker cable from the SSD. On the other end, a much wider cable plugs into the SSD for the U.2 multi-lane interface, with an additional cable for power. This is the fastest 2.5” SSD interface currently available to consumers, but that doesn't mean the drives are inherently faster. More on that momentarily.
SATA Express, meanwhile, communicates maximally through 2 PCI-e lanes on the motherboard, limiting the interface to 2GB/s on Gen3. SATA Express will become a dead and abandoned standard in short order, as the industry continues to ignore its existence and moves fully to M.2 and U.2 interfaces. SATA Express cannot communicate through 4 PCI-e lanes.
For reference, SATA has a maximum theoretical throughput of 600MB/s, which comes down to about 550MB/s after the overhead is accounted for. SATA does not utilize PCI-e, which is a small advantage for anyone maxing-out their chipset's lane count – but keep in mind that chipset storage lanes are not the same as GPU lanes, so even multi-GPU configurations may not conflict with NVMe or PCIe SSDs. Depends on the configuration, though.
(Above: U.2 adapter for M.2 slots allows for connection to U.2 SSDs -- some of which use faster controllers than M.2 SSDs)
M.2, then, is the most comparable to U.2. It's capable of the same four-lane throughput for storage devices, but takes a significantly larger footprint on the motherboard and limits users purely by physical space. U.2 interests us because it can be stacked where current SATA connectors are, PCI-e lanes allowing, and you could theoretically run several 2.5” U.2 SSDs.
Host, Video Editing: Steve "Lelldorianx" Burke
B-Roll: Keegan "HornetSting" Gallick
Supporting Research: Patrick "Mocalcium" Stone