M.2 And SATA Express, Discussed
M.2 PCIe
Intel ratchets up the utility and flexibility of Z97 in a small but profound way. This is achieved by making the chipset’s port 13 and 14 far more flexible than they were in the past. Previously, those two ports facilitated two of the PCH's six SATA interfaces. Now they're flexible, accommodating two big pieces of storage infrastructure: SATA Express and M.2 PCIe.
M.2 PCIe isn’t anything new. As far back as September of last year we were covering that in SanDisk A110 PCIe SSD: Armed With The New M.2 Edge Connector. More recently, we posted Plextor M6e 256 GB PCI Express SSD Review: M.2 For Your Desktop, testing the M.2 PCIe-based SSD on a half-length, half-height add-in board. We were told Plextor was also planning a version without the adapter, which suggested that there'd be motherboards with the corresponding two-lane slots. That day should be today. But as boards equipped with M.2 PCIe slots start selling, SSDs able to drop into them are few and far between.
It's actually pretty easy to mix up M.2 for PCIe and SATA. We're facing the same sort of confusion experienced when mSATA surfaced for mini-PCIe slots. Except this time, it's ever harder to distinguish between M.2 storage with SATA controllers and M.2 PCIe SSDs. So let's just forget about the SATA-based drives and focus on storage natively designed to drop onto the PCI Express bus through the M.2 form factor.
This form factor is flexible in that it can be molded into a wide range of single- or double-sided PCBs. An M.2 device is 22 mm wide, easily fitting a processor and NAND flash packages. Build a longer PCB and you get more space to add flash. Considering Samsung can cram its 1 TB 840 EVO mSATA into roughly the same space as a M.2 2260 (60 mm-long) form factor, something like a M.2 22110 (110 mm-long) gives you a ton of space to work with. And as manufacturing advances, increasing density, it's hard to imagine a day when M.2 will limit the upper bound of capacity.
| M.2 Real Estate (in mm) | 42 mm (M.2 2242) | 60 mm (M.2 2260) | 80 mm (M.2 2280) | 110 mm (M.2 22110) |
|---|---|---|---|---|
| Single-Sided | 924 mm2 | 1320 mm2 | 1760 mm2 | 2420 mm2 |
| Double-Sided | 1848 mm2 | 2640 mm2 | 3520 mm2 | 4840 mm2 |
Most M.2 PCIe SSDs will utilize two PCI Express lanes (in the case of Z97 Express, of course that means second-gen transfer rates). But Samsung's XP941 is unique in that it communicates over four. That makes it the most ideal candidate for testing ASRock's Z97 Extreme6 and its four-lane PCI Express 30 Ultra M.2 socket.
SATA Express
SATA Express replaces SATA 6Gb/s. The Serial ATA International Organization realized that doubling SATA's transfer rate again wasn't going to be practical. As Paul Wassenberg of Marvell fame told me last year at Flash Memory Summit 2013, SATA Express makes a lot more sense.
As the working group scaled up SSDs from one to eight PCIe lanes in testing, power consumption went through the roof as lanes were attached. But with just two lanes at third-gen transfer rates, power didn’t increase much compared to a SATA 6Gb/s-connected equivalent drive, even as performance was vastly superior. We know from the challenges presented by 12 Gb/s SAS that a cost-effective implementation would be difficult to achieve for SATA. Meanwhile, SATA Express wouldn't be as problematic. Given its PCI Express roots, however, cabling was the challenge to address.
Unlike M.2 PCIe SSDs, which can span up to four lanes, SATA Express uses just two. But whereas a M.2 PCIe-based drive is basically stuck to the motherboard, SATA Express employs cables to make more remote connections, just like SATA. This poses a few practical issues. An external PCIe-based SSD needs a signal from the clock generator. Carrying that signal over distance requires shielding and a beefier (more expensive) cable. So, to combat a prohibitively pricey implementation, the signal can be provided by the solid-state device itself.
Gain a Port, Lose a Port
Based on Intel's implementation of SATA Express in Z97, if you utilize the new technology, you lose access to two of the storage controller's SATA 6Gb/s ports and the M.2 interface. If you instead choose to go with M.2 (the devices are more plentiful, after all), you can't use SATA Express.
To help add a bit of clarity, I created the flow chart above to clarify M.2 PCIe and SATA Express, along with the AHCI and NVMe interface specifications.
On Z97, the PCH-provided M.2 and SATA Express ports are mutually exclusive. You cannot use both simultaneously. Asus is adding third-party SATA Express controllers to some boards, so obviously those are able to operate independently. And then there's ASRock's solution: borrowing four lanes from the CPU's PCI Express controller to create the Ultra M.2 slot. Let's look at that in more depth...
