Why SSDs are Transitioning from SATA to PCIe in the Next Gen Form Factor

By Wesley Fenlon

SATA Express, a PCI Express standard for solid state memory, is paving the way for data rates of up to 4 GB/s.

Solid state storage is getting too fast for SATA. The most common interface for both SSDs and hard drives is serial ATA--it's been a computer mainstay since it replaced IDE about a decade ago. SATA revision 3.0 offers data speeds of approximately 600 MB/s, far faster than any 7200 RPM hard drive can read or write. But SSDs are now pushing up against that limit. They can go faster. What do we do? Turn to a new interface.

Judging by Apple's new Mac Pro, 2013 MacBook Airs and the way Ultrabooks are headed, that new interface will be PCI Express. PCIe and a new SSD product line called NGFF, or Next Gen Form Factor, are going to make Ultrabooks and flash memory-equipped laptops dramatically faster over the next two years. Let's look at how and why.

Photo credit: iFixit

A presentation from 2012's Flash Memory Summit does a good job of addressing why the industry is transitioning away from the SATA interface, at least in thin laptops. It's the data rate, as we already said--but why hasn't there been a new iteration of SATA to keep ahead of flash memory speeds? The presentation explains that "Enabling SATA beyond 600 MB/s is a long term development effort. Single lane scaling beyond ~8Gbps is challenging & requires trade-offs. Multi-lane SATA requires a new connector and modified chipset SATA controllers to make multi-lane software transparent."

PCIe doesn't have those issues. "To enable higher speed client SSDs in near term ('13/'14), PCIe is the only choice," the presentation states. "PCIe has bandwidth lead (1GB/s with Gen3). PCIe has multi-lane for scalability (x2, x4, ...). Software compatible PCIe SSDs can be built as a single port AHCI device.

Until now, mSATA has been a common SSD solution for Ultrabooks and other laptops. But beyond the speed concerns, mSATA presents some problems. At a whole five millimeters, it's too thick, and it's difficult to add extra NAND to. That's why the inudstry is moving to the Next Gen Form Factor, also called SATA Express, which is designed to support a number of sizes and interface via PCIe.

Photo credit: Anandtech

Anandtech wrote a bit about NGFF last fall, noting that it supports two sockets, one for SATA and PCIE x2 interfaces perfect for lower-end SSDs and other add-in cards like Wi-Fi. The second socket supports PCI Express up to 4 GB/s, which is crazy fast. Four gigs a second. Perhaps even crazier: SATA Express cards are even smaller than mSATA cards.

The Flash Summit Presentation also shows how this standard is going to allow for much more flexibility than mSATA, so while all NGFF cards will be thinner and narrower than mSATA, some will be longer. That's to support a standardized approach to SSD sizes. The smaller cards will be for minor storage and caching and will be either 42 or 60 millimeters long. 80 and 110 millimeter cards will hold more flash storage.

TweakTown points out that flash storage makers will need new controllers to take advantage of SATA Express, and the technology isn't in place yet to capitalize on that 4 GB/s bandwidth. Anandtech also discovered that Apple was using a custom design for its new Haswell-powered MacBook Airs, not the M.2 NGFF which will hopefully become standardized in the next year or so. On the bright side, Apple's flash solution spiked to read speeds near 800 MB/s and write speeds in the 750 MB/s range.

If we can expect speeds that fast or faster from virtually all Ultrabooks in 2014, if not late 2013, NGFF will be our new best friend.