It seems that solid state drives are getting too fast, at least for the SATA III standard. As SSDs continue to increase in speed, the need for a new interface to accommodate high data transfer rates looks like the next step in storage technology. So how much speed are we talking about? SATA III has a maximum throughput of 6 Gb/s, which works to about 550 MB/s in the real-world. Meanwhile, SSDs are capable of surpassing 2,000 MB/s when not held back by the SATA III.
SATA III is plenty fast for traditional spinning hard drives, but magnetic platters aren’t the future of storage. Solid state storage demands a new interface that will push the performance envelope forward into the future. Several interfaces and standards have been developed in the past few years, though it is not quite clear which will become dominant as solid state storage becomes the norm. We take a look at the top contenders and examine where they can succeed and fail.
The Serial ATA International Organization, the powers behind SATA, provided a successor in the form of SATA Express. It is a combination of PCI Express and SATA buses, supporting both types of devices. In addition, it also works with older SATA II and SATA III devices. On the performance side, SATA Express utilizes two PCI-e lanes to for speeds up to 10 Gb/s in PCI-e 2.0 or 16 Gb/s in PCI-e 3.0.
Looking at the SATA Express connector on the motherboard above, you could confuse it for two SATA slots and maybe a third odd-looking connector off to the side. You can see that connector is reminiscent of IDE cables of old, which may not be a good thing. We don’t need to bring long and clunky connectors back in vogue.
Thankfully for space-conscious builders, the number of motherboards with SATA Express support is very low. And the number of SATA Express drives? Well, there have been no SATA Express SSDs released to date. The two lane PCI-e limit likely has something to do with that, as other SATA successors support at a minimum two.
PCI-Express Expansion Cards
SSDs in the form of PCI-e expansion cards have been on the market for several years now, though they have mainly been for enterprise-grade applications. The size of a half-height half-length (HHHL) GeForce or Radeon video card, expansion card SSDs aren’t exactly compact. But they don’t have the limitations of SATA drives and can use up to 16 PCI-e lanes—though most use only four.
The expansion card form factor also allows for more NAND chips to be put on the card for even higher SSD capacities. It isn’t uncommon to see one terabyte or larger capacities with these video cards. And they can even accommodate giant heat sinks, just like video cards. But to replace SATA III as the go-to standard for SSDs, PCI-e likely won’t cut it in large part because they can’t be used with laptops.
The form factor formerly known as SFF-8639, U.2 is SATA Express without the wide clunky connector and two-lane PCI-e limitation. U.2 supports up to four PCI-e lanes for fast transfer speeds and already sees use in the enterprise space. SFF-8639’s rebranding was a part of SATA revision 3.2 and was largely because U.2 is more consumer-friendly.
The U.2 interface on the storage device side resembles a standard SATA connector except a little wider. Moving to the motherboard side of the cable though, U.2 features a rather unique connector that isn’t found on many mainstream motherboards. However, U.2 connections can be added via PCI-e or M.2 slots.
In addition to rebranding SFF-8639 asU.2, SATA revision 3.2 also standardized the M.2 connector for storage devices. Formerly known as Next Generation Form Factor (NGFF), the M.2 connection standard can use two or four PCI-e lanes and was designed to be used for multiple purposes. The standard can be used for video cards, sound cards, and more. It wouldn’t be too far off to call it a miniaturized PCI-e slot.
On the motherboard, an M.2 slot resembles a very short RAM or PCI-e 1x slot. An M.2 SSD looks similar to a stick of DDR RAM, with NAND module on both surfaces. The pins are located on the end, instead of running along the length of the module. And similar to RAM and PCI-e expansion cards, M.2 cards are notched along their connectors. These notches are known as keys and there are several key configurations, but SSDs primarily use the B or M key.
Using PCI-e lanes addresses the interface limits of SATA, but it does little to address the limitations of the AHCI (Advanced Host Controller Interface) standard. A specification from 2004 and defined by Intel, it covers how SATA drives operate. But designed for hard drives, AHCI just can’t keep up with SSD performance.
The alternative is the newer NVMe (Non-Volatile Memory Express) specification, which was developed jointly by storage leaders such as Intel and Samsung. NVMe was made specifically for SSDs to take advantage of PCI-e lanes and allows computers to boot from PCI-e and M.2 storage drives.
The main NVMe advantages over AHCI include lower latency, multi-core CPU support, and more command queues with greater command queue depth. These changes give it increased IOPS and better battery life. Fun fact: the iPhone 6s uses NVMe for storage.
The move away from SATA and AHCI has begun at the top, with the fastest and most expensive SSD models utilizing NVMe and PCI-e lanes. NVMe seems to be a given, as AHCI simply cannot keep up with the demands of SSDs. As for a standardized PCI-e interface, M.2 seems to be a promising contender while SATA Express is basically dead in the water. Expansion card and U.2 SSDs are still mainly desktop computer affairs as laptops can’t use them. For now, NVMe capable M.2 SSDs seem to hold the most promise.