Onsdag, 11 november 2015 10:29

SSD Interfaces - Where we are, where we are going, and why it doesn't include Bono

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SSD Interfaces: Where we are, where we are going and why it doesn’t include Bono

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It is the summer of 2015 and boy, for those who keep track of SSD technology, what a ride it has been up to this point. Around 2009, consumer-friendly SSD solutions and pricing catapulted the revolutionary storage technology into the fast lane. The SATA II (3 GBps) interface spec was quickly maxed out, and so was SATA III (6 Gbps); almost every mid to high-end SATA III SSD on the market today is limited by the 6Gbps interface. With no real successor in sight for years, vendors set out to seek other key differentiators—may it be a lower price per GB or additional features for increased durability, such as PFM+ (Power Failure Management Plus). Just recently though, new standards have emerged. NVMe™, M.2, U.2, and SATA Express are some of the terms you catch when you read about new product announcements. Unsure what this is all about? Is SATA obsolete now? What should I go for when considering an upgrade or first time investment in SSDs? Let’s have a look...


OCZ engineers reinventing the SSD wheel

What about SATA now?

SATA III has been the reigning king for a few years now and it took quite a while for a successor to appear. SATA revision 3.2 was the eventual outcome, known by its more popular name “SATA Express”. Sounds quick you say? It is, depending on the amount of PCIe lanes it gets from the motherboards, it supports 8-16Gbps. This is a vast improvement over SATA III but not quite as good as other alternatives that have emerged. This effectively renders the SATA Express standard pointless. There are some motherboards with SATA Express available, SSDs however, not so much. Like, not at all.

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M.2 (“M dot 2”) was originally dubbed “Next Generation Form Factor” (NGFF). It allows for very, very small drives (approx. 12-30mm x 16-110mm), which makes them a perfect fit for notebooks, and brings either SATA III or PCIe connectivity. The latter allows for up to 32Gbps, if your motherboard allows for the use of 4 PCIe lanes. It supports both AHCI (Advanced Host Controller Interface) and NVMe, but more about that below. With its PCIe interface, it offers virtually the same performance as a PCIe SSD with similar PCIe bandwidth. Note that some M.2 drives utilize a SATA interface and/or AHCI, making those SATA drives using the M.2 form factor with very little performance gain over a standard SATA III SSD.

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Prototype of OCZ RevoDrive 400 M.2 and adapter card at IDF 2015 (via techreport.com)


This name surfaced just recently; it was originally called SFF-8639. It was perceived that the original name wasn’t quite catchy enough (who would’ve thought?). The new name is easier to remember but may lead to misunderstandings, especially in music stores. As of today, U.2 is an interface for 2.5-inch SSDs sporting a PCIe interface. Unlike M.2, which is available in both mobile devices and desktops, U.2 is more likely to remain in the desktop and server domains. It has yet to reach wide availability, both in terms of motherboard support and SSD availability, but has major motherboard players supporting it. Until then, U.2 to M.2 adapters for current motherboard models are available from vendors such as MSI.

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MSI U.2 to M.2 adapter with cables


NVMe is the short handle for “Non-Volatile Memory Express” and effectively a replacement for the AHCI protocol, addressing the difference in how SSDs deal with data compared to HDDs. AHCI was introduced years back to replace the legacy IDE mode. AHCI was highly beneficial for HDDs, allowing them to queue their workloads more efficiently. With the years gone by and SSDs emerging, AHCI turned out to lack of efficiency when it comes to maximizing SSD performance. The NVMe protocol was developed to be THE specification for accessing SSDs. Compared to AHCI, it allows for larger command queues and utilizes the increased parallelism capabilities of modern SSDs. Much like with AHCI, Windows 7 (via additional update), Windows 8.1 and Windows 10 come with integrated NVMe drivers. Vendor-specific drivers however usually offer even better performance. Linux started supporting NVMe drivers as early as 2012 with constant development and improving ever since.

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The new NVMe protocol detailed with lots of boxes and arrows

That’s all great but what does this mean for me? What kind of SSD should I buy?

This entirely depends on the platform you plan to use it with and what you are going to do with it. With the introduction of new standards and form factors, your SATA III SSD doesn’t suddenly become the least bit worse, it’s just not the fastest anymore. If you want the best of the best, opt for a M.2 or U.2 drive (make sure your board supports M.2 32Gbps!) or a PCIe SSD with NVMe support. If you can settle for less, SATA III SSDs are still an excellent choice and not going away anytime soon.


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