A Custom Form Factor PCIe SSD

In the old days, increasing maximum bandwidth supported by your PATA/SATA interface was always ceremonial at first. Hard drives were rarely quick enough to need more than they were given to begin with, and only after generations of platter density increases would you see transfer rate barriers broken. Then came SSDs.

Not only do solid state drives offer amazingly low access latency, but you can hit amazingly high bandwidth figures by striping accesses across multiple NAND Flash die. A 256GB SSD can be made up of 32 independent NAND die, clustered into 8 discrete packages. A good controller will be able to have reads/writes in flight to over half of those die in parallel. The result is a setup that can quickly exceed the maximum bandwidth that SATA can offer. Today that number is roughly 500MB/s for 6Gbps SATA, which even value consumer SSDs are able to hit without trying too hard. Rather than wait for another rev of the SATA spec, SSD controller makers started eyeing native PCIe based controllers as an alternative.

You can view a traditional SSD controller as having two sides: one that talks to the array of NAND flash, and one that talks to the host system’s SATA controller. The SATA side has been limiting max sequential transfers for a while now at roughly 550MB/s. The SATA interface will talk to the host’s SATA interface, which inevitably sits on a PCIe bus. You can remove the middle man by sticking a native PCIe controller on the SSD controller. With SATA out of the way, you can now easily scale bandwidth by simply adding PCIe lanes. The first generation of consumer PCIe SSDs will use PCIe 2.0, since that’s what’s abundant/inexpensive and power efficient on modern platforms. Each PCIe lane is good for 500MB/s, bidirectional (1GB/s total). Apple’s implementation uses two PCIe 2.0 lanes, for a total of 1GB/s of bandwidth in each direction (2GB/s aggregate).

The move to a PCIe 2.0 x2 interface completely eliminates the host side bottleneck. As I pointed out in my initial look at the new MacBook Air, my review sample’s 256GB SSD had no problems delivering almost 800MB/s in peak sequential reads/writes. Do keep in mind that you’ll likely see slower results on the 128GB drive.

Users have spotted both Samsung and SanDisk based PCIe SSDs in the 2013 MacBook Airs. Thankfully Apple doesn’t occlude the controller maker too much in its drive names. An SM prefix denotes Samsung:

My review sample featured a Samsung controller. There’s very little I know about the new Samsung controller, other than it is a native PCIe solution that still leverages AHCI (this isn't NVMe). Within days of Apple launching the new MBAs, Samsung announced its first consumer PCIe SSD controller: the XP941. I can only assume the XP941 is at least somewhat related to what’s in the new MBA.

The Samsung controller is paired with a 512MB DDR3 DRAM and 8 Samsung 10nm-class (10nm - 20nm process node) MLC NAND devices. 

New PCIe SSD (top) vs. 2012 MBA SATA SSD (bottom) - Courtesy iFixit

Despite moving to PCIe, Apple continues to use its own proprietary form factor and interface for the SSD. This isn’t an M.2 drive. The M.2 spec wasn’t far enough along in time for Apple to use it this generation unfortunately. The overall drive is smaller than the previous design, partially enabled by Samsung’s smaller NAND packages.

Absolutely Insane Battery Life PCIe SSD Performance
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  • arkhamasylum87 - Monday, June 24, 2013 - link

    Broadwell will have a refreshed GPU architecture and with the process shrinks, the gains would be more amenable to all. Although the intent to raise the GPU perf a decent percentage with dedicating more than the half the die is a big time change at Intel.
  • rmr - Monday, June 24, 2013 - link

    Hi Anand,

    Good review! I'm waiting for the updated review (using the i7 processor). BTW will it be possible for you to test the Air with an older 802.11g router (since some people have been complaining in the Apple forums about the Air dropping Wifi connections)? I was planning to get a new MBA but I'll be mostly using it at locations with older .11g routers.


  • scyap - Monday, June 24, 2013 - link

    Did the writer mention what OSX version was used for testing? Or I missed out?
    If this review was using Mountain Lion, should I expect even better battery life in Mavericks?
  • xype - Tuesday, June 25, 2013 - link

    Yes, you should. Every dev I know who runs on Mavericks reports 10-20% better battery life (that's mighty subjective and unscientific, but I am sure AnandTech will do a proper test).

    Personally I'll make use of my dev account and upgrade as soon as all the dev tools I need are confirmed a running (homebrew-installed stuff mostly).
  • Ricopolo - Tuesday, June 25, 2013 - link

    Anand, love your site. But your constant use of acronyms (like PCH, TDP, etc) that are non-household terms can be quite an obstacle to lay people, who are interested in gaining a bit of insight in tech development. Can you put together a glossary for these acronyms and put it in the footer or somewhere obvious?
    Thanks a lot.
  • SkylerSaleh - Tuesday, June 25, 2013 - link

    PCH stands for Platform Controller Hub. It provides some needed utilities required to run the CPU correctly, such as display handling, connecting peripherals, DMI, etc. As a lay-mans example, the PCH supports the CPU's operation, similar to how your subconscious supports your operation. Taking over the responsibility of semi-voluntary/non-voluntary actions like breathing, so that you can think about other things.
    TDP stands for thermal design power. It is a measurement of the maximum amount of cooling that is needed to cool a chip in its worst case. As a rule of thumb, the higher the TDP, the more power the chip will use at peak. (However this is not a good measurement of power usage when idle.)
  • name99 - Tuesday, June 25, 2013 - link

    Google shows up the obviously correct results for both of these very high in its list.
    If you want even cleaner and simpler results search on wikipedia.

    I don't want to be a dick, but part of Anand being a fairly high-end site (including, for example, the variety of technical details which make it substantially more interesting than a Macworld or The Verge or WSJ review) is that it consists of an engineering aware community, which speaks in its natural language, just like any other community.

    If you wish to be part of that community, the solution is not to complain that they use unfamiliar language, but to familiarize yourself with that language. It's not hard --- more so than ever before in the past you can learn what you need to just by scanning Google and wikipedia. And if you want to understand more details, again it's easier than its ever been before --- just look over the review articles either on this site, on Ars Technica (for the simplest introduction) or on David Kanter's Real World Tech (for the most sophisticated introductions you can probably get for free on the internet).
  • jb510 - Wednesday, June 26, 2013 - link

    Well said. It's worth noting that even some of us very technical folks have to look up a lot of terms and acronyms reading here as the spectrum of content is so broad. However, I greatly prefer the clean without reference to terms style of AT to the cumbersome reading if copy that is interrupted constantly to define things. Unless you're reading the print edition of AT, the whole internet is just one click away.
  • robco - Tuesday, June 25, 2013 - link

    The updated Airs seem pretty sweet. I use my laptop as my primary machine, so I'll probably wait for the rMBP to get updated. I'm also curious to see how well the Iris 5100 graphics compare to the HD 5000. An IPS display would have been a nice upgrade, but I can see why Apple decided to make upgrades in other areas instead.

    The WiFi snafu is interesting. I'm not sure if Apple missed it, or figured it would be a while before most users would upgrade to ac and decided to go ahead and ship it in time for WWDC. As for battery life, even the numbers under heavy workload are impressive for such a small machine. I'm curious to see how well the battery life numbers improve for other ultrabooks as they switch to Haswell running different OSes. I would also like to see if or how well battery live improves after OS 10.9 is released this fall.
  • Abelard - Tuesday, June 25, 2013 - link

    Thanks for the thorough review, Anand. The battery life you were getting is very impressive.

    I'm curious how the MBA will perform running OS X Mavericks, though. Developers and early adopters seem to be reporting battery life improvements. It's possible Mavericks could squeeze another hour or two out of the 2013 MBA.

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