The More Complicated (yet predictable) SSD Lottery

Apple continues to use a custom form factor and interface for the SSDs in the MacBook Air. This generation Apple opted for a new connector, so you can't swap drives between 2011 and 2012 models. I'd always heard reports of issues with the old connector from a manufacturing standpoint, so the change makes sense. The new SSD connector looks to be identical to the one used by the Retina Display equipped MacBook Pro, although rest of the SSD PCB is different.


The Toshiba Branded SandForce SF-2200 controller in the 2012 MacBook Air - iFixit

As always there are two SSD controller vendors populating the drives in the new MacBook Air: Toshiba and Samsung. The Samsung drives use the same PM830 controller found in the 2012 MacBook Pro as well as the MacBook Pro with Retina Display. The Toshiba drives use a rebranded SandForce SF-2200 controller. Both solutions support 6Gbps SATA and both are capable of reaching Apple's advertised 500MB/s sequential access claims.

While in the past we've recommended the Samsung over the Toshiba based drives, things are a bit more complicated this round because of the controller vendor Toshiba decided to partner with.


The write/recycle path in NAND flash based SSD

Samsung's PM830 works just like any other SSD controller. To the OS it presents itself as storage with logical block addresses starting from 0 all the way up to the full capacity of the drive. Reads and writes come in at specific addresses, and the controller maps those addresses to blocks and pages in its array of NAND flash. Every write that comes in results in data written to NAND. Those of you who have read our big SSD articles in the past know that NAND is written to at the page level (these days pages are 8KB in size), but can only be erased at the block level (typically 512 pages, or 4MB). This write/erase mismatch combined with the fact that each block as a finite number of program/erase cycles it can endure is what makes building a good SSD controller so difficult. In the best case scenario, the PM830 will maintain a 1:1 ratio of what the OS tells it to write to NAND and what it actually ends up writing. In the event that the controller needs to erase and re-write a block to optimally place data, it will actually end up writing more to NAND than the OS requested of it. This is referred to as write amplification, and is responsible for the performance degradation over time that you may have heard of when it comes to SSDs.


Write Amplification

For most client workloads, with sufficient free space on your drive, Samsung's PM830 can keep write amplification reasonably low. If you fill the drive and/or throw a fragmented enough workload at it, the PM830 doesn't actually behave all that gracefully. Very few controllers do, but the PM830 isn't one of the best in this regard. My only advice is to try and keep around 20% of your drive free at all times. You can get by with less if you are mostly reading from your drive or if most of your writes are just big sequential blocks (e.g. copying big movies around). I explain the relationship between free space and write amplification here.

Write Amplification vs. Spare Area, courtesy of IBM Zurich Research Laboratory

The Toshiba controller works a bit different. As I already mentioned, Toshiba's controller is actually a rebranded SandForce controller. SandForce's claim to fame is the ability to commit less data to NAND than your OS writes to the drive. The controller achieves this by using a hardware accelerated compression/data de-duplication engine that sees everything in the IO stream.

The drive still presents itself as traditional storage with an array of logical block addresses. The controller still keeps track of mapping LBAs to NAND pages and blocks. However, because of the compression/dedupe engine, not all data that's written to the controller is actually written to NAND. Anything that's compressible, is compressed before being written. It's decompressed on the fly when it's read back. All of the data is still tracked, the drive still is and appears to be the capacity that is advertised (you don't get any extra space), you just get extra performance. After all, writing nothing is always faster than writing something.

Writing less data to NAND can improve performance over time by keeping write amplification low. There are also impacts on NAND endurance, but as I've shown in the past, endurance isn't a concern for client drives and usage models. Writing less also results in a slight reduction in component count: there's no external DRAM found on SandForce based drives. The PM830 SSD features a 256MB DDR2 device on-board, while the Toshiba based drive has nothing - just NAND and the controller. This doesn't end up making the Toshiba drive substantially cheaper as SandForce instead charges a premium for its controller. In the case of the PM830, both user data and LBA-to-NAND mapping tables are cached in DRAM. In the case of the Toshiba drive, a smaller on-chip cache is used since there's typically less data being written to the NAND itself.

SandForce's approach is also unique in that performance varies depending on the composition of the data written to the drive.

PC users should be well familiar with SandForce's limitations, but this is the first time that Apple has officially supported the controller under OS X. As such I thought I'd highlight some of the limitations so everyone knows exactly what they're getting into.

Any data that's random in composition, or already heavily compressed, isn't further reduced by Toshiba's SandForce controller. As SandForce's architecture is designed around the assumption that most of what we interact with is easily compressible, when a SF controller encounters data that can't be compressed it performs a lot slower.

Apple SSD Comparison - 128KB Sequential Read (QD1)
Special thanks to AnandTech reader KPOM for providing the 256GB Samsung results

Apple SSD Comparison - 4KB Random Read (QD3)

The performance impact is pretty much limited to writing. We typically use Iometer to measure IO performance as it's an incredibly powerful tool. You can define transfer size, transfer locality (from purely sequential all the way to purely random) and even limit your tests to specific portions of the drive, among other features. Later versions of Iometer introduced the ability to customize the composition of each IO transfer. For simplicity, whenever Iometer goes to write anything to disk it's a series of repeating bytes (all 0s, all 1s, etc...). Prior to SandForce based SSDs this didn't really matter. SandForce's engine will reduce these IOs to their simplest form. A series of repeating bytes can easily be represented in a smaller form (one byte and a record of how many times it repeats). Left at its default settings, SandForce drives look amazing in Iometer - even faster than the PM830 based Samsung drive that Apple uses. Even more impressive, since very little data is actually being written to the drive, you can run default Iometer workloads for hours (if not days) on end without any degradation in performance. Doing so only tells us part of the story. While frequently used OS and application files are easily compressed, most files aren't.

Thankfully, later versions of Iometer include the ability to use random data in each transfer. There's still room for some further compression or deduplication, but it's significantly reduced. In the write speed charts below you'll see two bars for the Toshiba based SSD, the one marked incompressible uses Iometer's random data setting, the other one uses the default write pattern.

Apple SSD Comparison - 128KB Sequential Write (QD1)

Apple SSD Comparison - 4KB Random Write (8GB LBA Space - QD3)

When fed easily compressible data, the Toshiba/SandForce SSD performs insanely well. Even at low queue depths it's able to hit Apple's advertised "up-to" performance spec of 500MB/s. Random write performance isn't actually as good as Samsung's, but it's more likely to maintain these performance levels over time.

Therein lies the primary motivator behind SandForce's approach to flash controller architecture. Large sequential transfers are more likely to be heavily compressed (e.g. movies, music, photos), while the small, pseudo-random accesses are more likely easily compressible. The former is rather easy for a SSD controller to write at high speeds. Break up the large transfer, stripe it across all available NAND die, write as quickly as possible. The mapping from logical block addresses to pages in NAND flash is also incredibly simple. Fewer entries are needed in mapping tables, making the read and write of these large files incredibly easy to track/manage. It's the small, pseudo-random operations that cause problems. The controller has to combine a bunch of unrelated IOs in order to get good performance, which unfortunately leaves the array of flash in a highly fragmented state - bringing performance down for future IO operations. If SandForce's compression can reduce the number of these small IOs (which it manages to do very well in practice), then the burden really shifts to dealing with large sequential transfers - something even the worst controllers can do well.

It's really a very clever technology, one that has been unfortunately marred by a bunch of really bad firmware problems (mostly limited to PCs it seems).

The downside in practice is the performance when faced with these incompressible workloads. Our 4KB random write test doesn't actually drop in performance, but if we ran it for long enough you'd see a significant decrease in performance. The sequential write test however shows an immediate reduction of more than half. If you've been wondering why your Toshiba SSD benchmarks slower than someone else's Samsung, check to see what sort of data the benchmark tool is writing to the drive. The good news is that even in this state the Toshiba drive is faster than the previous generation Apple SSDs, the bad news is the new Samsung based drive is significantly quicker.

What about in the real world? I popped two SSDs into a Promise Pegasus R6, created a RAID-0 array, and threw a 1080p transcode of the Bad Boys Blu-ray disc on the drive. I then timed how long it took to copy the movie to the Toshiba and Samsung drives over Thunderbolt:

Real World SSD Performance with Incompressible Data
Copy 13870MB H.264 Movie 128GB Toshiba SSD 512GB Samsung SSD
Transfer Time 59.97 s 31.59 s
Average Transfer Rate 231.3 MB/s 439.1 MB/s

The results almost perfectly mirrored what Iometer's incompressible tests showed us (which is why I use those tests so often, they do a good job of modeling the real world). The Samsung based Apple SSD is able to complete the file copy in about half the time of the Toshiba drive. Pretty much any video you'd have on your machine will be heavily compressed, and as a result will deliver the worst case performance on the Toshiba drive.

Keep in mind that to really show this difference I had to have a very, very fast source for the transfer. Unless you've got a 6Gbps SSD over USB 3.0 or Thunderbolt, or a bunch of hard drives you're copying from, you won't see this gap. The difference is also less pronounced if you're copying from and to the same drive. Whether or not this matters to you really depends on how often you move these large compressed files around. If you do a lot of video and photo work with your Mac, it's something to pay attention to.

There's another category of users who will want to be aware of what you're getting into with the Toshiba based drive: anyone who uses FileVault or other full disk encryption software.

Remember, SandForce's technology only works on files that are easily compressed. Good encryption should make every location on your drive look like a random mess, which wreaks havoc on SandForce's technology. With FileVault enabled, all transfers look incompressible - even those small file writes that I mentioned are usually quite compressible earlier.

After enabling FileVault I ran our Iometer write tests on the drives again, performance is understandably impacted:

Apple SSD Comparison - 128KB Sequential Write (QD1)

Also look at what happens to our 4KB random write test if we repeat it a few times back to back:

Impact of FileVault on SandForce/Toshiba SSD

That trend will continue until the drive's random write performance is really bad. Sequential write passes will restore performance up to ~250MB/s, but it takes several passes to get it there:

Recovering Performance with Sequential Writes after Incompressible Rand Write

If you're going to be using FileVault, stay away from the Toshiba drive.

This brings us to the next problem: how do you tell what drive you have?

As of now Apple has two suppliers for the SSD controllers in all of its 2012 Macs: Toshiba and Samsung. If you run System Information (click the Apple icon in the upper left > About this Mac > System Report) and select Serial ATA you'll see the model of your SSD. Drives that use Toshiba's 6Gbps controller are labeled Apple SSD TSxxxE (where xxx is your capacity, e.g. TS128E for a 128GB drive), while 6Gbps Samsung drives are labeled Apple SSD SMxxxE. Unfortunately this requires you to already purchase and open up your system. It's a good thing that Apple stores are good about accepting returns.

There's another option that seems to work, for now at least. It seems as if all 256GB and 512GB Apple SSDs currently use Samsung controllers, while Toshiba is limited to the 64GB and 128GB capacities. There's no telling if this trend will hold indefinitely (even now it's not a guarantee) but if you want a better chance of ending up with a Samsung based drive, seek out a 256GB or larger capacity. Note that this also means that the rMBP exclusively uses Samsung controllers, at least for now.

I can't really blame Toshiba for this as even Intel has resorted to licensing SandForce's controllers for its highest performing drives. I will say that Apple doesn't seem to be fond of inconsistent user experiences across its lineup. I wouldn't be surprised if Apple sought out a third SSD vendor at some point.

The Display Performance
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  • Osamede - Tuesday, July 17, 2012 - link

    This Y5 was highly portable. If you never used one you have no idea. It was wedge shaped. And the shape allowed for durablity. It was military grade and rated to survive a drop of several feet. And yes STILL only 3.5 lbs, In 2004.

    Besides its now 2012, give us a call when Apple manages to bring to market a 14" laptop with optical drive at 3.5 lbs.. Only God knows the kind of hype we would have heard it it was Apple doing what Panasonic managed damn near a decade ag.

    There was also the even smaller Panasonic W series and before that Sharp had some really good products with the Actius series at really good prices. And now thats going back to probably 1998 or even earlier

    Of course Apple fanboys imagine that the world began when Apple launched the MacBook Air. Pschew.....
    Reply
  • KPOM - Tuesday, July 17, 2012 - link

    Apple isn't going to make any more notebooks with optical drives. Once the classic MacBook Pro is phased out, optical drives are history at Apple. If you notice, most Ultrabooks lack them, as well.

    A wedge design topping out at 1.9" thick is still pretty bulky. Obviously it was designed for a niche audience, since they aren't selling them anymore. From the specs in 2007 it had a Core Duo 1.66GHz, topped out at 1.5GB RAM, and was very expensive. The keyboard also looks cramped.

    http://www.trustedreviews.com/Panasonic-ToughBook-...

    I'm not disputing that there were ultraportables before the MacBook Air. Heck, I've even pointed to the PowerBook Duo. I've always been a fan of ultraportables and though they were a long-neglected category. Apple itself didn't have one for 2 years after the 12" Powerbook was dropped.

    Most ultraportables were slow sellers that were quietly abandoned. To its credit, Apple stuck with the MacBook Air. A lot of Apple fans were even calling for Apple to drop the MacBook Air as late as September 2010. However, they stuck with it and continued to perfect the design. It's indisputable that the release of the $999 11.6" MacBook Air in October 2010 launched the concept of a mainstream ultraportable.
    Reply
  • Osamede - Friday, July 20, 2012 - link

    Nice try with the red herring about optical drive.

    The Panasonic was 3.5 lbs with Optical dive. Take that weight out and you have a 14" laptop with 1400 x 1050 at 3.2 lbs.....in 2006.

    And yet according to you we are all supposed to roll over in supplication because in 2012 Apple is selling 13" laptops that weight 3lbs. Spare me the hype. Even The 2010 Sony Z is already a superior laptop compared to even todays "latest" MacBook Air, let alone the 7 year old Panasonic model.

    Some of you swallow marketing BS and think it is fact.
    Reply
  • KPOM - Friday, July 20, 2012 - link

    Red herring? You are the one claiming that Apple should be putting in optical drives if they want to brag. My point is that optical drives are mostly passé. Intel hasn't made it part of the Ultrabook standard. Sony's Vaio Z is an impressive machine, but at more than twice the cost of the base MacBook Air it had better be.

    I'm not saying we need to roll over in supplication. What I am saying is that Apple transformed the ultraportable from a niche device to a mainstream device. My whole point, which your red herring about Panasonic Toughbooks and Sony Vaios ignores, is that ultraportables had been around for a decade but weren't popular devices until Apple got the formula right. Maybe Panasonic could have made a 14" laptop with 1400x1050 at 3.2lbs in 2006, but they didn't. Maybe Sony could have made a $999 Sony Vaio in 2010, but they didn't. Apple DID make a $999 MacBook Air in 2010. Apple DID make an ultraportable with a full size keyboard and super slick trackpad in 2008, and they DID make it affordable in 2010, and they DID take the drastic step of dropping hard drives and going exclusively with SSD storage in 2010.
    Reply
  • phillyry - Tuesday, March 26, 2013 - link

    You can't just say that this other laptop was lighter than it actually was because it would be lighter if it didn't have an optical drive that it did have. Confused much? That's because it doesn't make sense. Your logic is flawed. It either had the drive and the weight and bulk that go with it (as it did) or it didn't (as in your imaginary best case). Reply
  • phillyry - Tuesday, March 26, 2013 - link

    Also, Apple is phasing out the optical drive. No red herring there. Reply
  • KPOM - Tuesday, July 17, 2012 - link

    "Of course Apple fanboys imagine that the world began when Apple launched the MacBook Air. Pschew..... "

    Actually a lot of "Apple fanboys" HATED the original MacBook Air.

    Read some of the comments to this article: http://www.macworld.com/article/1131583/macbookair...

    As for optical drives, it's 2012. Did you complain when Apple stopped supporting the floppy drive, too? I don't see any optical drives on Microsoft's Surface Pro.
    Reply
  • phillyry - Tuesday, March 26, 2013 - link

    The devices you refer to were not mainstream. Reply
  • will54 - Monday, July 16, 2012 - link

    "While 1440 x 900 is a bit much on a 15-inch MacBook Pro, I'd say it's near perfect on the 13-inch Air. If Apple were to do the Retina treatment on here, it'd be magnificent." This line is a bit confusing, can you explain further? Reply
  • Tegeril - Tuesday, July 17, 2012 - link

    I assume he intended to mean that the 15" screen is too large for 1440x900 as the pixels are quite large, but on a 13" screen it is ideal. Reply

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