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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  • KPOM - Tuesday, July 17, 2012 - link

    If you want to tinker with the system, get to know Terminal. It's a fully functional, full-blown Unix command line. Lots of Mac "tricks" involve using Terminal.

    There is a lot of eye candy in OS X, and unlike Windows, the standard utilities don't include lots of customizations, as that's a deliberate design decision. However, the functionality is there. OS X provides X Window support as well.

    Apple isn't a Windows OEM. They aren't obligated to optimize their PCs to run Windows. They make it available as a backstop. It isn't a matter of "forcing" people to us OS X. No one is "forced" to use OS X or buy a Mac. Boot Camp makes it less "risky" for someone with a lot of legacy software to make the switch since it provides a way for it to work. It doesn't promise and Apple doesn't advertise a system optimized to run Windows.
    Reply
  • Karltheghost - Wednesday, July 18, 2012 - link

    I think you're still getting me wrong. . .
    I live in Europe and here apple isn't as strong as in the USA for example. 90% are using Windows here. However, there are many people who would like to buy a macbook to run another OS on it, because they like the hardware. And actually it is officialy sold to possibly be used with Windows (If you go into a store, ask the guy at the counter if you can use windows the answer is yes) . But that's only possible with huge restrictions. They would sell a ton more machines if they would give full support for other OS.

    And the other matter according OSX that you've overlooked is their privacy policy and license agreement. Data privacy is quite a big deal to many persons here and to me it's one, if not the main reason not to use OSX
    Reply
  • pmhparis - Wednesday, July 18, 2012 - link

    You vastly overstate the problems with using Windows on Apple Hardware to the point that I see sour grapes in your comments.

    I'm coming from a background of years of Windows PCs. For the last few years I was a regular user of VMWare to get access to the different OS environments I need.

    I bought a rMBP to replace a Dell e6500 that was coming to the end of it's rope and my collegues are all drooling over how relatively Parralels makes using Windows VMs in OSX. The fact that it just works largely overcomes the minor problem of any performance hit. Besides, the rMBP is brand new and thus faster than all the other PCs so the Windows VMs are still faster than their PCs.

    I very rarely see the need to actually boot to windows using bootcamp as the windows bootcamp partition is shared with a VM instance btw so it's the same "machine" whether in a VM or in parallels, When using bootcamp the battery drains faster but even so the rMBP has a longer battery life.

    As someone actually using windows on a a rMBP the only reasons I can see for your comments are all irrational
    Reply
  • Karltheghost - Wednesday, July 18, 2012 - link

    "Besides, the rMBP is brand new and thus faster than all the other PCs so the Windows VMs are still faster than their PCs."

    But what you don't mention is, that it is still slower than it would be with decent drivers . . . or still slower than a PC Laptop with similar hardware.
    I don't want to run Windows in a VM, i want Windows (or Linux) ONLY, no OSX at all.

    "When using bootcamp the battery drains faster "
    This sounds like a restriction to me. I don't know about the new rMBP but with the 2011er MBP it was like 30 or 40% battery capacity just vanished.

    Don't get me wrong, if i would get the same battery life and computing power running Windows (or Linux) on the MBA, i would buy it. But as it seems to me i may be overstating a bit but you are trying to sweep this (nevertheless existing) problem under the carpet
    Reply
  • KPOM - Friday, July 20, 2012 - link

    But again, you don't seem to "get" that Apple doesn't want to sell you a Mac so that you can run Windows on it. They try to sell an integrated experience.

    Would the iPhone sell more if they let you run Android on it? Do you think Apple would ever do that? Would you expect them to?
    Reply
  • phillyry - Tuesday, March 26, 2013 - link

    Battery life doesn't magically disappear. It's just not as good under Window 7 as it is under OS X. Reply
  • phillyry - Tuesday, March 26, 2013 - link

    But the point was that some people just want the hardware.

    The point was also made that the sales reps at the Apple Stores say that if runs Windows.

    Hopefully we all know to take the words of sales reps with a grain of salt.

    Win7 runs alright and they did improve the battery life with the last update but the trackpad lacking all the OS X gestures is a real nag.
    Reply
  • phillyry - Tuesday, March 26, 2013 - link

    "a friend of mine payed 170€ just for a new iphone battery". Seriously?

    Where I live you can walk in to any Apple store with any iOS device and, if they can't fix it on the spot (okay, appointment required), then they'll offer you a replacement of the device for $100.
    Reply
  • KoolAidMan1 - Wednesday, July 18, 2012 - link

    Not at all, its just that the hater posts are irrational as usual.

    As for being disposable as you said in a post below, nonsense. Macs hold their resale value VERY well, I wish that my PC parts could be sold for as much after 2-3 years. The longest I've ever held any of my machines were my Macs (6 years for one of my production desktops).

    The rest of your post is based around anecdotal evidence. I don't know a single person who returned their Macs after only a few weeks based around "flaws" that you don't go into. Again, quite the opposite, longevity and high resale value are two huge things you're getting with one.
    Reply
  • Karltheghost - Wednesday, July 18, 2012 - link

    Fanboys are irrational as well ;)

    Actually there are PC parts which can be resold for a good price, look at the Thinkpads or Toughbooks.

    Of course it's anecdotal evidence, like i said so in my posts . . . but actually you'll find some manufactor whose "official" reputation is a lot better than personal experience in every market. Usually where you've got so aggressive Marketing, that objectivity is lost. For example there is FOX for mountainbike suspension or beats in the audio sector. And it seems to me it is apple in the computer sector . . .

    The flaws are mainly compatibility issues, if you have expensive software that is supposed to run on mac and if you install it, it is heavily crippled you sure are not very enthusiastic
    Reply

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