More/Better Internal Storage on the Toshiba AC100 – Part 2

Following my research for the previous article about the performance of SD/CF/USB flash modules, the only conclusion I could reach is that most of them are pretty dire. The only notable exception among the SD cards seems to be the latest generation of the SanDisk Extreme Pro (95MB/s) cards that just about managed to squeeze out enough performance on random writes to match a 7200rpm disk. Still, this is pretty dire compared to any reasonable SSD, so I wanted to see what else could be done about installing extra storage with good performance into an AC100.

What I came across is this: SuperTalent RC8 USB stick. It may look like a USB stick, but it is actually a full-on SSD, featuring a SandForce 1200 flash controller. I figured this was worth a shot, even though the4 specifications indicate it is rather large (far too large to fit inside an AC100 in it’s standard form). Stripped out of the casing, however, it looks like RC8 might just be fittable inside the AC100.

This is what I ended up with. There appears to be only one place inside an AC100 where a bare RC8 circuit board could be fitted. You will need the following:

1) P3MU mini-PCIe USB break-out module

2) SuperTalent RC8 USB stick

3) Custom made USB cable (male and female type A USB connectors, some single core wire, and some skill with a soldering iron)

Measure out exactly how long you need the cable to be – there is no room to tuck away excess able inside an AC100. Here is what my cable layout ended up looking like.

AC100 motherboard with P3MU and custom USB cable fitted

AC100 motherboard with P3MU and custom USB cable fitted

This is what it looks like with the top panel fitted. Note the large cut-out that has been made below the mini-PCIe slot access hole.

AC100 modified to receive RC8 USB SSD

AC100 modified to receive RC8 USB SSD

And again with the screws fitted. Note that one of the screw holes is in the area that had to be cut out. This shouldn’t affect the structural integrity of the AC100, though. Also note that the right speaker cable has been re-routed slightly to now go over the LED ribbon cable.

AC100 modified to receive RC8 SSD

AC100 modified to receive RC8 SSD

This is what it looks like with the RC8 attached. Now you can see why the cut-out in the top panel was exactly the shape it was – I specifically cut out the minimum possible amount to allow the RC8 to fit.

Toshiba AC100 with the SuperTalent RC8 USB SSD installed

Toshiba AC100 with the SuperTalent RC8 USB SSD installed

I also put a piece of thin transparent sticky tape over it to hold in in place, just to make sure nothing can short out against the underside of the keyboard.

Toshiba AC100 with the SuperTalent RC8 SSD

Toshiba AC100 with the SuperTalent RC8 SSD

And that is pretty much it. Put the keyboard back in and bolt it all together. The metal part of the USB connector will sit a tiny bit above the line of the panel, but the only way you’ll notice it once you put the keyboard back on is by knowing that there is a tiny bulge there.

Your AC100 should now be able to handle ~ 2000 IOPS on both random reads and random writes, along with much better life expectancy that having proper flash management brings.

At this point I would like to point out just how impressed I am with the SuperTalent RC8 USB SSD. Not only is the performance fenomenal (for a USB stick at least), but it really behaves like a SATA SSD – to the point where you can use tools like hdparm and smartctl on it (yes, it even supports SMART).

Flash Module Benchmark Collection: SD Cards, CF Cards, USB Sticks

Having spent a considerable amount of time, effort, and ultimately money trying to find decently performing SD, CF and USB flash modules, I feel I really need to ensure that I make the lives of other people with the same requirements easier by publishing my findings – especially since I have been unable to find a reasonable comprehensive data source with similar information.

Unfortunately, virtually all SD/microSD (referred to as uSD from now on), CF and USB flash modules have truly atrocious performance for use as normal disks (e.g. when running the OS from them on a small, low power or embedded device), regardless of what their advertised performance may be. The performance problem is specifically related to their appalling random-write performance, so this is the figure that you should be specifically paying attention to in the tables below.

As you will see, the sequential read and write performance of flash modules is generally quite good, as is random-read performance. But on their own these are largely irrelevant to overall performance you will observe when using the card to run the operating system from, if the random-write performance is below a certain level. And yes, your system will do several MB of writing to the disk just by booting up, before you even log in, so don’t think that it’s all about reads and that writes are irrelevant.

For comparison, a typical cheap laptop disk spinning at 5400rpm disk can typically achieve 90 IOPS on both random reads and random writes with typical (4KB) block size. This is an important figure to bear in mind purely to be able to see just how appalling the random write performance of most removable flash media is.

All media was primed with two passes of:

 dd if=/dev/urandom of=/dev/$device bs=1M oflag=direct

in order to simulate long term use and ensure that the performance figures reasonably accurately reflect what you might expect after the device has been in use for some time.

There are two sets of results:

1) Linear read/write test performed using:

dd if=/dev/$device of=/dev/null    iflag=direct
dd if=/dev/zero    of=/dev/$device oflag=direct

The linear read-write test script I use can be downloaded here.

2) Random read/write test performed using:

iozone -i 0 -i 2 -I -r 4K -s 512m -o -O +r +D -f /path/to/file

In all cases, the test size was 512MB. Partitions are aligned to 2MB boundaries. File system is ext4 with 4KB block size (-b 4096) and 16-block (64KB) stripe-width (-E stride=1,stripe-width=16), no journal (-O ^has_journal), and mounted without access time logging (-o noatime). The partition used for the tests starts at half of the card’s capacity, e.g. on a 16GB card, the test partition spans the space from 8GB up to the end. This is in done in order to nullify the effect of some cards having faster flash at the front of the card.

The data here is only the first modules I have tested and will be extensively updated as and when I test additional modules. Unfortunately, a single module can take over 24 hours to complete testing if their performance is poor (e.g. 1 IOPS) – and unfortunately, most of them are that bad, even those made by reputable manufacturers.

The dd linear test is probably more meaningful if you intend to use the flash card in a device that only ever performs large, sequential writes (e.g. a digital camera). For everything else, however, the dd figures are meaningless and you should instead be paying attention to the iozone results, particularly the random-write (r-w). Good random write performance also usually indicates a better flash controller, which means better wear leveling and better longevity of the card, so all other things being similar, the card with faster random-write performance is the one to get.

Due to WordPress being a little too rigid in it’s templates to allow for wide tables, you can see the SD / CF / USB benchmark data here. This table will be updated a lot so check back often.

More/Better Internal Storage on the Toshiba AC100

One of the unfortunate things about the AC100 is that the internal storage isn’t removable, and thus isn’t easily upgradable or replaceable. The latter could be an issue in the longer term because it is flash memory, so it will eventually wear out, and I since it is relatively basic eMMC, I don’t expect the flash controller to be particularly advanced when it comes to wear leveling and minimizing write amplification. Using the SD slot is an option, but if we are running the operating system from it, we cannot use it for removable media, which could be handy. We could use a USB stick instead, but then we lose the only USB port on the machine. There is no SATA controller inside the AC100.

What can be done about this? Well, models that have a 3G modem have it on a mini-PCIe USB card. Even though Tegra 2 has a PCIe controller built into it, the mini-PCIe slot isn’t fully wired up – only USB lines are connected. Since most of us can tether a data connection via our phones, and since this is more cost effective than paying for two separate mobile connections, the 3G module isn’t particularly vital. The main issue that the slot only has USB wired up. So what we would need is a USB mini-PCIe SSD. Is there such a thing? It turns out that there is. I have been able to find two:

  1. EMPhase Mini PCIe USB S1 SSD
  2. InnoDisk miniDOM-U SSD

The specification of the two modules is virtually identical (both use SLC flash among other similarities), so I decided to investigate both of them. Unfortunately, having contacted an EMPhase re-seller, they called me back having spoken to the manufacturer and talked me out of buying one, citing unspecified issues.

My local InnoDisk re-seller was more interested in selling me a product, but there were two reasons why despite very good pre-sales service I ultimately decided against buying one of these. The first and foremost was the performance specification. According to the manufacturer’s own figures, the random access performance with 4KB blocks is 1440 random read IOPS and 30 random write IOPS. Considering the price per GB of these modules is approximately 4x that of similarly performing SLC SD cards, this module was discarded on the basis of cost effectiveness.

Having discarded the above modules, there are still a few alternative options available. The low risk, tidy options include an SD mini-PCIe USB adapter and a micro-SD mini-PCIe USB adapter. They are very reasonably priced so I got one of each for testing, and I am pleased to say that they work absolutely fine in the AC100. Here is what they look like fitted into the AC100.

Dual micro-SD mini-PCIe USB Adapter

Dual micro-SD mini-PCIe USB Adapter

SD mini-PCIe USB Adapter

SD mini-PCIe USB Adapter

The SD cards will appear as USB disks. If you use the dual micro-SD adapter you can RAID the two cards together.

Unfortunately, I have found that the best results are achieved using a single SD card, purely because I haven’t found any micro-SD cards that have reasonable performance when it comes to random-write IOPS. SD cards fare a little better, but the best SD card I have found in terms of random write IOPS still tops out at a mere 19 random write IOPS using 4KB blocks. Still, it is 2/3 of the marketed figures for the InnoDisk SSD at 4x lower price per GB, and the performance just about scrapes past what I would consider minimal requirements for reasonable use.

I am currently putting together a list of SD, micro SD and USB flash devices and consistent benchmark performance figures for them, which should hopefully help you to choose the ones most suitable for your application. I hope to have the article up reasonably soon, but don’t expect it too soon – benchmarking SD cards takes a long time to do properly.