Storage Performance: Slightly Faster Than USB 1.0?!
Though there's not a lot of user-accessible space on the Kindle Fire, it's a real pain to fill it up over USB.
| USB File Transfer2.8 GB H.264 encoded MP4 Movie | Avg. Transfer Rate | Time |
|---|---|---|
| Amazon Kindle Fire (OS Level File Transfer) | 2.76 MB/s | 17:14.615 |
| Apple iPad 2 (iTunes) | 19.19 MB/s | 02:29.090 |
| Motorola Xoom (OS Level File Transfer) | 13.18 MB/s | 03:36.990 |
The process is excruciatingly slow, with a top sequential speed somewhere around 2.7 MB/s. If you're moving small files, expect initial speeds around 1.2 MB/s. Meanwhile, other tablets that support USB 2.0 frequently hit average speeds above 10 MB/s.
Hardware isn't the problem. Amazon employs a Samsung 8 GB KLM8G2FEJA eMMC NAND package. You find the same product in competing tablets, like Acer's 8 GB A100. Motorola employs a 32 GB eMMC NAND chip from Toshiba with similar specs. And yet, transfers are much faster on the Xoom.
The zippy transfer rate on the iPad 2 in the table above shouldn't come as a surprise. Apple is the only major tablet manufacturer to use vanilla MLC NAND, which is found in the zippy SSDs we all know and love. But that also means the A5 contains extra logic to add block management and ECC.
Since Tegra 2 and OMAP lack the same circuitry as Apple's A5, nearly all Android-based tablets use a simpler storage implementation called eMMC, which embeds block management and ECC onto the NAND itself. The difference is highlighted in the slide above.
Since everything is managed at the NAND level, the operating system doesn't have to bother issuing commands like secure erase or TRIM. The MMC controller in the storage device handles all of that. However, this also means that eMMC NAND is blind to much of what the operating system is doing. The result is a significant amount of performance overhead.
| Header Cell - Column 0 | Amazon Kindle Fire | Apple iPad 2 | Motorola Xoom |
|---|---|---|---|
| NAND Chip | Samsung KLM8G2FEJA | Toshiba TH58NVG7D2FLA89 | Toshiba THGBM2G8D8FBAIB |
| NAND Bus | eMMC v4.41 | Toggle 1.0 | eMMC v4.4 |
| NAND Bus Speed | 104 MB/s | 133 MB/s | 104 MB/s |
Of course, the difference between eMMC and regular NAND isn't limited to block management. The bus interface is also different. Whereas Toggle Mode 1.0 is limited to 133 MB/s, for instance, eMMC tops out at 104 MB/s. There's a new revision of eMMC that bumps speeds up to 200 MB/s, but it's currently too new and too expensive for tablet manufactures to implement.
It's not clear if there's a way to address the Kindle Fire's low transfer speeds through firmware. Other Android tablet vendors implement Microsoft's Media Transfer Protocol, whereas Amazon chooses the more generic USB Mass Storage Class (MSC) driver. That's good news for Mac users, since you don't need a special program to transfer files.
It's possible that the problem is with Amazon's USB MSC implementation. The only way to transfer files is to put the Fire in Mass Storage mode preventing you from using the tablet at the same time. However, it also suggests that Mass Storage mode is a hosted layer above the operating system.
RIM got around this issue with its Blackberry line by implementing a pass-through mode for transfers. It's likely that Amazon will have to do something similar in order to speed up USB performance. Currently, transferring files to your Kindle Fire over your home network using Astro (along with its SMB module) is likely faster than using USB.
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