[Asus  Eee Pad Transformer Prime] slybarman:
I read an article the other day off Flipboard (and it is driving me crazy because I cannot find it now), but it was discussing the IO controller of the various ASUS tablets. It mentioned that the controller in the Nexus 7 was newer and better than the one found in the Transformer Prime. It made it sound like the IO issue in the Prime was more a hardware issue than strictly a software issue. The article went on the say that tablets in general really need SSD and that was the future, etc.

Edit: D'oh! Here it is:

http://www.anandtech.com/show/6073/the-google-nexus-7-review/6

More recently users have been noticing very poor performance with ASUS Android tablets whenever there's substantial background IO going on, particularly multitasking while writes are taking place (e.g. downloading a torrent and browsing the web). The slowdowns range from multi-second long pauses to sluggish response time. The video below shows you what can happen on a Transformer Pad Infinity while downloading a bunch of large files in the background:

Even though downloading a large file is a largely sequential write operation, any other action performed while that download is in progress will effectively turn the IO stream from purely sequential to pseudo-random. Most of these eMMC and other cheap MLC NAND flash controllers are really glorified SD cards, primarily optimized for reading and writing large images as if they were used in a camera. What they're not designed for is to run and be used in a full blown multitasking OS. Some manufacturers seem to do a better job of picking their storage solution, and the Kingston eMMC in the Nexus 7 is faster than the Hynix e-NAND ASUS has used in previous tablets. Neither is perfect however. There's a tangible impact on simple multitasking if you're downloading a lot of files or installing apps in the background. Even operations in memory are quite negatively affected by background IO. Take a look at how SunSpider performance is hurt by a background file download:

SunSpider 9.1 Performance

Default	With 2MB/s Background Download

Google Nexus 7 (8GB)	1665.9 ms	1984.9 ms

To put all of this in perspective, I turned to Androbench, a storage benchmark that allows an ok amount of flexibility in configuring the test environment. At its default settings the benchmark varies a bit too much for my liking, but if you up the buffer sizes to 100MB it helps smooth out some of the variance.

Here we're looking at both sequential and random IO, the two sides we pay attention to in our SSD reviews as well. It's not clear to me whether or not Androbench is clamping the random IO to a particular range of addresses or not, but the results are telling nonetheless:

Androbench (100MB read/write)

256KB Sequential Read	256KB Sequential Write	4KB Random Read	4KB Random Write

ASUS Transformer Pad Infinity (64GB)	15.9 MB/s	7.13 MB/s	4.90 MB/s	0.22 MB/s

Google Nexus 7 (8GB)	23.1 MB/s	8.43 MB/s	4.77 MB/s	0.32 MB/s

Samsung Galaxy Nexus (16GB)	26.8 MB/s	7.29 MB/s	7.46 MB/s	0.23 MB/s

Sequential read/write speed isn't bad, but it's the random write speed that's really a problem. We're talking about write speeds of a couple hundred KB/s. Remember what I said earlier about how multitasking can take an otherwise sequential IO stream and make it look fairly random? I suspect the low random write performance is one reason we're seeing significant slowdowns with background IO. Not all NAND controllers do well with concurrent reads and writes, which could be another contributing factor to poor performance.

Note that for light usage this isn't a problem. Similar to the first generation of affordable MLC SSDs for PCs, as long as you're doing a lot of reading you'll be ok. It's really for the heavier usage models that this is a problem. On a tablet however, simple background installation or downloading of files counts as heavy these days.

The real solution to this problem is to integrate better NAND flash controllers on-board, or even onto the SoC itself. Tablet makers are still mostly focused on cost cutting, but eventually we'll see real SSDs with good controllers in these things. Windows 8 tablets, at least the high end ones due out later this year will be among the first to incorporate real SSDs.