256M was the biggest available in volume at design time..

Anyway, I suspect the difference in performance is mostly because ubifs can manage the nand intelligently. The mmc type chips are designed for single-tasking camera/media use, they hide the flash/nand behind a dumb controller which is fully optimized to be cheap and simple for large file use. It doesn't really handle the random updates of 4k blocks here and there very well.. Its native block size is something like 256k, so a random 4k write gets translated into a 256k read-modify-write cycle. It doesn't take much I/O before the emmc is saturated and the reads don't go through and the app running from /opt has a small pause/lag...

I notice this often with gpodder downloading something while I'm browsing the web with disk cache enabled. The 3 of gpodder, browser, swap hitting the emmc at the same time makes the average wait (as measured by iotop) for a request to complete easily exceed 500ms. That's noticeable latency.


On my sheevaplug I compared ext3 and nilfs2 on a OCZ Rally2 dual channel usb flash drive. It has over 20megabytes/s sequential write speed, compared to the promised 6 megabytes/sec of class 6 memory cards. The benchmark consisted of copying my 2 gigabyte squid webcache directory. ext3: 500kbyte/s nilfs2: 5000kbyte/s. A factor of 10 difference! nilfs2 is by accident and its nature faster on flash.. I hope to test LogFS some day too, it has actually been designed with flash in mind..