I used to work a bit with mobile devices containing raw NAND flash and noticed that it was pretty slow, with speeds somewhere around 7 MiB/s for sequential read and 3.5 MiB/s for sequential write. This is considerably slower than normal HDs, to say nothing of SSDs.
The fact that raw NAND is so much slower than SSDs is kind of surprising though since SSDs are (as I understand it) just raw NAND flash chips with a controller.
Why are SSDs so much faster than the raw flash chips used in mobile devices (or the eMMC that has mostly replaced it but is still way slower than SSDs)? Some possible reasons that I can think of:
- Cost. SSDs contain NAND chips of much higher quality than what is used in mobile devices. This seems odd since some pretty high-end devices (Nexus One for instance) used relatively slow NAND chips at a time when there were really fast SSDs available.
- Power usage. High performance NAND flash uses too much power to be usable on mobile devices. (I have no idea if this is actually the case.)
- High performance SSDs read and write from many flash chips in parallell (kind of like RAID or dual-channel memory) which gives a big speed up. This requires many flash-chips which would simply not fit in a mobile device. (Again, I have no idea if this is how it works.)
Best Answer
3 is the answer. SSD controllers (and high-end USB flash drive controllers) are basically RAID controllers. This is one of the reasons that larger flash drives tend to be faster than smaller ones. Mobile devices usually don't have many flash chips and so their storage is slower.