thanks. that's a useful observation and the same problem also affects new harddisks
with blocksizes > 512bytes (AFAIK the flash erase block size is typicall 64K).
I think the the eMMC has a different wear leveling strategy than consumer flash media
but I may be wrong.
I've just uploaded a bonnie++ port to extras-devel so that you can
start performing benchmarks with differents fs and devices (NAND, eMMC, MMC).