Has anybody thought about MMC aware IO scheduler? Such scheduler would try to merge writes on the same eraseblock even if there were gaps between the writes by reading the gaps first. Such merges would also be considered for writes on separate erase blocks if they are close enough. You may ask why bother. I have tested a board with eMMC with 512MiB erase block which can write 20 MiB/s when writes are contiguous with block size of 512 MiB. Write time is then 20 ms per erase block at mmc driver from startting to send the read command to mmc_request_done(). But if the writes are to randomly chosen erase block, write times jumps to 250 ms and is pretty constant regardless of the write size from 512 bytes to 512 KiB or offset or alignment. Reading a random erase block takes 30ms. Read speed is< not constant but grows slowly from 16KiB @ 11 MiB/s to 2 MiB @ 24 MiB/s. The device seems also to like sequential reads. With these figures the cost of reading a gap between two writes takes <= 25ms, writing a whole erase block takes 270ms, altogether about 300ms. Writing two any size writes separately to the same erase blcok would take 510ms. The same speedup applies roughly to writing two writes to consecutive erase blocks with gap between the writes. Jarkko Lavinen -- To unsubscribe from this list: send the line "unsubscribe linux-mmc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
