I used to think that whatever blocks the block layer sends down to the device driver to write actually does get written, even if the 2 writes are for the same LBA. (By calling a sync between 2 writes to the same area on the disk). My doubt is that does monmitoring IO stats from iostat, vmstat, proc/diskstats or /sys/sda/stat, give a count of the num ber of blocks sent to the disk for write?