I found the value would repeated addition when use vfs.dev.read[,ops,] and vfs.dev.write[,ops,] to count all device iops. In my environment /dev/sda has two partitions /dev/sda1 and /dev/sda2, it should count only /dev/sda is ok, but it will /dev/sda plus /dev/sda1 and /dev/sda2. the result is doble, it work incorrect.