Hi, Apologies in advance, as I'm sure this question has been asked many times and there is a standard answer, but I can't seem to find it on forums or this mailing list. I've always observed this behavior using 'iostat', when looking at READ throughput numbers, the value is about 4 times more than the real throughput number. Knowing this, I typically look at the member devices to determine what throughput is actually being achieved (or from the application driving the I/O). Looking at the sectors read field in the 'stat' file for an MD array block device: # cat /sys/block/md127/stat && sleep 1 && cat /sys/block/md127/stat 93591416 0 55082801792 0 93 0 0 0 0 0 0 0 0 0 0 93608938 0 55092996456 0 93 0 0 0 0 0 0 0 0 0 0 55092996456 - 55082801792 = 10194664 10194664 * 512 = 5219667968 5219667968 / 1024 / 1024 = 4977 This device definitely isn't doing 4,977 MiB/s. So now my curiosity is getting to me: Is this just known/expected behavior for the MD array block devices? The numbers for WRITE sectors is always accurate as far as I can tell. Or something configured strangely on my systems? I'm using vanilla Linux 5.4.12. Thanks, Marc
