On Mon, Mar 30, 2020 at 1:55 PM Marc Smith <msmith626@xxxxxxxxx> wrote: > > 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 for the report. Could you please share output of mdadm --detial /dev/md127 and cat /proc/mdstat Song
