On Thu, Apr 9, 2020 at 3:11 AM Song Liu <song@xxxxxxxxxx> wrote:
>
> 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
>
# mdadm --detail /dev/md127
/dev/md127:
Version : 1.2
Creation Time : Tue Mar 17 17:23:00 2020
Raid Level : raid6
Array Size : 17580320640 (16765.90 GiB 18002.25 GB)
Used Dev Size : 1758032064 (1676.59 GiB 1800.22 GB)
Raid Devices : 12
Total Devices : 12
Persistence : Superblock is persistent
Update Time : Thu Apr 9 13:07:12 2020
State : clean
Active Devices : 12
Working Devices : 12
Failed Devices : 0
Spare Devices : 0
Layout : left-symmetric
Chunk Size : 64K
Consistency Policy : resync
Name : node-126c4f-1:P2024_126c4f_01 (local to host
node-126c4f-1)
UUID : ceccb91b:1e975007:3efb5a9d:eda08d04
Events : 79
Number Major Minor RaidDevice State
0 8 0 0 active sync /dev/sda
1 8 16 1 active sync /dev/sdb
2 8 32 2 active sync /dev/sdc
3 8 48 3 active sync /dev/sdd
4 8 64 4 active sync /dev/sde
5 8 80 5 active sync
6 8 96 6 active sync /dev/sdg
7 8 112 7 active sync /dev/sdh
8 8 128 8 active sync /dev/sdi
9 8 144 9 active sync /dev/sdj
10 8 160 10 active sync /dev/sdk
11 8 176 11 active sync /dev/sdl
> and
>
> cat /proc/mdstat
# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4]
md127 : active raid6 sda[0] sdl[11] sdk[10] sdj[9] sdi[8] sdh[7]
sdg[6] sdf[5] sde[4] sdd[3] sdc[2] sdb[1]
17580320640 blocks super 1.2 level 6, 64k chunk, algorithm 2
[12/12] [UUUUUUUUUUUU]
unused devices: <none>
Thanks; please let me know if there is any more detail I can provide.
--Marc
>
> Song
