Comment 0 for bug 1790181

For one block device on one of my servers munin shows abnormally high latency.

When querying /proc/diskstats manually, the device in question shows:
   8 32 sdc 973668 361965 64248290 10224376 755130 518162 22349256 1347171224 63 54302956 1357481880

Sanity check:
If I divide the 7th value by the 4th value I get a avg read latency of about 10.5 ms. That is somewhat in line with the Avg. Read IO Wait Time of 11.35 ms that munin reports.

The problem:
If I divide the 11th value by the 8th value I get an avg write latency of 1789 ms. That is also close to the Avg. Write IO Wait Time of 1.60 s reported by munin. Now this is an SD card and it is slow, but not THAT slow..

To confirm unexpected values, I measured some activity:

    $ dd if=/dev/urandom of=testfile bs=1M count=100; time sync;
    cat /proc/diskstats
    100+0 records in
    100+0 records out
    104857600 bytes (105 MB, 100 MiB) copied, 18,7652 s, 5,6 MB/s

    real 0m8,132s
    user 0m0,000s
    sys 0m0,007s

The reported time spent writing went from 1353896856 before the test to 1356759112 after the test.
The difference is 2862256 ms or about 2800 seconds that supposedly passed in just over 26 real time seconds.
I repeated the test with count=1 and got 5780 ms IO write time reported in .7 s realtime.

#1297522 describes a similar error but I have no idea if the cause is related.

System information:
Description: Ubuntu 18.04.1 LTS
Release: 18.04

Linux 4.15.0-30-generic #32-Ubuntu SMP Thu Jul 26 17:42:43 UTC 2018 x86_64 x86_64 GNU/Linux

/dev/sdc is a Dell dual SD card module in RAID 1 mode with two Lexar 8GB U3 cards.
The system is a Dell R530.