Public bug reported:
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.
** Affects: linux (Ubuntu)
Importance: Undecided
Status: New
** Attachment added: "munin graph of average IO latency by disc"
https://bugs.launchpad.net/bugs/1790181/+attachment/5183049/+files/sdc-month.png
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https://bugs.launchpad.net/bugs/1790181
Title:
/proc/diskstats shows weird values
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