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[Chris J Arges <1297522@xxxxxxxxxxxxxxxxxx>]

SRU submitted for this.

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https://bugs.launchpad.net/bugs/1297522

Title:
  Since Trusty /proc/diskstats shows weird values

Status in “linux” package in Ubuntu:
  Fix Released
Status in “linux” source package in Trusty:
  In Progress

Bug description:
  SRU Justification:

  Impact: Tools that rely on diskstats may report incorrect data in
  certain conditions. In particular diskstats in a VM may report
  incorrect statistics.

  Fix: 0fec08b4ecfc36fd8a64432343b2964fb86d2675 ( in 3.14-rc1 )

  Testcase:
    - Install a VM with the affected kernel
    - Run cat /proc/diskstats | awk '$3=="vda" { print $7/$4, $11/$8 }'
    - If the two values are much larger compared to the v3.14-rc1 kernel in the same VM, we have failed. For example in a failing case I see: "132.44 5458.34"; in a passing case I see: "0.19334 5.90476".

  --

  After upgrading some virtual machines (KVM) to Trusty I noticed really
  high I/O wait times, e.g. Munin graphs now show up to 200 seconds(!)
  read I/O wait time. See attached image. Of course real latency isn't
  higher than before, it's only /proc/diskstats that shows totally wrong
  numbers...

  $ cat /proc/diskstats | awk '$3=="vda" { print $7/$4, $11/$8 }'
  1375.44 13825.1

  From the documentation for /proc/diskstats field 4 is total number of
  reads completed, field 7 is the total time spent reading in
  milliseconds, and fields 8 and 11 are the same for writes. So above
  numbers are the average read and write latency in milliseconds.

  Same weird numbers with iowait. Note the column "await" (average  time
  in milliseconds for I/O requests):

  $ iostat -dx 1 60
  Linux 3.13.0-19-generic (munin) 	03/25/14 	_x86_64_	(2 CPU)

  Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
  vda               2.30    16.75   72.45   24.52   572.79   778.37    27.87     1.57  620.00  450.20 1121.83   1.71  16.54

  Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
  vda               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00

  Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
  vda               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00

  Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
  vda               0.00    52.00    0.00   25.00     0.00   308.00    24.64     0.30 27813.92    0.00 27813.92   0.48   1.20

  I upgraded the host system to Trusty too, however there
  /proc/diskstats output is normal as before.

  $ uname -r
  3.13.0-19-generic

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