kernel-packages team mailing list archive

[Chris J Arges <1346687@xxxxxxxxxxxxxxxxxx>]

This is a change in the kernel between 3.13 and 3.13-rc1. It also
persists in the most recent kernel version. I believe this is still an
invalid bug as what benefit would setting an I/O scheduler to anything
be if the device is virtual? On a physical machine you are still able to
set the I/O scheduler properly.

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https://bugs.launchpad.net/bugs/1346687

Title:
  Ubuntu 14 KVM Guest I/O Elevator Non-configurable

Status in “linux” package in Ubuntu:
  Invalid

Bug description:
  Ubuntu 14.04 guest runs pretty much standard standard configuration:
  KVM hypervisor, ACPI and APIC enabled, virtio disk bus, raw storage
  format...

  While running this project, I am also involved in testing SSD drives
  across different Linux distributions and their support for discard
  option (TRIM).

  To cut the story short, Ubuntu-based KVM guest did not set any
  scheduler on the boot disk vda):

  # lsblk -io KNAME,TYPE,SCHED,ROTA,DISC-GRAN,DISC-MAX
  KNAME TYPE SCHED ROTA DISC-GRAN DISC-MAX
  sr0 rom deadline 1 0B 0B
  vda disk 1 0B 0B
  vda1 part 1 0B 0B
  vda2 part 1 0B 0B
  vda5 part 1 0B 0B
  dm-0 lvm 1 0B 0B
  dm-1 lvm 1 0B 0B

  That is in stark contrast with other distributions running almost
  identical KVM guest configuration (with the exception of the root file
  system using BTRFS instead of EXT4 on SUSE).

  Oracle Linux 6.5:

  # lsblk -io KNAME,TYPE,SCHED,ROTA,DISC-GRAN,DISC-MAX
  KNAME TYPE SCHED ROTA DISC-GRAN DISC-MAX
  sr0 rom deadline 1 0B 0B
  vda disk deadline 1 0B 0B
  vda1 part deadline 1 0B 0B
  vda2 part deadline 1 0B 0B
  dm-0 lvm 1 0B 0B
  dm-1 lvm 1 0B 0B

  OpenSUSE 13.1:

  # lsblk -io KNAME,TYPE,SCHED,ROTA,DISC-GRAN,DISC-MAX
  KNAME TYPE SCHED ROTA DISC-GRAN DISC-MAX
  sr0 rom cfq 1 0B 0B
  vda disk cfq 1 0B 0B
  vda1 part cfq 1 0B 0B
  vda2 part cfq 1 0B 0B
  dm-0 lvm 1 0B 0B
  dm-1 lvm 1 0B 0B

  Indeed, checking elevator capabilities for boot disk on Ubuntu KVM
  guest showed:

  # cat /sys/block/vda/queue/scheduler
  none

  Other Linux distributions show more options:

  # cat /sys/block/vda/queue/scheduler
  noop deadline [cfq]

  As well, attempts to change the elevator on Ubuntu guest fail. For
  example:

  # echo noop > /sys/block/vda/queue/scheduler

  # echo $?
  0

  # cat /sys/block/vda/queue/scheduler
  none

  Setting it globally in /etc/default/grub and updating it via update-
  grub2 fails too.

  In the meantime, to automate check of elevators (schedulers) and
  discard support for SSD drives and thin-provisioned volumes on Linux,
  I wrote simple Perl script:

  http://www.circlingcycle.com.au/Unix-sources/Linux-check-IO-scheduler-
  and-discard-support.pl.txt

  Part of the results on RHEL 6.5 server would look like:

  INFO: I/O elevator (scheduler) and discard support summary
  INFO: Hard Disk sdb configured with I/O scheduler "cfq"
  INFO: SSD sda configured with I/O scheduler "deadline" supports discard operation
  INFO: Hard Disk sdc configured with I/O scheduler "cfq"
  INFO: Hard Disk sdi configured with I/O scheduler "cfq"
  INFO: Hard Disk sdh configured with I/O scheduler "cfq"

  The Ubuntu KVM guest runs latest paches:

  Linux ubuntu14-vm2.circlingcycle.com.au 3.13.0-24-generic
  #47-Ubuntu SMP Fri May 2 23:30:00 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

  Regards,

  Dusan Baljevic VK2COT

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