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[Andy Whitcroft <apw@xxxxxxxxxxxxx>]

The underlying issue is that the blk-mq support currently does _not_
have support for IO schedulers, so when this method is selected we can
no longer use the IO schedulers at all.  This is not ready to be a
default selection without IO scheduler support so we will flip the
default back to off in the next upload.  It can of course be reneabled
via the kernel command line (at least) for those who want to experiment
with it.

** Package changed: ubuntu => linux (Ubuntu)

** Changed in: linux (Ubuntu)
       Status: New => Triaged

** Changed in: linux (Ubuntu)
     Assignee: (unassigned) => Andy Whitcroft (apw)

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https://bugs.launchpad.net/bugs/1397061

Title:
  CONFIG_SCSI_MQ_DEFAULT default changed preventing use of IO schedulers

Status in “linux” package in Ubuntu:
  Triaged

Bug description:
  In kernel 3.18RC1 this kernel config parameter is: # CONFIG_SCSI_MQ_DEFAULT is not set
  In kernel 3.18RC2 and beyond, the kernel config parameter is: CONFIG_SCSI_MQ_DEFAULT=y

  This results in loss of the ability to set the IO scheduler via
  /sys/block/sda/queue/scheduler.

  Now we get:

  doug@s15:~$ cat /sys/block/sda/queue/scheduler
  none

  Where we are used to getting:

  doug@s15:~/temp2$ cat /sys/block/sda/queue/scheduler
  noop [deadline] cfq

  From the add a CONFIG_SCSI_MQ_DEFAULT option commit message:

  > Add a Kconfig option to enable the blk-mq path for SCSI by default
  > to ease testing and deployment in setups that know they benefit
  > from blk-mq.

  How do we know that all systems benifit from blk-mq?

  It seems complicated to have to re-compile the kernel to get the other IO scheduler options back.
  Why isn't this option done similar to the others? I.E.

  doug@s15:~/temp2$ cat /sys/block/sda/queue/scheduler
  noop [deadline] cfq blk-mq

  (and I realize that is actually an upstream question.)

  By the way, my system does seem to benefit from blk-mq, I just didn't
  understand why I couldn't observe and change the IO scheduler anymore,
  and so isolated the change.

  Experimental data:

  Random seeks in a large file:
  blk-mq: 104 seeks per second average
  deadline: 74 seeks per second average
  cfq: 74 seeks per second average
  noop: 74 seeks per second average

  Kernel compile:
  deadline: 23 minutes 37.4 seconds
  blk-mq: 23 minutes 35.4 seconds

  Note 1: Please do not ask for all of my apport stuff, it is not needed for this bug report.
  Note 2: on IRC "apw" asked me to enter this bug report

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