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This bug was fixed in the package linux - 3.18.0-8.9

---------------
linux (3.18.0-8.9) vivid; urgency=low

  [ Leann Ogasawara ]

  * Release Tracking Bug
    - LP: #1407692
  * rebase to v3.18.1
  * ubuntu: AUFS -- Resolve build failure union has no member named
    'd_child'

  [ Upstream Kernel Changes ]

  * arm64: optimized copy_to_user and copy_from_user assembly code
    - LP: #1400349
  * x86, kvm: Clear paravirt_enabled on KVM guests for espfix32's benefit
    - LP: #1400314
    - CVE-2014-8134
  * rebase to v3.18.1
 -- Leann Ogasawara <leann.ogasawara@xxxxxxxxxxxxx>   Mon, 05 Jan 2015 09:12:32 -0800

** Changed in: linux (Ubuntu)
       Status: Fix Committed => Fix Released

** CVE added: http://www.cve.mitre.org/cgi-
bin/cvename.cgi?name=2014-8134

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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:
  Fix Released

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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