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[Joseph Salisbury <joseph.salisbury@xxxxxxxxxxxxx>]

This issue appears to be an upstream bug, since you tested the latest
upstream kernel. Would it be possible for you to open an upstream bug
report[0]? That will allow the upstream Developers to examine the issue,
and may provide a quicker resolution to the bug.

Please follow the instructions on the wiki page[0]. The first step is to
email the appropriate mailing list. If no response is received, then a
bug may be opened on bugzilla.kernel.org.

Once this bug is reported upstream, please add the tag: 'kernel-bug-
reported-upstream'.

[0] https://wiki.ubuntu.com/Bugs/Upstream/kernel

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https://bugs.launchpad.net/bugs/1531951

Title:
  A severe performance degradation while syncing md raid after the
  upgrade to 15.10

Status in linux package in Ubuntu:
  Triaged

Bug description:
  I run Ubuntu from the fakerade (imsm) raid5 volume. If the volume is
  syncing, the system response is severely degraded. There is no problem
  if I boot the same system from the previous kernel 3.19.0-32. Some
  numbers:

  [md125_resync] kernel thread CPU consumption: 94 per-mill compared to 10 in 3.19 kernel
  [md125_raid5] kernel thread CPU consumption: 243 per-mill compared to 86 in 3.19 kernel
  1MB direct read from md array: 0.5s compared to 0.01s in 3.19 kernel
  1MB direct read from md raid member: 2.2s compared to 0.02s in 3.19 kernel

  As a result, it's practically impossible to work in 4.2 kernel if md
  raid is syncing (even the boot takes several minutes here), whereas in
  3.19 I don't feel any performance degradation at all, regardless the
  raid volume is syncing or not.

  ProblemType: Bug
  DistroRelease: Ubuntu 15.10
  Package: linux-image-4.2.0-22-generic 4.2.0-22.27
  ProcVersionSignature: Ubuntu 4.2.0-22.27-generic 4.2.6
  Uname: Linux 4.2.0-22-generic x86_64
  NonfreeKernelModules: nvidia
  ApportVersion: 2.19.1-0ubuntu5
  Architecture: amd64
  AudioDevicesInUse:
   USER        PID ACCESS COMMAND
   /dev/snd/controlC1:  palo       3869 F.... pulseaudio
   /dev/snd/pcmC0D0p:   palo       3869 F...m pulseaudio
   /dev/snd/controlC0:  palo       3869 F.... pulseaudio
  CurrentDesktop: Unity
  Date: Thu Jan  7 18:51:39 2016
  HibernationDevice: RESUME=UUID=f9900a93-c16d-4cae-9098-194fdf3abbfe
  InstallationDate: Installed on 2011-09-16 (1574 days ago)
  InstallationMedia: Ubuntu 11.04 "Natty Narwhal" - Release amd64 (20110427.1)
  MachineType: System manufacturer System Product Name
  ProcFB: 0 VESA VGA
  ProcKernelCmdLine: BOOT_IMAGE=/vmlinuz-4.2.0-22-generic root=/dev/mapper/vg00-lv_ubuntu_root ro init=/sbin/upstart
  RelatedPackageVersions:
   linux-restricted-modules-4.2.0-22-generic N/A
   linux-backports-modules-4.2.0-22-generic  N/A
   linux-firmware                            1.149.3
  RfKill:
   
  SourcePackage: linux
  UpgradeStatus: Upgraded to wily on 2016-01-02 (4 days ago)
  dmi.bios.date: 02/23/2009
  dmi.bios.vendor: American Megatrends Inc.
  dmi.bios.version: 2102
  dmi.board.asset.tag: To Be Filled By O.E.M.
  dmi.board.name: P5Q-PRO
  dmi.board.vendor: ASUSTeK Computer INC.
  dmi.board.version: Rev 1.xx
  dmi.chassis.asset.tag: Asset-1234567890
  dmi.chassis.type: 3
  dmi.chassis.vendor: Chassis Manufacture
  dmi.chassis.version: Chassis Version
  dmi.modalias: dmi:bvnAmericanMegatrendsInc.:bvr2102:bd02/23/2009:svnSystemmanufacturer:pnSystemProductName:pvrSystemVersion:rvnASUSTeKComputerINC.:rnP5Q-PRO:rvrRev1.xx:cvnChassisManufacture:ct3:cvrChassisVersion:
  dmi.product.name: System Product Name
  dmi.product.version: System Version
  dmi.sys.vendor: System manufacturer

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