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[Colin Ian King <1242812@xxxxxxxxxxxxxxxxxx>]

@Daniel, these commits are present in Trusty.

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https://bugs.launchpad.net/bugs/1242812

Title:
  ext4 random block I/O write performance regression with 3.11 Saucy
  Kernel

Status in “linux” package in Ubuntu:
  Fix Released

Bug description:
  Commit e7ea81db5(ext4: restructure writeback path) introduced a
  performance regression with random writes. Using tools such as
  bonnie++, writes with dd or even stress testing with tools such as
  'stress' one can observe a 10%-20% performance regression.

  SRU Justification:

  Commit e7ea81db5(ext4: restructure writeback path) introduced a
  performance regression with random writes. Using tools such as
  bonnie++, writes with dd or even stress testing with tools such as
  'stress' one can observe a 10%-20% performance regression.

  Impact:

  Write performance is diminished causing a noticeable regression
  compared to previous released kernels.

  Fix:

  Two patches are required:

  a) upstream fix 9c12a83 which fixes the overly aggressive writing back of pages which ultimulately resulted in more seeking and
  less performance.

  b) commit aeac589a7 from the dev branch of kernel/git/tytso/ext4.git
  which ensures no more pages than nr_to_write can be added to the
  extent for mapping.

  Testcase:

  Using stress-ng on a 2 CPU machine run:

  stress-ng --hdd 2 --hdd-ops 200000

  (see: git://kernel.ubuntu.com/cking/stress-ng.git)

  With the fix, this consistently runs ~10-20% faster than the non-fixed
  kernel.

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