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[Launchpad Bug Tracker <1701132@xxxxxxxxxxxxxxxxxx>]

This bug was fixed in the package livecd-rootfs - 2.408.25

---------------
livecd-rootfs (2.408.25) xenial; urgency=medium

  * Sort and list dependencies nicely
  * Depend on packages instead of installing them at run time
  * Use xz -T4 instead of pxz.
    Supported xz versions before 5.2 accept the -T4 option but ignore it.
    Also depend on xz-utils instead of downloading pxz. (LP: #1701132)

 -- Balint Reczey <rbalint@xxxxxxxxxx>  Wed, 06 Dec 2017 11:34:04 +0100

** Changed in: livecd-rootfs (Ubuntu Xenial)
       Status: Fix Committed => Fix Released

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https://bugs.launchpad.net/bugs/1701132

Title:
  Please use xz instead of pxz

Status in livecd-rootfs package in Ubuntu:
  Fix Released
Status in livecd-rootfs source package in Xenial:
  Fix Released

Bug description:
  [Impact]

   * Livecd-rootfs switches from using pxz to xz
   * Pxz is installed at run-time instead of being a package dependency and pxz is in universe despite livecd-rootfs being in main. Both of those are undesired and fixing them makes livecd-rootfs easier to maintain following best practice and policy in Ubuntu.

  [Test Case]

   * Livecd-rootfs in Xenial uses (p)xz only for compressing raspi2 images.
     To test the change install updated livecd-rootfs on armhf and create images setting the following environment variables: ARCH=armhf SUBARCH=raspi2 PROJECT=ubuntu-cpc.
     Notice that the xz-compressed rapi2 image is still created.
   * Test-build Xenial image and make sure they are not broken. A subset of those images are built in the autopkgtest suite.

  [Regression Potential]

   * It is a known and accepted regression that raspi2 image generation takes more wall-clock time after switching to xz because while Xenial's xz version does accept the -T4 option it does not compress in a parallel manner.
   * The original change in Artful set -0 compression for xz which resulted in ~35% bigger compressed images. This regression is fixed in LP: #1732681 which is scheduled to be back-ported together with switching to xz.
   * In case of this change causes regression it would manifest itself as failing build for raspi2, failing build for ubuntu-cpc images or increased raspi2 image size.

  [Original bug report text]

  Livecd-rootfs (main) 2.447 uses "pxz -T4" for xz compression, while pxz is not in main.
  Xz 5.2 also supports multi-threaded compression thus xz could easily replace current pxz usage.

  A quick test on compressing livecd.ubuntu-cpc.img (created by ubuntu-
  image) shows that there is very little difference in compressed file
  size while xz's -T4 brings substantial speedup in wall clock time:

  $ cat test-xz
  #!/bin/sh

  TESTFILE=$1
  echo "Compr.\tThreads\tWall.t\tUser.t\tSys.t\tRes.t" > times.txt
  echo "Wall.t\tUser.t\tSys.t\tRes.t" > decompression-times.txt
  echo "Size(kB)" > sizes.txt
  for threads in 1 4; do
      for compression in $(seq 0 9) $(seq -f '%0.fe' 0 9); do
          /usr/bin/time -a -o times.txt -f "-$compression\t -T$threads\t%e\t%U\t%S\t%M" -- xz -T$threads -$compression -k $TESTFILE
          /usr/bin/time -a -o decompression-times.txt -f "%e\t%U\t%S\t%M" -- xz -t $TESTFILE.xz
          ls -k -s $TESTFILE.xz | cut -d' ' -f1 >> sizes.txt
          rm $TESTFILE.xz
      done
  done
  paste times.txt decompression-times.txt sizes.txt > time-size.txt

  $ cat time-size.txt
  Compr.  Threads Wall.t  User.t  Sys.t   Res.t   Wall.t  User.t  Sys.t   Res.t   Size(kB)
  -0       -T1    63.48   63.07   0.40    4932    1.25    1.16    0.08    2432    416324
  -1       -T1    102.90  102.26  0.63    11064   1.53    1.26    0.14    3252    414624
  -2       -T1    131.04  130.40  0.62    18544   1.43    1.27    0.09    4224    414616
  -3       -T1    146.22  145.61  0.60    33992   1.43    1.24    0.12    6364    414616
  -4       -T1    142.24  141.76  0.47    50488   1.34    1.21    0.10    6424    414584
  -5       -T1    147.81  147.26  0.53    97616   1.37    1.22    0.10    10420   414564
  -6       -T1    148.61  148.06  0.54    97608   1.35    1.23    0.10    10536   414564
  -7       -T1    154.29  153.70  0.58    191800  1.44    1.22    0.14    18820   414564
  -8       -T1    155.82  155.16  0.65    380336  1.53    1.27    0.13    35208   414564
  -9       -T1    194.08  193.18  0.88    691128  1.59    1.31    0.13    67700   414444
  -0e      -T1    76.81   76.31   0.48    6296    1.27    1.21    0.05    2536    416256
  -1e      -T1    116.49  115.96  0.53    15068   1.28    1.20    0.07    3232    414568
  -2e      -T1    137.13  136.62  0.50    26892   1.30    1.20    0.10    4292    414564
  -3e      -T1    144.34  143.84  0.49    50556   1.31    1.21    0.09    6340    414564
  -4e      -T1    146.93  146.48  0.44    50412   1.30    1.20    0.10    6340    414568
  -5e      -T1    150.59  150.04  0.53    97552   1.32    1.22    0.10    10416   414564
  -6e      -T1    153.40  152.77  0.62    97564   1.32    1.22    0.09    10404   414568
  -7e      -T1    157.10  156.46  0.62    191956  1.35    1.26    0.08    18548   414568
  -8e      -T1    160.36  159.66  0.69    380248  1.46    1.25    0.13    35040   414568
  -9e      -T1    198.32  197.42  0.88    691256  1.49    1.19    0.14    67648   414448
  -0       -T4    21.97   86.83   0.40    24896   1.28    1.22    0.05    2448    416368
  -1       -T4    35.73   140.12  0.45    73456   1.29    1.22    0.06    3136    414640
  -2       -T4    38.15   149.36  0.54    141044  1.30    1.21    0.08    4160    414624
  -3       -T4    40.74   153.95  0.55    276336  1.30    1.22    0.08    6216    414620
  -4       -T4    40.02   152.20  0.63    341980  1.38    1.23    0.11    6352    414588
  -5       -T4    41.68   153.74  1.10    676656  1.49    1.27    0.10    10336   414564
  -6       -T4    42.70   157.64  1.19    677144  1.48    1.25    0.13    10436   414564
  -7       -T4    47.70   157.42  2.24    1330432 1.60    1.32    0.08    18480   414560
  -8       -T4    74.56   178.48  13.32   1676392 1.69    1.34    0.10    35000   414560
  Command terminated by signal 9  Command exited with non-zero status 1   0
  -9       -T4    19.60   41.90   5.28    1656776 0.01    0.00    0.00    2076    416296
  -0e      -T4    30.23   117.70  0.56    30596   1.29    1.22    0.06    2444    414584
  -1e      -T4    37.16   144.83  0.51    90304   1.31    1.21    0.10    3272    414572
  -2e      -T4    38.73   151.84  0.50    174124  1.32    1.24    0.08    4296    414568
  -3e      -T4    40.78   156.10  0.65    342012  1.33    1.27    0.05    6432    414572
  -4e      -T4    42.03   161.22  0.62    342108  1.33    1.24    0.09    6396    414564
  -5e      -T4    43.10   160.05  1.08    676732  1.47    1.30    0.07    10448   414568
  -6e      -T4    43.17   159.25  1.31    676584  1.55    1.27    0.12    10328   414564
  -7e      -T4    48.12   162.80  2.09    1330552 1.60    1.29    0.12    18596   414568
  -8e      -T4    77.13   182.30  13.98   1673832 1.69    1.34    0.10    34920   0
  Command terminated by signal 9  Command exited with non-zero status 1
  -9e      -T4    18.92   44.75   4.78    1662540 0.02    0.00    0.00    2196

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