[Bug 1831736] Re: [MIR] lz4 by default

[Dimitri John Ledkov]

** Description changed:

  Use `lz4 -9 -l` compression for initramfs by default as discussed on
  ubuntu-devel.
  
  This would also pull the lz4 package into main
  
  https://lists.ubuntu.com/archives/ubuntu-devel/2019-June/040726.html
+ 
+ [Regression Potential]
+ 
+ We are trying to optimize for total boot speed, but performing a micro-
+ optimization upon time to create/unpack kernel/initrd is an insufficient
+ benchmark for total boot speed. This is because it ignores time to load
+ the kernel/initrd, and whether the firmware/bootloader were able to
+ stream decompress it whilst loading it. I.e. it is argued that in the
+ real world, subsecond decompression gains are irrelevant if UEFI
+ firmware, tftp boot, etc. take a lot longer than that to read extra 10s
+ of MBs of boot material.
+ 
+ [TODO]
+ Measure pure i/o load speed with stopwatch, to figure out MB/s speed of loading initrds/kernel off FAT32, EXT4, TFTP, HTTP.
+ Re-evaluate if we should provide different compression mechanisms:
+ - ie. gzip instead of lz4 for most cases (revert)
+ - ie. xz for painful i/o cases (e.g. netboot)

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https://bugs.launchpad.net/bugs/1831736

Title:
  [MIR] lz4 by default

Status in initramfs-tools package in Ubuntu:
  Fix Released
Status in linux package in Ubuntu:
  Confirmed
Status in live-build package in Ubuntu:
  Fix Released
Status in livecd-rootfs package in Ubuntu:
  Fix Released
Status in lz4 package in Ubuntu:
  Fix Released
Status in partman-auto package in Ubuntu:
  Triaged
Status in ubuntu-release-upgrader package in Ubuntu:
  Triaged

Bug description:
  Use `lz4 -9 -l` compression for initramfs by default as discussed on
  ubuntu-devel.

  This would also pull the lz4 package into main

  https://lists.ubuntu.com/archives/ubuntu-devel/2019-June/040726.html

  [Regression Potential]

  We are trying to optimize for total boot speed, but performing a
  micro-optimization upon time to create/unpack kernel/initrd is an
  insufficient benchmark for total boot speed. This is because it
  ignores time to load the kernel/initrd, and whether the
  firmware/bootloader were able to stream decompress it whilst loading
  it. I.e. it is argued that in the real world, subsecond decompression
  gains are irrelevant if UEFI firmware, tftp boot, etc. take a lot
  longer than that to read extra 10s of MBs of boot material.

  [TODO]
  Measure pure i/o load speed with stopwatch, to figure out MB/s speed of loading initrds/kernel off FAT32, EXT4, TFTP, HTTP.
  Re-evaluate if we should provide different compression mechanisms:
  - ie. gzip instead of lz4 for most cases (revert)
  - ie. xz for painful i/o cases (e.g. netboot)

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