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

[Dimitri John Ledkov]

However 298.1MB is larger than old installs.

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Title:
  [MIR] lz4 by default

Status in Release Upgrader:
  New
Status in initramfs-tools package in Ubuntu:
  Fix Released
Status in linux package in Ubuntu:
  Fix Released
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:
  Invalid
Status in ubuntu-release-upgrader package in Ubuntu:
  Invalid

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)

  I booted grub2 and measured loading largish amount of files, ie. $
  date; initrd (hd0,gpt5)/initrd.img;  initrd (hd0,gpt5)/initrd.img;
  initrd (hd0,gpt5)/initrd.img; initrd (hd0,gpt5)/initrd.img; initrd
  (hd0,gpt5)/initrd.img; date

  To get a rough speed between 30 and 44 MB/s of loading these files off
  ext4 on nvme.

  With lz4 initrd taking 67M, and gzip initrd taking 59M, the grub i/o
  penalty is 0.18s whilst I gain over a second in faster decompression
  time. Overall a win.

  xz initrd is 36M meaning saving e.g. 0.8s of i/o time whilst gaining
  2.4s of decompression time, meaning overall worse than gzip.

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