[Bug 1931725] Re: initramfs-tools: use zstd as the default compression method

[Dimitri John Ledkov]

** Changed in: initramfs-tools (Ubuntu)
       Status: New => Fix Committed

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Title:
  initramfs-tools: use zstd as the default compression method

Status in initramfs-tools package in Ubuntu:
  Fix Committed
Status in linux package in Ubuntu:
  New

Bug description:
  Turns out that loading is always the slow part in loading initramfs
  into memory and decompressing it since decompression is always the
  final 10-20% or so of the task.  It therefore makes sense to use a
  good compressor that shrinks the initramfs as much as possible with
  little decompression overhead.

  Benchmarking zstd vs lz4 shows that while zstd can be ~5x slower in
  decompression, the image size is much smaller with zstd than lz4, and
  since ~80-90% of the boot time is loading the image it makes sense to
  use zstd.

  Attached is a libreoffice spread sheet showing typical load and
  decompression times for a fairly standard 3.4 GHZ intel box with data
  for load times for a 5400 RPM, 7200 RPM and SATA SSD drives.

  The conclusions from the test results (attached) show:

  1. Loading time is always significantly slower than decompression time.
  2. ZSTD is 5x slower than LZ4 in decompression speed but produces far
  better compressed images
  3. Given that loading time is the major factor in loading +
  decompression, ZSTD is best for kernel and initramfs boot timings.

  (Also refer to https://kernel.ubuntu.com/~cking/boot-speed-eoan-5.3
  /kernel-compression-method.txt for some raw data on drive load speeds
  for the same UEFI box I did a couple of years ago).

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