** Also affects: ubuntu-release-upgrader
Importance: Undecided
Status: New
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https://bugs.launchpad.net/bugs/1831736
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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