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Is it possible to switch to zstd or lz4 initrd/kernel compression for faster boot? (lemmy.ca)

submitted 1 year ago by [email protected] to c/pop_os

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I'm exploring ways to shave a few seconds off of my boot time, and I came across a post that stated, "my initrd is pretty small--doesn’t really load much--and Arch also defaults to using zstd which is also faster to decompress versus gzip."

What compression does Pop! use for initrd and the kernel? When I run ls -al /boot, I see files such as 14M vmlinuz-6.4.6-76060406-generic and 119M initrd.img-6.4.6-76060406-generic. Are these compressed?

Lastly, is there a way to choose the compression of these boot files without a custom kernel build? Or is what I'm trying to do "off the beaten path" and going to lead to "you have to compile your own kernel from here on out"?

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[–] Spider89 6 points 1 year ago (2 children)

Go to /etc/initramfs-tools/initramfs.conf and change:

compress=gzip

compress=ztsd

and run:

sudo update-initramfs -u -k all

[–] mmstick 2 points 1 year ago (1 children)

There's nothing to change. zstd is already the default. Has been from the beginning.

[–] Spider89 1 points 1 year ago (1 children)

Then the kernel should be ztsd. Try running file and see the output.

[–] [email protected] 1 points 1 year ago (1 children)

How would I check? Like this?

$ zstd -l vmlinuz-6.4.6-76060406-generic 
Frames  Skips  Compressed  Uncompressed  Ratio  Check  Filename
File "vmlinuz-6.4.6-76060406-generic" not compressed by zstd

[–] Spider89 2 points 1 year ago (1 children)

No. Like this:

file /boot/vmlinuz-6.4.6-76060406-generic

[–] [email protected] 1 points 1 year ago (1 children)

Wow, this told me much more than I expected; however, I'm still not sure if it's zstd:

/boot/vmlinuz-6.4.6-76060406-generic: Linux kernel x86 boot executable bzImage, version 6.4.6-76060406-generic ([email protected]) #202307241739~1694621917~22.04~ac5e1a8 SMP PREEMPT_DYNAMIC Wed S, RO-rootFS, swap_dev 0XD, Normal VGA

bzImage sounds like...bzip2, maybe?

[–] Spider89 1 points 1 year ago (1 children)

No. BZ stands for Big zImange. The kernel is compressed.

To see what compression was used:

zgrep CONFIG_KERNEL_ /proc/config.gz

Try file on the initrd instead.

[–] [email protected] 1 points 1 year ago (1 children)

Ah, thanks! Slightly different location, but basically the same. Here we go:

$ grep CONFIG_KERNEL_ /boot/config-6.4.6-76060406-generic
# CONFIG_KERNEL_GZIP is not set
# CONFIG_KERNEL_BZIP2 is not set
# CONFIG_KERNEL_LZMA is not set
# CONFIG_KERNEL_XZ is not set
# CONFIG_KERNEL_LZO is not set
# CONFIG_KERNEL_LZ4 is not set
CONFIG_KERNEL_ZSTD=y

So the kernel is "zstd" compressed.

OTOH, I'm not sure if this means anything about initrd (ASCII cpio archive??)

$ file /boot/initrd.img-6.4.6-76060406-generic 
/boot/initrd.img-6.4.6-76060406-generic: ASCII cpio archive (SVR4 with no CRC)

[–] Spider89 1 points 1 year ago

Looks like your initrd isn't compressed. Huh?

[–] [email protected] 1 points 1 year ago

When I check this file, it is already set at COMPRESS=zstd. However, I'm not sure if it's working as I think, because the vmlinuz-6.4* kernel file is not a zstd file? Maybe it uses zstd for just a portion of the binary...