Go to /etc/initramfs-tools/initramfs.conf and change:
compress=gzip
to
compress=ztsd
and run:
sudo update-initramfs -u -k all
this post was submitted on 20 Sep 2023
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There's nothing to change. zstd is already the default. Has been from the beginning.
Then the kernel should be ztsd. Try running file and see the output.
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
No. Like this:
file /boot/vmlinuz-6.4.6-76060406-generic
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?
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.
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)
Looks like your initrd isn't compressed. Huh?
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...
Pop already uses zstd for its initramfs. You can check by looking at /etc/initramfs-tools/initramfs.conf.
There was an issue where they were considering switching to xz to save space, but when I tried that, it actually made things slower, so I'd avoid that if possible:
I'd be curious to know how uncompressed fairs if you test it. But i think you're really getting into minimal gains territory...
I wouldn't focus on faster decompression.
Disk read speed is almost always the bottleneck. I think you'll find that smaller file sizes, even if the decompress takes slightly longer, are faster overall because they save disk I/O.
YMMV based on your hardware, of course.