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Now initramfs files are compressed with gzip
But we can use another compression, i.e. LZO, LZ4, and ZSTD.
Then which one is the best?
compression ratio
ZSTD > GZIP > LZO > LZ4
decompression speed
LZ4 > LZO = ZSTD > GZIP
I suggest choosing ZSTD, instead of GZIP since
- better compression ratio than current GZIP
- also better decompression speed than current GZIP
With gzip
$ file /boot/initrd.img-5.9.0-*
/boot/initrd.img-5.9.0-3-amd64: gzip compressed data, from Unix, original size modulo 2^32 197680640
/boot/initrd.img-5.9.0-4-amd64: gzip compressed data, from Unix, original size modulo 2^32 197594112
$ du -h /boot/initrd.img-5.9.0-*
59M /boot/initrd.img-5.9.0-3-amd64
59M /boot/initrd.img-5.9.0-4-amd64
With zstd
$ file /boot/initrd.img-5.9.0-*
/boot/initrd.img-5.9.0-3-amd64: Zstandard compressed data (v0.8+), Dictionary ID: None
/boot/initrd.img-5.9.0-4-amd64: Zstandard compressed data (v0.8+), Dictionary ID: None
$ du -h /boot/initrd.img-5.9.0-*
40M /boot/initrd.img-5.9.0-3-amd64
40M /boot/initrd.img-5.9.0-4-amd64
Yes, 59MB initramfs file becomes 40MB (2/3)! plus bonus, better decompression speed.
However, there's a problem to do so - zstd package's Priority is not standard one - it's "optional".
> Package: gzip
> Version: 1.10-2
> Priority: required
> Package: zstd
> Version: 1.4.5+dfsg-4
> Priority: optional
It means there is not zstd package in every environment. And, just raise its Priority
means bloat minimal system size that is a problem we want to avoid.
Now initramfs-tools has a hack to avoid zstd (and other compression tools)
absence in mkinitramfs command.
> if ! command -v "${compress}" >/dev/null 2>&1; then
> compress=gzip
> echo "No ${compress} in ${PATH}, using gzip"
> fi
Set COMPRESS=zstd in /etc/initramfs/initramfs.conf and no zstd installed,
there's no problem.
> $ sudo update-initramfs -u
> update-initramfs: Generating /boot/initrd.img-5.9.0-4-amd64
> No zstd in /usr/bin:/sbin:/bin, using gzip
(above message is patched with #971270)
So, just set default compression as zstd can bring a better result for many users,
IMO (Not sure raising zstd package Priority (to standard) is required or not).