Compress initramfs

In some cases, the /boot partition is too small for the initramfs to fit. This makes it impossible to install multiple kernels, so the solution is to change the compression method.

Edit /etc/initramfs-tools/initramfs.conf and change the compression method to xz (xz has a higher compression ratio, but slows down the decompression speed).

sudo vim /etc/initramfs-tools/initramfs.conf

- MODULES=most
+ MODULES=dep

- COMPRESS=lz4
+ COMPRESS=xz

Install xz-utils and update initramfs.

Ubuntu

sudo apt install xz-utils
sudo update-initramfs -u -k all