What's taking space are mainly these:
-rw-r--r-- 1 root root 153M heinä 10 14:22 initrd.img-5.11.0-22-generic
-rw-r--r-- 1 root root 151M heinä 23 13:13 initrd.img-5.11.0-25-generic
apt-get upgrade fails:
update-initramfs: Generating /boot/initrd.img-5.11.0-25-generic
Error 24 : Write error : cannot write compressed block
E: mkinitramfs failure cpio 141 lz4 -9 -l 24
update-initramfs: failed for /boot/initrd.img-5.11.0-25-generic with 1.
dpkg: error processing package initramfs-tools (--configure):
installed initramfs-tools package post-installation script subprocess returned error exit status 1
Errors were encountered while processing:
initramfs-tools
E: Sub-process /usr/bin/dpkg returned an error code (1)
How to fix this:
I suggest that apt-get/dpkg handles upgrading outside /boot. Two kernels seem to fit there if the updating is done cleverly. Or if only one kernel fits, then keep only on kernel in /boot.
Thanks,
Ilkka