I got this message from WHM. Do I have to fix it? Thank you.
The filesystem “/dev/sda1”, which is mounted at “/boot”, has reached “critical” status because it is 95.63% full.
My disk is:
Device Size Used Available Percent Used Mount Point
/dev/sda1 99M 90M 4.4M 96% /boot
/dev/sda2 451G 63G 366G 15% /
/usr/tmpDSK 4.0G 435M 3.4G 12% /tmp
Obviously the message just tells you that the disk partition /dev/sda1 is 95.65% full. This partition is mounted as /boot, and the contents probably won't change much. The folder /boot usually only contains information for the bootloader, nothing more. Therefore it isn't very critical in my eyes.
You could also look inside this folder to look its contents up, but it should look something like this if the grub bootloader is used:
drwxr-xr-x 6 root root 4096 6. Mär 09:25 grub
-rw-r--r-- 1 root root 18777509 19. Jun 19:45 initramfs-linux-fallback.img
-rw-r--r-- 1 root root 3577244 19. Jun 19:45 initramfs-linux.img
-rw-r--r-- 1 root root 4517872 8. Jun 08:42 vmlinuz-linux
In case you are using GRUB (otherwise there might be a different directory): the /boot/grub folder contains the current grub configuration (which menu entries are displayed, menu timeout, ...), which is derived from your /etc/defaults/grub configuration. The tools update-grub/grub-mkconfig have to be called after changing the configuration in order to update the /boot/grub folder.
The /boot/initramfs-*.img are the initial ram disk images, which contain modules loaded at startup. They can be modified by editing the /etc/mkinitcpio.conf configuration file and running mkinitramfs with appropriate parameters afterwards. The fallback image should always stay there in case something goes wrong with the normal boot. The vmlinuz-linux file is basically the kernel executable.