This “mkinitcpio” hook mounts the root file system from a subdirectory rather than the root directory of a partition. The “dir=” kernel parameter specifies which subdirectory. A leading slash (/) is allowed but not necessary.
This allows multiple separate operating systems to be installed in a single partition. For example a single partition could contain multiple OS filesystems and other shared files:
- /os/arch-x86-64/bin/
- /os/arch-x86-64/etc/
- . . .
- /os/arch-i686/bin/
- /os/arch-i686/etc/
- . . .
- /home/user/
- /os/debian/ (potentially)
- /os/win/WINDOWS/ (if a common filesystem was supported)
It is achieved by using a “bind mount” of the subdirectory.
Add “dir” to the HOOKS list in /etc/mkinitcpio.conf:
HOOKS+=" dir"
Add a “dir=” parameter to the kernel command line (for example in /boot/grub/menu.lst). The path is relative to the file system of the “root” device. For example:
APPEND root=LABEL=label dir=/os/arch-x86-64 ro
It’s also useful to mount the whole partition somewhere as well using /etc/fstab, although this means that all the files under the mounted OS directory will be visible under both mount points. Although I have not seen any serious problems, some programs complain about a circular file system loop:
find: File system loop detected
du: WARNING: Circular directory structure.
This almost certainly means that you have a corrupted file system.
NOTIFY YOUR SYSTEM MANAGER.