Published at LXer:
In Linux 2.6.24.5, the problem is explained in init/do_mounts.c. Following the execution path, you can see that the first root filesystem mount happens in mount_root(). First, the device name is converted to a (major,minor) device number pair, then /dev/root is created with that (major,minor) pair. Finally, /dev/root is mounted as the root filesystem.
The catch to this is that the only partition accessible is the one created for /dev/root. The way around it is to make some other mount the first mount, then switch to the XFS later in the boot process. This is exactly what an init ramdisk gives us.
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