Section 9.9. Root Disk Options | Linux Kernel in a Nutshell (In a Nutshell (OReilly))

These options control how the kernel finds and handles the filesystem that contains the root filesystem.

Mount the root device read-only on boot.: The default for the kernel is to mount the root device as read-only at boot time. This option ensures that this is the mode the kernel uses. It overrides the rw command-line option, if it had been specified earlier on the boot command line.

Specify the root filesystem to boot from.

root= device

Tell the kernel which disk device the root filesystem image is on. device can be specified in one of the following ways:

nnnn: A device number in hexadecimal represents the major and minor number of the device in the internal format that the kernel expects. This method is not recommended unless you have access to kernel internals.
/dev/nfs: Use the NFS disk specified by the nfsroot boot option as the root disk.
/dev/ <diskname>: Use the kernel disk name specified by <diskname> as the root disk.
/dev/ <diskname><decimal>: Use the kernel disk name specified by <diskname> and the partition specified by <decimal> as the root disk.
/dev/ <diskname>p <decimal>: Use the kernel disk name specified by <diskname> and the partition specified by <decimal> as the root disk. This is the same as above, but is needed when <diskname> ends with a digit.

Time to delay before attempting to mount the root filesystem.
rootdelay= n: Wait n seconds before trying to mount the root filesystem. This can be useful if the root filesystem is on a USB or FireWire device, as those disk devices take a bit longer to be discovered by the kernel.

The root filesystem mount options.
rootflags= options: Mount options that the kernel should use in mounting the root filesystem. The options value depend on the filesystem type; see the documentation for the individual types for details on what is valid.

The root filesystem type.
rootfstype= type: Try to mount the root filesystem as this type of filesystem. For instance, rootfstype=ext3.

Mount the root device read-write on boot.: The default for the kernel is to mount the root device as read-only at boot time. This option mounts the root device as read-write instead.