Filesystem labels can be used to specify a partition before
its respective device number is known and is accessible through a fixed
mountpoint name. This becomes essential when the devices are reordered. You
would need to manually adjust /etc/fstab and boot loader
configuration files. Even changing an IDE drive from master to slave requires
this.
When mounting a device, all partitions (that would appear in
/proc/partitions) are probed for the filesystem label. It will
postload modules if necessary. This even works when the kernel mounts the root
partition. Of course it is best to have the code for the root filesystem
in the kernel.
The command to give a filesystem label is fs dependant. Some fs also can only be assigned a label if they are not mounted. Reiser3 and XFS are known to require the unmounted state. Ext2 and Ext3 do not require unmount, but that should not make you preferring them. Here are the different fs commands for a fictive home partition:
reiserfstune -l home /dev/hdc1
xfs_admin -L home /dev/hdc1
e2label /dev/hdc1 home
If your root fs is one of those who can only be tuned in unmounted or read-only state, you will need to boot into another system, i.e. rescue CD, live CD, another Linux on disk, etc.
After the filesystem(s) have been given a label, you could mount them with e.g.:
mount LABEL=home /home
The same applies to fstab where our example home
partition entry could now look like this:
LABEL=home /home xfs defaults,usrquota,grpquota 0
0