CAUTION: Make a backup of your DATA before running these commands.
If you ever need to extend your running OS mounted root partition that is using LVM, run these commands after adding a new disk / partition to your system.
Use the following commands to check for existing LVM partitions:
pvdisplay -v vgdisplay -v
First of all, check if the resize_inode option is active.
# tune2fs -l /dev/mapper/VolGroup-lv_root | grep resize_inode
If that option is present in the partition, you can continue, if not, you need to unmount the partition.
cfdisk /dev/sdb pvcreate /dev/sdb1 vgextend VolGroup /dev/sdb1 lvresize --resizefs -l +100%FREE /dev/mapper/VolGroup-lv_root
Use df to check for the results, no need to reboot!