Resize Root Filesystem in Logical Volume

For CentOS and other linux distros which using LVM, sometimes it’s necessary to resize the root partition. Unfortunately, we can’t ‘just’ resize the mounted partition. By using step-by-step below, we can resize the root partition using Linux live CD, in this case I use Fedora 15 beta live CD. Here’re the steps:
1. Boot from live CD.
2. Open terminal window
3. Do not mount the filesystems
4. At the shell type: lvm
5. Within lvm shell, run following commands:
lvm> vgscan
lvm> vgchange –available y VolGroup00
lvm> lvscan # should show the LV as “ACTIVE”; the devices in /dev it mentions should now exist
lvm> exit
6. Now resize the filesystem with this command (e.g. using device /dev/VolGroup00/LogVol00):
e2fsck -f /dev/VolGroup00/LogVol00 # check filesystem
resize2fs /dev/VolGroup00/LogVol00 nnG # resize the fs to nn GB; nn should be less than the current fs size
e2fsck -f /dev/VolGroup00/LogVol00 # recheck filesystem
7. Run lvm again and type:
lvm> lvreduce -LnnG /dev/VolGroup00/LogVol00 # resize the LV to nn GB; nn should be the same as above
lvm> exit
8. Do another filesystem check:
e2fsck -f /dev/VolGroup00/LogVol00
9. Reboot and remove CD.

If you get the warning of resizing currently ACTIVE LV, please do the following from command line:
lvchange -a n VolGroup00/LogVol00 # to deactivate LV
and after operation above do:
lvchange -a y VolGroup00/LogVol00 # to reactivate LV

Credit to: coreychch
Mostly taken from: http://www.linuxquestions.org/questions/linux-newbie-8/lvm-and-shrinking-the-root-filesystem-327292/