My root partition recently ran out of space and I needed to resize it.
So I booted into a live usb and then, after activating my volume groups:
$ sudo vgchange -a y
I looked up lvm resize online and came up with the command:
$ sudo lvextend -L+10G /dev/mapper/big-root
Next I rebooted and ran "df -h". It was the same size and I was annoyed and confused. After some routine poking around I noticed the man page suggested the use of a "-r" flag to actually resize the file system. Who knew?
$ sudo lvextend -r -L+10G /dev/mapper/big-root
PS: Oh and if you need to reduce the size of the partition because you ran the command without modifying the filesystem and you don't want to learn how to use "resizefs"
$ sudo lvreduce -L-10G /dev/mapper/big-root
UPDATE: A reader has suggested that the previous code block might be misleading. To reduce the size of a lvm partition, you should use the following command with the "-r" flag or you will risk losing data. The file system must be resized before the lvm partition's size is reduced. However in the previous example, my filesystem was the size of the partion after reduction.
$ sudo lvreduce -r -L-10G /dev/mapper/big-root
Note: Commandlinefu says it better and faster: http://www.commandlinefu.com/commands/view/12195/lvextend-logical-volume