Resizing online is possible and it is even better with the new 3.3 Linux kernel
You can extend an ext4 file system – even if it is your current root and your OS is running. On the other hand, online shrinking is only supported from version 3.3 of the Linux kernel.
In this example, I will show you how I extended a real life, live ~5 GB virtual disk (with an ext4 file system) to 10 GB.
Read more about the new 3.3 and 3.4 Linux kernel versions at http://www.ibm.com/developerworks/library/l-33linuxkernel/ (thanks Joro).