Recently ext4 had been added to Scientific Linux 5.3 and CentOS 5.3 as a tech. preview. Being curious as I am, I enabled ext4 (well ext4dev on my /shared partition) to see what its like and how well it performs. To convert an existing ext2/3 partition to an ext4dev partition you first need to be running the 2.6.18-128.el5 or higher kernel. Then you need to do yum install e4fsprogs Following http://ext4.wiki.kernel.org/index.php/Ext4_Howto#For_people_running_RHEL will get you most of the way there... Here's what I did

tune4fs -O extents,uninit_bg,dir_index /dev/DEV
tune4fs -E test_fs /dev/DEV
fsck.ext4 -pf /dev/DEV
...edit /etc/fstab and changed ext3 to ext4dev on /dev/DEV...

Note, doing the above will break backwards compatibility, and you will not be able to remount it as ext3. Now all I need to do is wait for my test partition to explode in my face and then loose all my data.

Part of the reason for testing and familiarising with this is that at the place I work, it would be nice to be able to create filesystems 16tb or bigger which currently ext3 and xfs has issues with.