If you recently installed CentOS 5.6 or updated to CentOS 5.6 you can now utilize the ext4 filesystem.
To do the conversion or create a new ext4 filesystem you need the e4fsprogs tool kit from yum.
#yum -y install e4fsprogs
I did this conversion on a test system with a non-root filesystem to avoid any possible problems. I also backed up the filesystem just in case something went terribly wrong.
First you need to un-mount the filesystem, as it cannot be in use.
#cd /; umount /dev/VolGroup00/LogVol00
Now you can run the tune4fs command to convert the filesystem to ext4.
#tune4fs -O extents,uninit_bg,dir_index /dev/VolGroup00/LogVol00
Now that the filesystem is ext4 it is no longer able to be mounted as ext3, so change its entry in the fstab.
Now because we used the option uninit_bg we need to run fsck on the new ext4 filesystem. This is a good idea to do anyways, but is a requirement here.
#e4fsck -fDC0 /dev/VolGroup00/LogVol00
e4fsck will complain about "One or more block group descriptor checksums are invalid", this is totally normal.
Before doing these steps on the root filesystem (/) I would recommend you read over the Ext4 Howto on kernel.org. I re-wrote the steps for more accurate information pertaining specifically to CentOS 5.6 and my system.
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