I believe the default OS file system in CentOS 5 is ext3.
Is it possible to use ext4 on Centos 5.x, or only on 6.x? Can I upgrade an existing ext3 filesystem to ext4?
In addition to Kenny Rasschaert's answer:
You can find a nice explanation here:
Be warned that it can be dangerous and you should make a backup before attempting to do this.
Converting a root filesystem to ext4
Converting a root filesystem is a bit more tricky because you cannot unmount it, as your system is running on it. Nevertheless it is still possible to do it without using an external bootable media. You should do this in a single-user mode.
First step is to modify your /etc/fstab file to say "ext4" instead of "ext3" for root partition. This is important because you will be operating on a read-only filesystem later, and will not be able to make the change, and this would result in your system unable to mount a root filesystem on next boot.
Let us assume that root partition is /dev/sda1, so your /etc/fstab should look something like this.
Now remount the root filesystem read-only.
Then run a filesystem check on the root filesystem.
It will tell you to reboot the system. That may be a good idea, so simply boot into single-user mode and remount it read-only again. It is fine even though we have already modified /etc/fstab, because ext3 can be mounted as ext4 without conversion.
Next, enable all the ext4 features on the root filesystem.
And run run a filesystem check on the root filesystem again. It will find and fix errors. This is normal.
You can now reboot to your new ext4 system, and enjoy faster filesystem check times, better performance, and all the improvements of ext4. Well, almost; read the next section.
According to the Red Hat docs, ext4 is supported in version 5.6 and above.