...use tune2fs -l /dev/hda5 | grep features to get a list of your filesystem features. If you have "dir_index" in there then you're all set. Ignore any "needs_recovery" - that is good for mounted filesystems (it just means "not cleanly unmounted")

To add dir_index, do the following.

• Unmount the filesystem. If this is your primary filesystem, use a bootable CD like Knoppix or some Linux install CD.
• Make sure the filesystem is really not mounted. Just for safety of your data. Run e2fsck /dev/hda5 - this should take less than one second if your filesystem is clean and is needed anyway otherwise...
• Execute tune2fs -O +dir_index /dev/hda5 to enable the feature for your filesystem
• Execute e2fsck -Df /dev/hda5. This will force a filesystem check with directory optimization (!) on your filesystem.
• Mount your system (or reboot, if you were using a live CD)