As long as you are converting a filesystem that can be unmounted, it is fairly simple procedure. In this example we will be converting a /dev/sdc1 partition mounted as /home directory.
First, unmount the partition.
=====>> umount /dev/sdc1
Next, run a filesystem check on it to make sure it is in sane condition. We are still on ext3.
=====>> fsck.ext3 -pf /dev/sdc1
Enable new features of ext4 on the filesystem.
=====>> tune2fs -O extents,uninit_bg,dir_index /dev/sdc1
Option "extents" enables the filesystem to use extents instead of bitmap mapping for files, "uninit_bg" reduces file system check times by only checking used portions of the disk, and "dir_index" allows storing the contents of large directories in a htree for faster access. Option "dir_index" is also supported by ext3, so you may already be using it, but it makes no harm to specify it here.
Run a filesystem check. It will find errors. It is normal. Let it fix them. You may want to run the check twice to make sure that the filesystem is now clean.
=====>> fsck.ext4 -yfD /dev/sdc1
The "-D" parameter will actually enable the "dir_index" option by rebuilding directory index. It can be rebuilt (optimized) at any later time by running the check with the parameter.
Now edit your /etc/fstab file to say "ext4" instead of "ext3" for /home. Other options may differ for your system.
/dev/sdc1 /home ext4 defaults 0 2
Try to mount your new ext4 filesystem.
=====>> mount /home
If it succeeds, congratulations. If not, do not panic. You have not lost your data. And you have a backup after all, right? Make sure you have all the latest tools listed in prerequisites. Get them form Debian unstable or experimental if needed. Upgrade and try again.
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