To convert a file system from ext3 to ext4, use
tune2fs -O extents /dev/DEVand remount the file system as ext4. (Two
e2fsckruns are recommended before and after
tune2fs.) Some documentations also include the
-E test_fsoption. This is not necessary now since ext4 is no longer experimental.
Finally do not forget to modify
An ext4 file system created this way is not a “true” ext4 - the extents feature, the main advantage of ext4 comapred to ext3, is not automatically applied to old files. New files created afterwards are in the extents format.
Unlike the 100% backward compatibility of ext3 with ext2, an ext4 file system can no longer be mounted as if it were an ext3, unless the extents feature is disabled. (If you want to disable extents, why not simply use ext3?)