One thing comes to my mind with respect to people trying this out. I recall reading that e4defrag was going be the proposed way to convert files to using extents after migrating from ext3. But I get "Operation not supported" when I try to defrag files older than the date of migration, so it looks like e4defrag doesn't work on non-extents files yet (that or something strange is going on).
Testers should be aware of this so they won't expect to be able to migrate to ext4 then immediately run e4defrag and have it work. If you migrated from ext3, the older files won't defrag.
One thing comes to my mind with respect to people trying this out. I recall reading that e4defrag was going be the proposed way to convert files to using extents after migrating from ext3. But I get "Operation not supported" when I try to defrag files older than the date of migration, so it looks like e4defrag doesn't work on non-extents files yet (that or something strange is going on).
Testers should be aware of this so they won't expect to be able to migrate to ext4 then immediately run e4defrag and have it work. If you migrated from ext3, the older files won't defrag.