Is there a way to move an ext4 filesystem between physical disks transparently for the applications, i.e. while preserving file descriptors? There's no LVM nor hardware RAID, unfortunately.
I was thinking in the direction of creating an md mirror and breaking it after the sync, but it seems that it requires unmounting the disk or partition at least once. Converting to LVM on the fly would be ideal, but I haven't found any evidence that it can be done.