When a new submirror is attached (added) to a mirror, all the data from another submirror in the mirror is automatically written to the newly attached submirror. Once the mirror resynchronization is done, the new submirror is readable. A submirror remains attached to a mirror until it is explicitly detached.
If the system crashes while a resynchronization is in progress, the resynchronization is restarted when the system finishes rebooting.