The online backup seems to occur in 3 or 4 stages (not clear from the logs).
In the last stage, dor a large repository (40GB) threads seem to hang if anyone tries to login. However, the online backup will eventually finish and the hung threads are released to do work. No need to restart the server.
The start of the online backup looks like this (in system out log) :-
SystemOut O dd.mm.yyyy hh:mm:ss *INFO * Backup: Read size (nnnnnnnnn bytes) after nnnnnn ms (Backup.java, line 169)
The end of the online backup looks like this :-
SystemOut O dd.mm.yyyy hh:mm:ss *INFO * Backup: Copied last stage after nnnnnnnnnn ms (Backup.java, line 198)
(working on externalising the Data Store to reduce the online backup time by 70%).