Does Oracle write to data files in begin/hot backup mode

Oracle will stop updating file headers, but will continue to write data to database files even if a table space is in backup mode.

In backup mode, Oracle will write out complete changed blocks to the Redo log files.

Normally only changes are logged to the Redo logs; this is done to enable reconstruction of a block if only half of it was backed up (split blocks). Because of this, one should notice increased log activity and archiving during online backups