Why is excessive redo generated during an Online/Hot Backup in Oracle

       There is not excessive redo generated, there is additional information

logged into the online redo log during a hot backup the first time a block is

modified in a tablespace that is in hot backup mode.

In hot backup mode only 2 things are different:

No 1->               The first time a block is changed in a datafile that is in hot backup mode,

                   the ENTIRE BLOCK is written to the redo log files, not just the changed bytes.

                   Normally only the changed bytes (a redo vector) is written. In hot backup mode,

                   the entire block is logged the FIRST TIME. This is because you can get into a

                   situation where the process copying the datafile and DBWR are working on the

                   same block simultaneously.

and 2nd is :->

         The datafile headers which contain the SCN of the last completed checkpoint

          are NOT updated while a file is in hot backup mode. This lets the recovery

         process understand what archive redo log files might be needed to fully recover

         this file.

  To limit the effect of this additional logging, you should ensure you only place one tablepspace at a time in backup mode and bring the tablespace out of backup mode as soon as you have backed it up. This will reduce the number of blocks that may have to be logged to the minimum possible.

Please share your really important  comments and views  for improvement......   Thanks :)