RMAN backup statistics

  v$RMAN_BACKUP_JOB_DETAILS contains information about backup durations, I/O rates.  It's a good starting point from which you can begin turning the backup.
col input_bytes_display format a18
col output_bytes_display format a18
col time_taken_display format a18
set linesize 132
set pagesize 50
SELECT session_recid, input_bytes_display, output_bytes_display,time_taken_display, end_time
FROM v$rman_backup_job_details
ORDER BY end_time

SESSION_RECID INPUT_BYTES_DISPLA OUTPUT_BYTES_DISPL TIME_TAKEN_DISPLAY END_TIME
------------- ------------------ ------------------ ------------------ ---------
         7049     6.99G            965.00M          00:09:38           13-FEB-11
         7057     6.94G            948.13M          00:08:51           14-FEB-11
         7065    10.28G              1.81G          00:11:22           15-FEB-11
         7073     9.96G              1.73G          00:30:26           16-FEB-11
         7081     6.85G            784.77M          00:10:24           17-FEB-11
         7089     6.82G            784.52M          00:08:34           18-FEB-11
         7097     6.93G            814.08M          00:09:03           19-FEB-11
         7105     7.00G            830.98M          00:08:50           20-FEB-11
         7113     6.97G            822.40M          00:09:00           21-FEB-11
         7121     6.97G            821.77M          00:10:11           22-FEB-11
         7129     6.98G            822.81M          00:10:40           23-FEB-11

 Based on the result above, we should investigate what happened on 16-FEB-11 that backup took longer than usual.  As basic RMAN tuning, we have two recomendations for improving RMAN backup

• Use incremental backup with block change tracking enabled.
• Adjust multipexing of backup sets by adjusting filesperset (limit the number of datafiles in each backupset), maxopenfiles (limit the number of files that can be opened for reads simutaneously), diskratio (read datafiles from specified number of disks)