RMAN Cumulative and differential level 1taking too much time

Question

hi,
I am attempting to HOT backup my 600 GB database to backup into Tape using NMO 5 EMC Networker 7.6.
My networker server is on Win Serv 2003.
My oracle database is on RHEL 4.5 Architecture ia64
Oracle DB Version 10.2.0.4.0
Using ASM
Using EMC Storage as Databse storage
Using tape backup media type LTO-Ultrium-5
No of chaneels used same for bothLevel 0 & 1 is 4
there are 60 Datafiles fior the database
i am atttempting incremental backup[Hot] backup
for Incrementa Level 0 is taking 90 Minutes to complete.
BUT leve1 backup [Both differential and cumulative] are taking almost the same time as taken for Level 0 backup
almost 80Mins.
but the backup Set size for Level 0 is almost 500 GB and Sizes for any Level 1 backup not more than 200MB.
i am confused if both LEVEL 0 AND LEVEL 1 BACKUP should take the same span of time.
please help to reduce the time to complete the Level 1 backups..
thanks in advance

Answer 1

RMAN incremental level 1 and up will have to verify every block in the data files to identify if any modifications have occurred. The time it takes to complete the incremental backup will depend on how much changed. Are you using the latest patches? There are known bugs that can affect performance problems with RMAN backup and recovery. Otherwise, check the Oracle documentation to troubleshoot RMAN.
Block change tracking as already mantioned, introduced in 10g, can greatly speed up your incremental level 1 and up backups.
From what I understand:
SQL> ALTER DATABASE ENABLE BLOCK CHANGE TRACKING USING FILE '/mydir/rman_change_track.f';
As soon as block change tracking is enabled, Oracle starts to record every block that updates. The information is stored in a bitmap inside the BCT file. Every incremental backup causes a bitmap swtich in the BCT file.
If there exists a previous bitmap beside the current bitmap, then an incremental level 1 backup will only backup the blocks according to the current bitmap. Incremental level 1 backups are differential backups by default. If there is no previous bitmap, the RMAN backup will perform a conventional scan of the database as usual.
The bitmap logic applies also to cumulative level 1 incremental backups, which will use all the bitmaps recorded since the last bitmap switch from a level 0 incremental backup. Due to the limit of 8 bitmaps, a cumulative incremental level 1 backup will have to perform a conventional scan of the database, if you make a level 0 database backup followed by 7 differential incremental backups.