For a very large
database, the time it takes to scan the blocks in the database can easily exceed the time it takes
to perform the actual backup.
FIGURE 12-12 OEM backup summary
FIGURE 12-13 OEM backup script
Chapter 12: Using Recovery Manager (RMAN) 451
By enabling block change tracking, RMAN knows which blocks have changed by using a
change tracking file. Although there is some slight overhead in space usage and maintenance of
the tracking file every time a block is changed, the tradeoff is well worth it if frequent incremental
backups are performed on the database. In the following example, we create a block change
tracking file in the DATA disk group and enable block change tracking:
SQL> alter database enable block change tracking
2 using file '+DATA';
Database altered.
SQL> exit
[oracle@dw ~]$ asmcmd
ASMCMD> cd data/dw
ASMCMD> ls
CHANGETRACKING/
CONTROLFILE/
DATAFILE/
ONLINELOG/
PARAMETERFILE/
TEMPFILE/
spfiledw.ora
ASMCMD> cd changetracking
ASMCMD> ls ??“s
Block_Size Blocks Bytes Space Name
512 22657 11600384 25165824 ctf.270.632356105
ASMCMD>
The next time a backup is performed, RMAN will only have to use the contents of the file
ctf.270.632356105 (an OMF-named file in the DW/CHANGETRACKING directory of the DATA
disk group) to determine which blocks need to be backed up. The space needed for the block
change tracking file is approximately 1/250,000 the size of the database.
The dynamic performance view V$BLOCK_CHANGE_TRACKING contains the name and size
of the block change tracking file as well as whether change tracking is enabled:
SQL> select filename, status, bytes from v$block_change_tracking;
FILENAME STATUS BYTES
--------------------------------------------- ---------- ----------
+DATA/dw/changetracking/ctf.
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