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Tuesday, July 29, 2008

Oracle Block Change Tracking

RMAN block change tracking

With Oracle 10gr1 Enterprise Edition Oracle introduce Block change tracking feature which use to FAST / Speed up RMAN Incremental Backup.

What is BLOCK CHANGE TRACKING?

RMAN's change tracking feature for incremental backups improves incremental backup performance by recording changed blocks in each datafile in a change tracking file. If change tracking is enabled, RMAN uses the change tracking file to identify changed blocks for incremental backup, thus avoiding the need to scan every block in the datafile.

After enabling change tracking, the first level 0 incremental backup still has to scan the entire datafile, as the change tracking file does not yet reflect the status of the blocks. Subsequent incremental backup that use this level 0 as parent will take advantage of the change tracking file.

Source: Oracle documentation

1. Enable Block Change tracking and set location

Before enabling we must to set/specify block change tracking location.There is two way to specify block change tracking location1. using DB_CREATE_FILE_DEST parameter which use OMF file name.

SQL> alter system set db_create_file_dest='location' SCOPE=BOTH;

SQL> alter database enable block change tracking;

2. Manually specify location for the block change tracking

SQL>alter database enable block change tracking using file 'location';

2. Disable block change tracking

SQL> alter database disable block change tracking;

NOTE: BCT files deleted automatically by oracle when BCT is disabled.

3. Reset the location of Block change tracking file

There are two options for this

1. shutdown the database and set the new location

SQL> shutdown database

SQL> move BCT file to new location

SQL> startup mount

SQL> alter database rename file 'old_location' TO 'new_location';

SQL> alter database open;

or


2. disable the block change tracking / set the new location / enable BCT

SQL> alter database DISABLE block change tracking;

SQL> alter database enable block change tracking using FILE 'NEW_LOCATION';

After that RMAN use new location for the BCT.

4. Checking the information about block change tracking enable or disable

Check v$BLOCK_CHANGE_TRACKING view

5. BCT file is important for restore and recovery of database.

Answer: No, it is doesn't require for Database Recovery of database

6. What happen if BCT file is lost and corrupted.

Answer: That is very interesting case here, suppose oracle found if BCT file is corrupted or missing then oracle will automatically recreate new BCT file.

Let see some practial view...

1. Enable BCT

SQL> alter database enable block change tracking using file 'e:\bct.dbf';
Database altered.

2. Check the status and default size.

SQL> select * from v$block_change_tracking;
STATUS FILENAME BYTES

---------- ---------------------------------------- ----------

ENABLED E:\BCT.DBF 11599872

NOTE: Filesize created is 11 mb by default.

3. Now check alertsid log file what entries we found.

Mon Jul 28 20:06:53 2008

alter database enable block change tracking using file 'e:\bct.dbf'

Block change tracking file is current.

Starting background process CTWR

Mon Jul 28 20:06:54 2008

CTWR started with pid=20, OS id=464

Block change tracking service is active.

Completed: alter database enable block change tracking using file 'e:\bct.dbf'


4. Disable BCT

SQL> alter database disable block change tracking;
Database altered.

SQL> select * from v$block_change_tracking;
STATUS FILENAME BYTES

---------- ---------------------------------------- ----------

DISABLED

5. Check alertsid log file

Mon Jul 28 20:12:41 2008

alter database disable block change tracking

Mon Jul 28 20:12:41 2008

Block change tracking service stopping.

Stopping background process CTWR

Deleted file E:\BCT.DBF

Completed: alter database disable block change tracking

6. Suppose BCT file lost or missing then what oracle will do...

SQL> shutdown immediate;

Database closed.

Database dismounted.

ORACLE instance shut down.

SQL> host del c:\bct.dbf

SQL> startup

ORACLE instance started.
[output cut]

Database mounted.

Database opened.

Now check the alertlog file for more information about how oracle create new BCT file when missing or lost.

Mon Jul 28 20:16:54 2008

ALTER DATABASE OPEN

CHANGE TRACKING is enabled for this database, but the

change tracking file can not be found. Recreating the file.

Change tracking file recreated.

Block change tracking file is current.

See Also: Incremental Backup Algorithm

1 comment:

2gb micro sd karte said...

When data blocks change, shadow processes track the changed blocks in a private area of memory at the same time they generate redo. When a commit is issued, the BCT information is copied to a shared area in Large Pool called 'CTWR dba buffer'. At the checkpoint, a new background process, Change Tracking Writer (CTWR), writes the information from the buffer to the change-tracking file. If contention for space in the CTWR dba buffer occurs, a wait event called, 'Block Change Tracking Buffer Space' is recorded. Several causes for this wait event are poor I/O performance on the disk where the change-tracking file resides, or the CTWR dba buffer is too small to record the number of concurrent block changes.