块修改跟踪 （Block Change Tracking）

Using Block Change Tracking to Improve Incremental Backup Performance

The block change tracking feature for incremental backups improves backup performance by recording changed blocks for each data file.

About Block Change Tracking

If block change tracking is enabled on a primary or standby database, then RMAN uses a block change tracking file to identify changed blocks for incremental backups. By reading this small bitmap file to determine which blocks changed, RMAN avoids having to scan every block in the data file that it is backing up.

Block change tracking is disabled by default. Nevertheless, the benefits of avoiding full data file scans during backup are considerable, especially if only a small percentage of data blocks are changed between backups. If your backup strategy involves incremental backups, then block change tracking is recommended. Block change tracking does not change the commands used to perform incremental backups. The change tracking file requires no maintenance after initial configuration.

You can only enable block change tracking at a physical standby database if a license for the Oracle Active Data Guard option is enabled.

该特性一般是Rman备份时直接读取修改过的块而不是扫描所有数据文件块来进行增量备份；这些修改块记忆在一个位图文件中；

Space Management in the Block Change Tracking File

The change tracking file maintains bitmaps that mark changes in the datafiles between backups. The database performs a bitmap switch before each backup. Oracle Database automatically manages space in the change tracking file to retain block change data that covers the eight most recent backups. After the maximum of eight bitmaps is reached, the oldest bitmap is overwritten by the bitmap that tracks the current changes.

The first level 0 incremental backup scans the entire data file. Subsequent incremental backups use the block change tracking file to scan only the blocks that have been marked as changed since the last backup. An incremental backup can be optimized only when it is based on a parent backup that was made after the start of the oldest bitmap in the block change tracking file.

Consider the eight-bitmap limit when developing your incremental backup strategy. For example, if you make a level 0 database backup followed by seven differential incremental backups, then the block change tracking file now includes eight bitmaps. If you then make a cumulative level 1 incremental backup, then RMAN cannot optimize the backup, because the bitmap corresponding to the parent level 0 backup is overwritten with the bitmap that tracks the current changes.

Location of the Block Change Tracking File

One block change tracking file is created for the whole database. By default, the change tracking file is created as an Oracle managed file in the destination specified by the 

DB_CREATE_FILE_DEST

 initialization parameter. You can also place the change tracking file in any location that you choose, by specifying its name when enabling block change tracking. Oracle recommends against using a raw device (that is, a disk without a file system) as a change tracking file.

Note:

In an Oracle RAC environment, the change tracking file must be located on shared storage accessible from all nodes in the cluster.

RMAN does not support backup and recovery of the change tracking file. The database resets the change tracking file when it determines that the change tracking file is invalid. If you restore and recover the whole database or a subset, then the database resets the block change tracking file and starts tracking changes again. After you make a level 0 incremental backup, the next incremental backup can use change tracking data.

默认情况下“修改跟踪文件”在初始化参数db_create_file_dest指定的位置作为OMF被创建； 当然你也可以手工指定位置；在集群环境中该文件的位置要使所有节点都能够访问到；

Size of the Block Change Tracking File

The size of the block change tracking file is proportional to the size of the database and the number of enabled threads of redo. The size of the block change tracking file can increase and decrease as the database changes. The size is not related to the frequency of updates to the database.

Typically, the space required for block change tracking for a single instance is approximately 1/30,000 the size of the data blocks to be tracked. For an Oracle RAC environment, it is 1/30,000 of the size of the database, times the number of enabled threads.

The following factors that may cause the file to be larger than this estimate suggests:

• To avoid the overhead of allocating space as your database grows, the block change tracking file size starts at 10 megabytes. New space is allocated in 10 MB increments. Thus, for any database up to approximately 300 gigabytes, the file size is no smaller than 10 MB, for up to approximately 600 gigabytes the file size is no smaller than 20 megabytes, and so on.
• For each data file, a minimum of 320 kilobytes of space is allocated in the block change tracking file, regardless of the size of the data file. Thus, if you have a large number of relatively small data files, the change tracking file is larger than for databases with a smaller number of larger data files containing the same data.

Enabling and Disabling Block Change Tracking

You can enable block change tracking when the database is either open or mounted. This section assumes that you intend to create the block change tracking file as an Oracle Managed File in the database area, which is where the database maintains active database files such as data files, control files, and online redo log files. See "Overview of the Fast Recovery Area" to learn about the database area and fast recovery area.

To enable block change tracking:

1. Start SQL*Plus and connect to a target database with administrator privileges.
2. Ensure that the 

   DB_CREATE_FILE_DEST

    initialization parameter is set.

   SHOW PARAMETER DB_CREATE_FILE_DEST
         

   If the parameter is not set, and if the database is open, then you can set the parameter with the following form of the 

   ALTER SYSTEM

    statement:

   ALTER SYSTEM SET 
     DB_CREATE_FILE_DEST = '/disk1/bct/'
     SCOPE=BOTH SID='*';
         

3. Enable block change tracking.

   Execute the following 

   ALTER DATABASE

    statement:

   ALTER DATABASE ENABLE BLOCK CHANGE TRACKING;
         

   You can also create the change tracking file in a location that you choose yourself by using the following form of SQL statement:

   ALTER DATABASE ENABLE BLOCK CHANGE TRACKING 
     USING FILE '/mydir/rman_change_track.f' REUSE;
         

   The 

   REUSE

    option tells Oracle Database to overwrite any existing block change tracking file with the specified name.

Disabling Block Change Tracking

This section assumes that the block change tracking feature is currently enabled. When you disable block change tracking, the database removes the block change tracking file from the operating system.

To disable block change tracking:

1. Start SQL*Plus and connect to a target database with administrator privileges.
2. Ensure that the target database is mounted or open.

3. Disable block change tracking.

   Execute the following 

   ALTER DATABASE

    statement:

   ALTER DATABASE DISABLE BLOCK CHANGE TRACKING;      

使用 alter database 语句来启用和禁用该功能；

Checking Whether Change Tracking is Enabled

You can query the 

V$BLOCK_CHANGE_TRACKING

 view to determine whether change tracking is enabled, and if it is, the file name of the block change tracking file.

To determine whether change tracking is enabled:

Enter the following query in SQL*Plus (sample output included):

COL STATUS   FORMAT A8
COL FILENAME FORMAT A60
 
SELECT STATUS, FILENAME
FROM   V$BLOCK_CHANGE_TRACKING;

STATUS   FILENAME
-------- ------------------------------------------------------------
ENABLED  /disk1/bct/RDBMS/changetracking/o1_mf_2f71np5j_.chg
      

Changing the Location of the Block Change Tracking File

To move the change tracking file, use the 

ALTER DATABASE RENAME FILE

 statement. The database must be mounted. The statement updates the control file to refer to the new location and preserves the contents of the change tracking file. If you cannot shut down the database, then you can disable and enable block change tracking. In this case, you lose the contents of the existing block change tracking file.

To change the location of the change tracking file:

1. Start SQL*Plus and connect to a target database.
2. If necessary, determine the current name of the change tracking file:

   SQL> SELECT FILENAME FROM V$BLOCK_CHANGE_TRACKING;
         

3. If possible, shut down the database. For example:

   SQL> SHUTDOWN IMMEDIATE
         

   If you shut down the database, then skip to the next step. If you choose not to shut down the database, then execute the following SQL statements and skip all remaining steps:

   SQL> ALTER DATABASE DISABLE BLOCK CHANGE TRACKING;
   SQL> ALTER DATABASE ENABLE BLOCK CHANGE TRACKING USING FILE 'new_location';
         

   In this case you lose the contents of the block change tracking file. Until the next time you complete a level 0 incremental backup, RMAN must scan the entire file.
4. Using host operating system commands, move the change tracking file to its new location.
5. Mount the database and move the change tracking file to a location that has more space. For example:

   ALTER DATABASE RENAME FILE
      '/disk1/bct/RDBMS/changetracking/o1_mf_2f71np5j_.chg' TO 
      '/disk2/bct/RDBMS/changetracking/o1_mf_2f71np5j_.chg';
         

   This statement changes the location of the change tracking file while preserving its contents.
6. Open the database:

   SQL> ALTER DATABASE OPEN;
         

See Also:

Oracle Database SQL Language Reference to learn about the 

ALTER DATABASE

 statement and the 

ALTER SYSTEM

statement OCP 考题：

Q184. How is block change tracking  enabled?

A. With alter database enable block change tracking

B. With alter system enable block change tracking

C. With an init.ora parameter change

D. With an spfile parameter change

Answer: A