关于block change tracking

 

1.block change tracking介绍

Block chage tracking是Oracle10g的一个新特性，Block change tracking进程记录自从上一次备份以来数据块的变化，并把这些信息记录在跟踪文件中。RMAN使用这个文件判断增量备份中需要备份的变更数据。这极大的促进了备份性能，RMAN可以不再扫描整个文件以查找变更数据。

为此Oracle引入了一个新的后台进程，CTWR，其全称为Change Tracking Writer，用于记录变化的块并将变化写入相应的日志文件中。 

 

2.配置block change tracking

 

开启bct：

可以在数据文件，asm，裸设备上创建日志跟踪文件

 

alter database enable block change tracking using file '/oradata/block_track.log';

 

alter database enable block change tracking using file ‘+data’;

 

alter database enable block change tracking using file ‘/dev/rlvbct’;

 

通过查询v$block_change_tracking视图查询bct的使用情况。

 

禁用bct：

 

alter database disable block change tracking;

 

  
  
  
  

   
   
   
    修改块修改跟踪文件路径
  
  
  
  
　　修改块修改跟踪文件可以通过alter database rename file命令，但是只能在mount状态下操作。
  
  
  
  

   
   
   
    
  
  
  
  
  
  
  
  

   
   
   
   　　查看当前块修改跟踪文件路径：
  
  
  
  
  
  
  
  

   
   
   
   
 
    

      SQL> select filename from v$block_change_tracking; 
    
 
    

      FILENAME 
    
 
    

      -------------------------------------------------------------------------------------- 
    
E:/ORACLE/PRODUCT/10.2.0/DB_1/DATABASE/RMAN_INC.TRK 
   
  
  
  
  
  
  
  
  

   
   
   
   　　Shutdown 数据库并启动到mount状态：
  
  
  
  
  
  
  
  

   
   
   
   
 
    

      SQL> shutdown immediate 
    
 
    

      Database closed. 
    
 
    

      Database dismounted. 
    
 
    

      ORACLE instance shut down. 
    
 
    

      SQL> startup mount 
    
 
    

      ORACLE instance started. 
    
 
    

      Total System Global Area  167772160 bytes 
    
 
    

      Fixed Size                  1295608 bytes 
    
 
    

      Variable Size              88083208 bytes 
    
 
    

      Database Buffers           71303168 bytes 
    
 
    

      Redo Buffers                7090176 bytes 
    
Database mounted. 
   
  
  
  
  
  
  
  
  

   
   
   
   　　手工复制块修改文件到新路径下，然后执行alter database命令：
  
  
  
  
  
  
  
  

   
   
   
   
 
    

      SQL> alter database rename file 'E:/ORACLE/PRODUCT/10.2.0/DB_1/DATABASE/RMAN_INC.TRK' 
    
 
    

        2  to 'E:/oracle/oradata/jssweb/rman_inc.trk'; 
    
 
    

      Database altered. 
    
 
    

      SQL> alter database open; 
    
Database altered. 
   
  
  
  
  
  
  
  
  

   
   
   
   　　查看当前块修改跟踪文件路径：
  
  
  
  
  
  
  
  

   
   
   
   
 
    

      SQL> select filename from v$block_change_tracking; 
    
 
    

      FILENAME 
    
 
    

      ----------------------------------------------------- 
    
E:/ORACLE/ORADATA/JSSWEB/RMAN_INC.TRK 
   
  
  
  
  
  
  
  
  

   
   
   
   　　如果你的数 据库无法shutdown ， 其实也无所谓，先disable，然后执行enable时指定一个新路径也可以的:)，只不过，原来文件中记录的块修改的跟踪信息也就没有了。
  
  
  
  

3.How To Size the Block Change Tracking File [ID 306112.1]

 

In this Document
  Goal
  Solution
  
  
  
  

   
   
   
    
  
  
  
  
    
  
  
  
  

   
   
   
   Platforms: 1-914CU;
  
  
  
  
  
  
  
  

   
   
   
   
 
    
 
     
This document is being delivered to you via Oracle Support's Rapid Visibility (RaV) process and therefore has not been subject to an independent technical review.
 
    
 
   
  
  
  
  
 
  
  
  
  

   
   
   
    
  
  
  
  
  
  
  
  

   
   
   
   Applies to: 
  
  
  
  
Oracle Server - Enterprise Edition - Version: 10.1.0.2 to 10.2.0.1 - Release: 10.1 to 10.2
Oracle Server - Enterprise Edition - Version: 10.1.0.2 to 11.1.0.7   [Release: 10.1 to 11.1]
Information in this document applies to any platform.
 
  
  
  
  

   
   
   
   Goal
  
  
  
  
 
  
  
  
  

   
   
   
   This is a clarification of the documentation for sizing the Block Change Tracking File.
  
  
  
  
 
  
  
  
  

   
   
   
   Block Change Tracking is used to optimize incremental backups. A new file is introduced:
  
  
  
  
 
  
  
  
  

   
   
   
   - Changed blocks are tracked by the CTWR background process as redo is generated
   
   
   
   
- Database backups automatically use the change tracking file
  
  
  
  
Sizing of this file is important, and even though you can use V$BLOCK_CHANGE_TRACKING view
to monitor the usage, it may in some cases not be easy to change the size, ie when using raw devices.
 
  
  
  
  

   
   
   
   Solution
  
  
  
  
 
  
  
  
  

   
   
   
   The formula is:
  
  
  
  
 
  
  
  
  

   
   
   
    
  
  
  
  
 
  
  
  
  

   
   
   
   <size of change tracking file> = <# of redo threads> * (# of old backups + 2) * (size of db/250000)
  
  
  
  
 
  
  
  
  

   
   
   
    
  
  
  
  
 
  
  
  
  

   
   
   
   Let’s take an example of a 500 GB database, with only one thread, and having eight backups kept
   
   
   
   
in RMAN. The repository for this will require a block change tracking file of 20 MB.
   
   
   
   

   
   
   
   
Threads * (number of old backups+2) * (database size in bytes/250000) = 20 MB
   
   
   
   

   
   
   
   
The amount of data that each bit in the change tracking file covers is 32KB. That's 262144 bits.
   
   
   
   
That is where the 1/250000 figure comes from. This is the ratio of the size of *one bitmap* in the 
   
   
   
   
change tracking file, to the size of the database data that the bitmap covers. And, because w 
   
   
   
   
keep up to 8 levels of bitmap history in the file, plus the current bitmap, that reduces the ratio 
   
   
   
   
to about 1/30000, per enabled thread.
  
  
  
  
 
  
  
  
  

   
   
   
   Dividing 1TB by 30000, you get 30 MB, per TB, per enabled thread.
  
  
  
  
 
  
  
  
  

   
   
   
   So the parameters we use to determine the block change tracking file are:
   
   
   
   
- Database size
   
   
   
   
- Number of enabled threads
   
   
   
   
- Number of old backups. (The block change tracking file keeps a record of all changes between previous backups, up to eight in total)
  
  
  
  
  
  
  
  

   
   
   
    
  
  
  
  
  
  
  
  

   
   
   
   4.一些备忘知识点
  
  
  
  
  
  
  
  

   
   
   
    通过位图跟踪两次备份间变化的数据块；
  
  
  
  
  
  
  
  

   
   
   
    每次备份前进行位图切换；
  
  
  
  
  
  
  
  

   
   
   
    开发增量备份策略时，要考虑到8个位图的限制；
  
  
  
  
  
  
  
  

   
   
   
    在RAC环境中，change tracking file需要放在共享存储上；
  
  
  
  
  
  
  
  

   
   
   
    Change tracking file的大小和数据库的大小和enabled的redo thread的个数成正比；
  
  
  
  
  
  
  
  

   
   
   
    Change tracking file的大小和数据更新的频率无关；
  
  
  
  

  
  
  
  

   
   
   
    
  
  
  
  
  
  
  
  

   
   
   
    
  
  
  
  

 

为了优化增量备份，oracle引入了change tracking机制，下面是和change tracking有关的重要知识点，备忘。

1． 通过位图跟踪两次备份间变化的数据块；

2． 每次备份前进行位图切换；

3． 开发增量备份策略时，要考虑到8个位图的限制；

4． 在RAC环境中，change tracking file需要放在共享存储上；

5． Change tracking file的大小和数据库的大小和enabled的redo thread的个数成正比；

6． Change tracking file的大小和数据更新的频率无关；

7． 在mount或open状态下enable change tracking；

8．Enable change tracking

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

9．Disable change tracking

ALTER DATABASE DISABLE BLOCK CHANGE TRACKING;

10．     检查change tracking状态

SELECT STATUS, FILENAME FROM V$BLOCK_CHANGE_TRACKING;

11．     改变change tracking file的位置

1) 不关闭数据库的方式

SQL> ALTER DATABASE DISABLE BLOCK CHANGE TRACKING;

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

注意：这种方式会丢失change tracking file的内容

2) 关闭数据库的方式

SQL> SELECT FILENAME FROM V$BLOCK_CHANGE_TRACKING;(确定当前的文件名)

SQL> SHUTDOWN IMMEDIATE

用操作系统命令将文件move到新路径

SQL> ALTER DATABASE RENAME FILE

'/disk1/changetracking/o1_mf_2f71np5j_.chg' TO

'/disk2/changetracking/o1_mf_2f71np5j_.chg';

SQL> ALTER DATABASE OPEN;