oracle block change tracking 简称oracle bct，该功能在数据库rman增量备份过程中，确实能减少增量备份的时间，提高备份的效率，在做增量备份之前，我们需要做level 0级别的备份，在做下一次level 1级别备份前，会将2次备份之间的数据差异通过位图的形式，以日志的方式记录在bct日志中，这样在做增量备份时，不需要对每个daafile文件进行比对，减少了比对时间，提高了备份效率。
下面我们看下数据库的增量备份演进；
1、现实及历史
Oracle bct是在10g时提出的，当数据库进行增量备份时，记录每个块的变化情况记录到bct文件，这样在下一次增量备份时，不需要进行数据库所有文件的扫描，直接对变更的数据库进行备份，缩短了备份执行的时间及系统软硬件资源的消耗。
2、bct备份原理及基础
bct是基于bitmap file方式记录每个数据文件块的变化，启用了bct服务后，不需要其他维护操作，将上次level0级备份以来所变化的块进行记录，记录到bct文件中，文件中包含了8次bitmaps信息，所以增量备份时，需要记得这个前提，当超过8次时，bitmap信息将会覆盖，那么bct将不会对增量备份进行优化，将重新扫描所有的数据文件。同时bct文件的大小和数据库的大小及开启的线程redo的数据有关系，tracking file的大小会根据数据库大小的变化而变化，和更新频率没有关系。单实例的情况下大约为1/30000数据库的大小，如果是rac环境，需要考虑thread数量。
初始值为10m，每个datafile，在tracking file里最少分配320k的空间，所以如果有大量的小datafile，change tracking file也会相对较大。
3、bct策略需要注意事项
①开启bct功能，如果直接开启该功能，其文件存放位置和参数db_create_file_dest相关，需设置该参数。
②直接开始bct并指定具体位置；alter database enable block change tracking using file '/home/oracle/track.f';
SQL> select * from v$block_change_tracking;
STATUS FILENAME BYTES
-------- ------------------------------------------------------------ ----------
ENABLED /home/oracle/track.f 11599872
4、bct在还原库测试
①备份数据库（Oracle 11.2.0.4）
level0级别备份命令：
run{
allocate channel c1 type disk;
allocate channel c2 type disk;
allocate channel c3 type disk;
allocate channel c4 type disk;
backup as compressed backupset incremental level 0 database tag='level0' format '/home/oracle/backup/full_db_level0_%U_%T.bak' ;
sql 'alter system archive log current';
backup as compressed backupset archivelog all tag='arc_bak' format='/home/oracle/backup/archivelog_%U_%T' skip inaccessible filesperset 6 not backed up 1 times delete input;
backup current controlfile format '/home/oracle/backup/ctl_level0_%U_%T.bak';
backup spfile format '/home/oracle/backup/spfile_level0_%U_%T.bak';
release channel c1;
release channel c2;
release channel c3;
release channel c4;
}
开启bct，插入数据scott.it数据
SQL> select * from it;
ID
----------
1
2
100
SQL> commit;
进行level1级别备份命令：
run{
allocate channel c1 type disk;
allocate channel c2 type disk;
allocate channel c3 type disk;
allocate channel c4 type disk;
backup as compressed backupset incremental level 1 database tag='level1' format '/home/oracle/backup/full_db_level1_%U_%T.bak' ;
sql 'alter system archive log current';
backup as compressed backupset archivelog all tag='arc_bak' format='/home/oracle/backup/archivelog_%U_%T' skip inaccessible filesperset 6 not backed up 1 times delete input;
backup current controlfile format '/home/oracle/backup/ctl_level1_%U_%T.bak';
backup spfile format '/home/oracle/backup/spfile_level1_%U_%T.bak';
release channel c1;
release channel c2;
release channel c3;
release channel c4;
}
[oracle@db1 backup]$ ls -lrth
total 313M
-rw-r----- 1 oracle oinstall 1.1M Aug 22 13:18 full_db_level0_0q15q71j_1_1_20220822.bak
-rw-r----- 1 oracle oinstall 22M Aug 22 13:19 full_db_level0_0p15q71j_1_1_20220822.bak
-rw-r----- 1 oracle oinstall 96K Aug 22 13:19 full_db_level0_0r15q729_1_1_20220822.bak
-rw-r----- 1 oracle oinstall 74M Aug 22 13:19 full_db_level0_0o15q71j_1_1_20220822.bak
-rw-r----- 1 oracle oinstall 195M Aug 22 13:19 full_db_level0_0n15q71j_1_1_20220822.bak
-rw-r----- 1 oracle oinstall 13K Aug 22 13:19 archivelog_0s15q72p_1_1_20220822
-rw-r----- 1 oracle oinstall 2.5K Aug 22 13:19 archivelog_0t15q72p_1_1_20220822
-rw-r----- 1 oracle oinstall 9.4M Aug 22 13:19 ctl_level0_0u15q72q_1_1_20220822.bak
-rw-r----- 1 oracle oinstall 96K Aug 22 13:19 spfile_level0_0v15q72s_1_1_20220822.bak
-rw-r----- 1 oracle oinstall 608K Aug 22 13:30 full_db_level1_1015q7ms_1_1_20220822.bak
-rw-r----- 1 oracle oinstall 128K Aug 22 13:30 full_db_level1_1215q7ms_1_1_20220822.bak
-rw-r----- 1 oracle oinstall 664K Aug 22 13:30 full_db_level1_1115q7ms_1_1_20220822.bak
-rw-r----- 1 oracle oinstall 96K Aug 22 13:30 full_db_level1_1415q7mt_1_1_20220822.bak
-rw-r----- 1 oracle oinstall 1.1M Aug 22 13:30 full_db_level1_1315q7ms_1_1_20220822.bak
-rw-r----- 1 oracle oinstall 525K Aug 22 13:30 archivelog_1515q7mv_1_1_20220822
-rw-r----- 1 oracle oinstall 2.5K Aug 22 13:30 archivelog_1615q7mv_1_1_20220822
-rw-r----- 1 oracle oinstall 9.4M Aug 22 13:30 ctl_level1_1715q7n0_1_1_20220822.bak
-rw-r----- 1 oracle oinstall 96K Aug 22 13:30 spfile_level1_1815q7n2_1_1_20220822.bak
[oracle@db1 backup]$ rman target /
Recovery Manager: Release 11.2.0.4.0 - Production on 星期一 8月 22 13:32:10 2022
Copyright (c) 1982, 2011, Oracle and/or its affiliates. All rights reserved.
connected to target database: ORCL (DBID=1640816139)
RMAN> list backup summary;
using target database control file instead of recovery catalog
List of Backups
===============
Key TY LV S Device Type Completion Time #Pieces #Copies Compressed Tag
------- -- -- - ----------- --------------- ------- ------- ---------- ---
23 B 0 A DISK 22-8月 -22 1 1 YES LEVEL0
24 B 0 A DISK 22-8月 -22 1 1 YES LEVEL0
25 B 0 A DISK 22-8月 -22 1 1 YES LEVEL0
26 B 0 A DISK 22-8月 -22 1 1 YES LEVEL0
27 B 0 A DISK 22-8月 -22 1 1 YES LEVEL0
28 B A A DISK 22-8月 -22 1 1 YES ARC_BAK
29 B A A DISK 22-8月 -22 1 1 YES ARC_BAK
30 B F A DISK 22-8月 -22 1 1 NO TAG20220822T131922
31 B F A DISK 22-8月 -22 1 1 NO TAG20220822T131924
32 B 1 A DISK 22-8月 -22 1 1 YES LEVEL1
33 B 1 A DISK 22-8月 -22 1 1 YES LEVEL1
34 B 1 A DISK 22-8月 -22 1 1 YES LEVEL1
35 B 1 A DISK 22-8月 -22 1 1 YES LEVEL1
36 B 1 A DISK 22-8月 -22 1 1 YES LEVEL1
37 B A A DISK 22-8月 -22 1 1 YES ARC_BAK
38 B A A DISK 22-8月 -22 1 1 YES ARC_BAK
39 B F A DISK 22-8月 -22 1 1 NO TAG20220822T133008
40 B F A DISK 22-8月 -22 1 1 NO TAG20220822T133010
②恢复数据库
直接使用oracle数据库dummy方式启动到nomount状态，通过已有的备份恢复spfile文件
[oracle@db1 dbs]$ rman target /
Recovery Manager: Release 11.2.0.4.0 - Production on 星期一 8月 22 13:46:26 2022
Copyright (c) 1982, 2011, Oracle and/or its affiliates. All rights reserved.
connected to target database (not started)
RMAN> startup nomount;
startup failed: ORA-01078: failure in processing system parameters
LRM-00109: could not open parameter file '/u01/app/oracle/product/11.2.0/dbhome_1/dbs/initorcl.ora'
starting Oracle instance without parameter file for retrieval of spfile
Oracle instance started
Total System Global Area 1068937216 bytes
Fixed Size 2260088 bytes
Variable Size 285213576 bytes
Database Buffers 775946240 bytes
Redo Buffers 5517312 bytes
RMAN> restore spfile from '/home/oracle/backup/spfile_level1_1815q7n2_1_1_20220822.bak';
Starting restore at 22-8月 -22
using target database control file instead of recovery catalog
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=429 device type=DISK
channel ORA_DISK_1: restoring spfile from AUTOBACKUP /home/oracle/backup/spfile_level1_1815q7n2_1_1_20220822.bak
channel ORA_DISK_1: SPFILE restore from AUTOBACKUP complete
Finished restore at 22-8月 -22
SQL> startup nomount;
ORACLE instance started.
Total System Global Area 1185853440 bytes
Fixed Size 2252664 bytes
Variable Size 956301448 bytes
Database Buffers 218103808 bytes
Redo Buffers 9195520 bytes
SQL> quit
Disconnected from Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
[oracle@db1 ~]$ rman target /
Recovery Manager: Release 11.2.0.4.0 - Production on 星期一 8月 22 13:50:35 2022
Copyright (c) 1982, 2011, Oracle and/or its affiliates. All rights reserved.
connected to target database: ORCL (not mounted)
RMAN> restore controlfile from '/home/oracle/backup/ctl_level1_1715q7n0_1_1_20220822.bak';
Starting restore at 22-8月 -22
using target database control file instead of recovery catalog
allocated channel: ORA_DISK_1
channel ORA_DISK_1: SID=572 device type=DISK
channel ORA_DISK_1: restoring control file
channel ORA_DISK_1: restore complete, elapsed time: 00:00:01
output file name=/oradata/orcl/control01.ctl
output file name=/oradata/orcl/control02.ctl
Finished restore at 22-8月 -22
RMAN> alter database mount;
恢复数据库：restore & recover database；
run{
restore database;
recover database;
alter database open resetlogs;
}
检查数据情况：
[oracle@db1 ~]$ sqlplus / as sysdba
SQL*Plus: Release 11.2.0.4.0 Production on 星期一 8月 22 13:55:59 2022
Copyright (c) 1982, 2013, Oracle. All rights reserved.
Connected to:
Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
SQL> archive log list;
Database log mode Archive Mode
Automatic archival Enabled
Archive destination USE_DB_RECOVERY_FILE_DEST
Oldest online log sequence 1
Next log sequence to archive 1
Current log sequence 1
SQL> conn scott/oracle
Connected.
SQL> select * from it;
ID
----------
1
2
100
数据恢复完成，数据无丢失，此时bct状态为启用状态，数据库已经恢复过，其需要初始化才能发挥其备份优化的功能。 SQL> select * from v$block_change_tracking;
STATUS FILENAME BYTES
-------- ---------------------------------------- ----------
ENABLED /home/oracle/track.log 11599872
5、bct对数据库性能的影响
①OLTP类型业务，启用bct对数据库的性能的影响几乎为0.
②OLAP类型业务，对于数仓类型的业务，大量的数据录入，造成bct进程来不及进行写入到track.f日志中，造成拥塞，这时会反应到dbtime占用上，如果发现这种情况，可以反应出bct的开启对数据库性能的影响。
总结：bct功能使用在增量备份过程中，提高增量备份数据库的效率。一般推荐数据库的块变化量小于20%时应该激活BCT。同时bct在恢复库的过程中，如果track.f文件丢失，开启数据库会报错，只要在mount状态下将bct功能关闭，在开启open状态即可。
