Oracle RMAN: Fast Incremental Backups ~ Atikh's DBA blog

There are two ways to achieve fast incremental backups as mentioned in last section of this post

We will discuss these two methods in details

Block change tracking (BCT) file

While performing incremental backup RMAN search for modified blocks whose system change number (SCN) is higher than last incremental level backups incremental start SCN
In normal incremental backup strategy, in order to identify modified blocks it would require to read an entire datafile
If incremental backups needs to be fast enough we need to skip the scan of entire datafile to find modified blocks, oracle 10g introduced a more refined way of using block change tracking (BCT) file.
This file keeps entry of those blocks that are modified since last full backup
At the time of next incremental backup, RMAN would read details from this file only and avoid looking for whole datafile
BCT file uses bitmap structures to update and maintain the information from changed blocks
BCT file is neither default nor it gets generated at the time of database creation.
It can be viewed using database view v$block_change_tracking

SQL> select filename, status from v$block_change_tracking;

FILENAME STATUS

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

DISABLED

SQL>

We can enable block change tracking file using below command

SQL >alter database enable block change tracking using file '/u01/oracle/bct/config.f';

Database altered

SQL> select filename,status from v$block_change_tracking;

FILENAME STATUS

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

/u01/oracle/bct/config.f ENABLED

Default location for BCT file is $ORACLE_HOME/dbs
Default size would be 10M (1/30,000) of total database sizes at the time of database creation.
When BCT file is enabled, oracle database use the background process change tracking writer (CTWR) for recording the bitmaps of blocks being modified for datafile
The CTWR background process uses memory are CTWR dba buffer allocated from large pool

Current size is viewed using v$sgastat view

SQL> select pool,name, bytes from v$sgastat where name like 'CTWR%';

POOL NAME BYTES

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

large pool CTWR dba buffer 1525808

BCT file speeds up the already faster incremental backups even more, this file does not need an additional administration by DBA.

Multi Section Incremental backups

As we have discussed here, SECTION SIZE clause helps to improve the performance backups of huge size datafiles and databases, same can be used in Incremental backup strategy as well.
To make use of SECTION SIZE clause in level 1 incremental backups, the compatible parameter must be set to 12.0, for level 0 incremental backups compatible can be 11.0
Using below command we can complete multi section incremental backups

RMAN > backup incremental level 1 section 500m database;

Kindly comment below if any additional information is required

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Oracle RMAN: Fast Incremental Backups