Creating Fast Incremental Backups : 

We have seen how block change tracking is enabled.

Enable block change tracking on the physical standby database

The motive of Incremental backup is to backup only those blocks which are changed since the last zero or level 1 incremental backup.

Block change tracking provides a boost to this. If we enable block change tracking then Oracle Database tracks the physical location of all database changes in the change tracking file. And while taking backup RMAN takes care of it. RMAN determines changed block from change tracking file so it does not need to scan the whole database for changed block since the last backup.

How is the Block Change Tracking maintained?

A background process called CTWR works for it. When redo data is being written to redo log, CTWR awakes and writes physical location of that changed block in Change Tracking file.

Note: Change Tracking file only holds the physical location of the changed block and does not contain any data.

The size of the Change Tracking File depends on following aspects :

  1. Database size in bytes.
  2. Number of threads in RAC environment
  3. Number of old backups maintained by the change tracking file

The minimum size of change tracking file is 10 MB and when it reaches to 10 MB, new 10 MB will be allocated.

Monitoring Block Change Tracking :

We can check use block change tracking file from v$backup_datafile view with a used_change_tracking column.

Let us understand this with one example

Step 1: Take level zero backup.

Step 2: Do some DML operation on the database.

Step 3: Take increment level 1 backup.

I have a small database, but we can level zero backup took 35 seconds and level 1 backup has just taken 3 seconds.

Step 4 : Check v$backup_datafile view with used_change_tracking=’YES’

We can see here PCT_FOR_BACKUP is 32 % for specific files.

Stay tuned for More articles on Oracle DataGuard

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