Oracle DBA

In modern DBA practices, backup performance and efficiency are critical underpinnings of reliability and recovery SLAs. Oracle’s Block Change Tracking (BCT) feature exists to accelerate RMAN incremental backups by avoiding full datafile scans. In Oracle 19c, BCT continues to deliver value—and knowing how to enable, monitor, and disable it becomes essential for any serious DBA.

Below, we explore the concept, usage, and performance implications of BCT in Oracle 19c.

What Is Block Change Tracking (BCT)?

• Block Change Tracking was introduced in Oracle 10g as a feature to speed up RMAN incremental backups.
• When enabled, Oracle tracks which blocks have changed since the last backup, storing that information in a special file managed by the CTWR (Change Tracking Writer) process.
• During an incremental backup, RMAN uses the BCT file to determine which blocks should be backed up, rather than scanning entire datafiles. This selective block read can significantly reduce I/O and backup time.
• In effect, BCT transforms full scans into targeted reads of changed blocks, delivering a leaner, faster incremental backup workflow.

Enabling Block Change Tracking

You can turn on BCT in an Oracle 19c database with a simple ALTER command:

ALTER DATABASE ENABLE BLOCK CHANGE TRACKING 
  USING FILE '/u01/CLONE/datafile/blockfile.log';

After enabling, you can verify its status:

SELECT filename, status
FROM   V$block_change_tracking;

You can also check for the CTWR process in memory:

SELECT * 
FROM   V$SGASTAT 
WHERE  name LIKE '%CTWR%';

You may find an entry such as:

POOl   NAME                       BYTES     CON_ID
large pool   CTWR dba buffer       1961984   0

To confirm the CTWR session:

SELECT sid, program, status
FROM   v$session
WHERE  program LIKE '%CTWR%';

You might see something like:

SID  PROGRAM         STATUS
68   oracle (CTWR)    ACTIVE

Disabling Block Change Tracking

If you no longer wish to use BCT or want to test backup without it, you can disable it:

ALTER DATABASE DISABLE BLOCK CHANGE TRACKING;

After doing this, the CTWR session should disappear:

SELECT sid, program, status
FROM   v$session
WHERE  program LIKE '%CTWR%';

You would see no rows selected, indicating the CTWR process has been removed.

Why Use BCT — and When to Avoid It

Advantages:

1. Reduced I/O overhead
   Since RMAN only reads changed blocks, it avoids full datafile scans, leading to lower I/O and faster backup windows.
2. Faster incremental backups
   With fewer blocks to read, incremental workloads complete more quickly.

Caveats & Considerations:

• The BCT file itself consumes disk space and requires management.
• If there is corruption or issues with the BCT file, RMAN may fall back to scanning or error out.
• In environments with minimal changes (low block churn), the performance gain might be marginal.
• Always monitor the overhead introduced by the CTWR process and ensure it does not become a bottleneck.

Suggested Best Practices for DBAs

• Use BCT in your production databases where incremental backups are frequent and change rates are moderate to high.
• Monitor BCT file health and backup it regularly as part of your backup catalog.
• Keep an eye on the CTWR process resource usage (CPU, memory) to ensure it remains lightweight.
• During maintenance windows or testing, you may disable BCT to compare and validate backup performance.
• Document and automate your BCT enable/disable procedures as part of your DBA toolkit.