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Block Change Tracking Inside Out (Doc ID 1528510.1)

Last updated on MARCH 26, 2024

Applies to:

Oracle Database Backup Service - Version N/A and later
Oracle Database - Enterprise Edition - Version 10.1.0.3 and later
Oracle Database Cloud Schema Service - Version N/A and later
Oracle Database Exadata Express Cloud Service - Version N/A and later
Gen 1 Exadata Cloud at Customer (Oracle Exadata Database Cloud Machine) - Version N/A and later
Information in this document applies to any platform.

Purpose

Oracle RMAN was able to take incremental backups already in 9i. However, prior to introduction of Oracle 10g block change tracking (BCT), RMAN had to scan the whole datafile to and filter out the blocks that were not changed since base incremental backup and overhead or incremental backup was as high as full backup. Oracle 10g new feature, block change tracking,
minimizes number of blocks RMAN needs to read to a strict minimum. With block change tracking enabled RMAN accesses on disk only blocks that were changed since the latest base incremental backup.

This feature is widely known in the world of Oracle database administrators. However, hardly anything is available on internal implementation of block change tracking. This makes it difficult to evaluate the impact of enabling BCT in Oracle databases and quantify performance overhead.

This paper and presentation try to uncover internals of block change tracking and show which areas of Oracle database are involved, how processes work together, what are hidden limitations and impact of enabling block change tracking.

DISCLAIMER

I cannot provide any guarantee that presented material is absolutely correct. There is no publicly available documentation on change tracking internals (at least, I’m not aware of any) so this paper is based purely on experiments and research plus few hints from more knowledgeable peers that I’m very grateful for. Please take this material carefully and make sure you validate my assumptions before making critical decisions – either with your own tests or request Oracle support to provide more information if your business requires it.

Scope

 X$ tables provide wealth of information about Oracle internals and block change tracking in particular. The tables starting with X$KRC provide easy access to block change tracking file. Those tables are not simply exposing memory regions. When selecting from X$KRC tables, block change tracking file is accessed behind the scenes.

Extended SQL tracing using event 10046 was used extensively on foreground sessions as well as background processes such as DBWx, LGWR, CKPT, and CTWR. It's worth to mention here that DBWR doesn't post “db file parallel write” wait event with Async IO enabled (Oracle 10.2.0.2 on Linux 32 bit). Here is where strace utility comes handy and let researcher see all system
calls. In addition to strace, lsof utility can show open file descriptors for a given process.

To match content of X$KRC tables to the content of block change tracking file, hex dump or hex editor such as bvi is handy. There are two events reserved for block change tracking tracing – 19774 and 19775. ORADEBUG utility is also very helpful.

Details

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