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How to estimate RMAN incremental backup size using block change tracking file (Doc ID 1938079.1)

 

Oracle Database - Enterprise Edition - Version 10.1.0.2 and later

Zero Data Loss Recovery Appliance Software - Version 12.1.0.1.0 and later Oracle Database Cloud Schema Service - Version N/A and later Oracle Database Exadata Express Cloud Service - Version N/A and later Oracle Database Exadata Cloud Machine - Version N/A and later Information in this document applies to any platform. ***Checked for relevance on 7-Dec-2015***

GOAL

Customer wants to estimate their incremental backup size for disk / tape storage sizing purposes. The block change tracking (BCT) file tracks which database blocks were changed since the last RMAN backup, so that RMAN incremental backups only need to read and write the changed blocks. The change tracking file data can also be used to estimate future incremental backup size.

SOLUTION

Attached are two queries:

select file#,       blocks_changed,       block_size,       blocks_changed * block_size bytes_changed,       round(blocks_changed / blocks * 100, 2) percent_changedfrom v$datafile join     (select fno             file#,             sum(bct) blocks_changed      from (select distinct fno, bno, bct from x$krcbit            where vertime >= (select curr_vertime from x$krcfde                              where csno=x$krcbit.csno and fno=x$krcbit.fno))      group by fno order by 1)using(file#);

alter session set nls_date_format = 'yyyy-mm-dd hh24:mi:ss';set pagesize 9999select vertime, csno, fno, bno, bct from x$krcbitwhere vertime >= (select curr_vertime from x$krcfde                  where csno=x$krcbit.csno and fno=x$krcbit.fno)order by fno, bno;

The first gives a per-file summary of the number of blocks changed and the second lists all of the individual blocks that changed. The queries show all changes made since the last "bitmap switch". A bitmap switch occurs every time RMAN takes a backup. You can also force a bitmap switch to occur by calling dbms_backup_restore.bctswitch. So if you want to keep statistics for changes per day, week, whatever, then at the end of every interval you would first run the queries and capture the output, then call bctswitch. If you just want to track changes since the last backup, then you don't need to use bctswitch.

Ordinarily each change tracking bit covers 32KB of data (4 blocks if using the default 8KB block size). If you want each bit to only cover one block, set _bct_chunk_size=1 before enabling change tracking. This will give more accurate incremental size results, but BCT file size will also increase. When using change tracking solely as an incremental estimation method, we recommend setting _bct_chunk_size=1.

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