Re: Block Change Capture Questions

Ohhh, nice query Tim.

Jared Still
Certifiable Oracle DBA and Part Time Perl Evangelist Principal Consultant at Pythian
Oracle ACE Alumni
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On Tue, Feb 22, 2022 at 7:03 AM Tim Gorman <tim.evdbt_at_gmail.com> wrote:

> Scott,
>
> Block Change Tracking (BCT) has been a feature since 10.1, so it has been
> more than 15 years (at least) since it was introduced. It has been turned
> on for some extremely "busy" production databases, impact on transaction
> performance is negligible if anything, and the impact on incremental backup
> duration is generally dramatic. The BCT file itself should be treated like
> any other Oracle datafile, is self-purging, and is backed up by RMAN.
>
> BCT is not a perfect mechanism, and usage patterns where there are more
> than 7-8 incrementals between each full backup can overwhelm the purported
> 8-bit tracking byte, causing incremental backups to revert to pre-BCT
> behavior and duration. Periodically querying V$BACKUP_DATAFILE to check
> whether DATAFILE_BLOCKS *0.75 > BLOCKS_READ where USED_CHANGE_TRACKING =
> 'YES' can let you know if this is happening by checking if incremental
> backups are still generating a lot of I/O (i.e. 75% the size of the
> datafile at time of backup) despite BCT being enabled.
>
> Also, enabling BCT on a physical standby database is a trigger for
> licensing the Active DataGuard option, which can be an unpleasant surprise
> if not intended.
>
> Hope this helps,
>
> -Tim
>
>
>
> On 2/22/2022 6:39 AM, Scott Canaan wrote:
>
> We use CommVault for database backups. CommVault interfaces with RMAN.
> We do hot backups on prod databases, with one full backup each night and
> several incrementals throughout the day. Our CommVault administrator sent
> a message from CommVault that says:
>
>
>
> Severity: Minor
>
> Job ID: 4778016
>
> Event Date: Mon Feb 21 03:01:11 2022
>
> Program: ClOraAgent
>
> Client: rpiddevl2
>
> Description: Block Change Tracking is found DISABLED on Oracle
> DB [RPIDDEVL]. Incremental backups may run slow.
>
>
>
> He wants us to look into this. We have and are concerned that the
> overhead will be more than the benefits.
>
>
>
> Has anyone turned this on? If so, what was the overhead? Did it help
> with backups?
>
>
>
> Thank you,
>
>
>
> *Scott Canaan ‘88*
>
> *Sr Database Administrator *Information & Technology Services
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