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Use case for retention policy based on Redundancy

greeny
greeny Member Posts: 179 Blue Ribbon
edited Jan 15, 2020 4:36AM in Recovery Manager (RMAN)

Versions applicable : 11.2.0.4 and higher

RMAN retention policies allow you to specify how long you want to retain backups in the backup media.

RMAN> CONFIGURE RETENTION POLICY TO REDUNDANCY 2;

The above configuration means RMAN will retain maximum of two backup copies (Versions) irrespective of time period.

When a third copy is created , the 'first' oldest copy is made obsolete and made eligible for deletion.

Following is a statement from a book titled 'Oracle RMAN for Absolute beginners' (2014) by Darl Kuhn

" I find that a retention policy based on redundancy is easier to work with and more predictable with regard to how long backups are retained. If I set redundancy to 2, I know that RMAN won’t mark as obsolete the latest two backups. In contrast, the recovery window retention policy depends on the frequency of the backups and the window length to determine whether a backup is obsolete. "

Darl seems to prefer Redundancy over Recovery Window retention policy . But, I would like to know when exactly is it suitable to use 'Redundancy' retention policy ?

Is it when you have less space available in the backup media ? Or something to do with incremental backup types (Differential/Cumulative) which is used in conjunction ?

Answers

  • Dude!
    Dude! Member Posts: 22,829 Black Diamond
    edited Jan 15, 2020 4:36AM

    You will have to ask the author of the book. I can only give you my view, and to my knowledge the assessment is incorrect. There is no such thing like "marking" anything as obsolete. RMAN will use the retention policy to determine which backup or recovery data is currently outside the retention policy. Hence the report obsolete and delete obsolete command always re-evaluate the current situation. Also, RMAN never deletes anything automatically without your consent.

    The retention policy does not guarantee that data is deleted. You cannot use it to make sure you have enough capacity available for backup.

    Keep in mind that the default control_file_record_keep_time parameter defines a minimum of 7 days by default. This parameter applies to records in the control file that are circularly reusable, such as archive log records and various backup records. RMAN cannot evaluate data that is no longer recorded.

    Whether you use edundancy or recovery window retention depends on your restore and recovery requirements. Redundancy specifies that you have x number of backups available. it does not define how old these backups are. Again, keep the control_file_record_keep_time parameter in mind. A recovery window won't consider data as obsolete based on time evaluation, regardless of how many copies you may have. What you use depends on your restore and recovery requirements.

    greeny