Why are backups of a previous incarnation marked obsolete without regard to redundancy?

After incomplete recovery and opening a 9i database with resetlogs we ran a full backup which completed successfully. The backup includes a command to delete obsolete backups after it finishes:

delete noprompt obsolete device type sbt;

RMAN is configured to use REDUNDANCY 2:

RMAN configuration parameters are:
CONFIGURE RETENTION POLICY TO REDUNDANCY 2;

However all the backups from the old incarnation were immediately marked obsolete and deleted without regard to redundancy.

• Would this behaviour have been different if we had a RECOVERY WINDOW configured instead of REDUNDANCY 2?

• Is this behaviour the same in later versions of Oracle?

edit: added output of LIST INCARNATION:

RMAN> list incarnation;

List of Database Incarnations
DB Key  Inc Key DB Name  DB ID            CUR Reset SCN      Reset Time
------- ------- -------- ---------------- --- -------------- ----------
1       1       LIVE     3494832994       NO  1              19-JAN-04
2       2       LIVE     3494832994       NO  11966702870498 01-JAN-14
3       3       LIVE     3494832994       YES 12041003378277 04-JUL-18

For The Interested RMAN Beginners

The RMAN policies REDUNDANCY and RECOVERY WINDOW are mutually exclusive. This means you can either set one or the other.

Redundancy Policy

Having set REDUNDANCY 2 will always keep only the last two backups and deleting (or marking obsolete) any other previous backups, that are no longer required to bring back the database into a consistent state.

Note:

If you have LEVEL 1 and ARCHIVELOG backups in between the FULL and/or LEVEL 0 backups, then they should be retained until no longer required.

Additional Information

According to the PDF file Oracle Database - Backup and Recovery Reference your retention policy REDUNDANCY 2 should not have deleted your older backups:


The following scenario illustrates how redundancy works in an incremental
backup strategy. Assume that the redundancy level is 1. You run a level 0
database backup at noon Monday, a level 1 cumulative backup at noon on
Tuesday and Wednesday, and a level 0 backup at noon on Thursday.
Immediately after each daily backup you run a DELETE OBSOLETE. The
Wednesday DELETE command does not remove the Tuesday level 1 backup
because this backup is not redundant: the Tuesday level 1 backup could be used
to recover the Monday level 0 backup to a time between noon on Tuesday and
noon on Wednesday. However, the DELETE command on Thursday removes the
previous level 0 and level 1 backups.

The above example uses a value of 1 but is clearly stating that earlier backups of full backups are not deleted until they are larger than the value configured.

Answering your questions

-
It depends, .... (see 2. and "Possible Issues")

-
According to the documentation the retention policy setting still seems to be the same for all versions:

• Oracle 11c (11.1) : CONFIGURE

• Oracle 11c (11.2) : CONFIGURE

• Oracle 12c (12.1) : CONFIGURE

• Oracle 12c (12.2) : CONFIGURE

• Oracle 18c (18.x) : CONFIGURE

Possible Issues

• You encountered a bug during the backup and restores of your database instance.

Reproducing with Oracle 12c

Because Oracle RDBMS 9i is pretty much obsolete and our environment is nearly up-to-date, I was only able to re-iterate/reproduce the steps in a 12c environment.

RMAN Configuration

RMAN was set up using the defaults:

RMAN> show all;

RMAN configuration parameters for database with db_unique_name ORACLE12JN are:
CONFIGURE RETENTION POLICY TO REDUNDANCY 2;
CONFIGURE BACKUP OPTIMIZATION OFF; # default
CONFIGURE DEFAULT DEVICE TYPE TO DISK; # default
CONFIGURE CONTROLFILE AUTOBACKUP ON; # default
CONFIGURE CONTROLFILE AUTOBACKUP FORMAT FOR DEVICE TYPE DISK TO '%F'; # default
CONFIGURE DEVICE TYPE DISK PARALLELISM 1 BACKUP TYPE TO BACKUPSET; # default
CONFIGURE DATAFILE BACKUP COPIES FOR DEVICE TYPE DISK TO 1; # default
CONFIGURE ARCHIVELOG BACKUP COPIES FOR DEVICE TYPE DISK TO 1; # default
CONFIGURE MAXSETSIZE TO UNLIMITED; # default
CONFIGURE ENCRYPTION FOR DATABASE OFF; # default
CONFIGURE ENCRYPTION ALGORITHM 'AES128'; # default
CONFIGURE COMPRESSION ALGORITHM 'BASIC' AS OF RELEASE 'DEFAULT' OPTIMIZE FOR LOAD TRUE ; # default
CONFIGURE RMAN OUTPUT TO KEEP FOR 7 DAYS; # default
CONFIGURE ARCHIVELOG DELETION POLICY TO NONE; # default
CONFIGURE SNAPSHOT CONTROLFILE NAME TO 'C:\ORACLE_12\PRODUCT\12.2.0\DBHOME_1\DATABASE\SNCFORACLE12JN.ORA'; # default

RMAN Backups

RMAN backups were performed by issuing a simple backup database; command:

RMAN> backup database;

Starting backup at 17-JUL-18
using channel ORA_DISK_1
channel ORA_DISK_1: starting full datafile backup set
channel ORA_DISK_1: specifying datafile(s) in backup set
input datafile file number=00001 name=C:\ORACLE_12\ORADATA\ORACLE12JN\SYSTEM01.DBF
input datafile file number=00005 name=C:\ORACLE_12\ORADATA\ORACLE12JN\UNDOTBS01.DBF
input datafile file number=00003 name=C:\ORACLE_12\ORADATA\ORACLE12JN\SYSAUX01.DBF
input datafile file number=00007 name=C:\ORACLE_12\ORADATA\ORACLE12JN\USERS01.DBF
channel ORA_DISK_1: starting piece 1 at 17-JUL-18
channel ORA_DISK_1: finished piece 1 at 17-JUL-18
piece handle=C:\ORACLE_12\RECOVERY_AREA\ORACLE12JN\BACKUPSET\2018_07_17\O1_MF_NNNDF_TAG20180717T151608_FNVV0RH4_.BKP tag=TAG20180717T151608 comment=NONE
channel ORA_DISK_1: backup set complete, elapsed time: 00:00:15
Finished backup at 17-JUL-18

Starting Control File and SPFILE Autobackup at 17-JUL-18
piece handle=C:\ORACLE_12\RECOVERY_AREA\ORACLE12JN\AUTOBACKUP\2018_07_17\O1_MF_S_981731783_FNVV17QR_.BKP comment=NONE
Finished Control File and SPFILE Autobackup at 17-JUL-18

Verifying Backups, Check Obsolete Backups and List Incarnations

After a certain period of backups had been performed I double-checked the RMAN catalog:

RMAN> list backup summary;

List of Backups
===============
Key TY LV S Device Type Completion Time #Pieces #Copies Compressed Tag
------- -- -- - ----------- ------------------- ------- ------- ---------- ---
3 B F A DISK 2018-07-16 10:03:01 1 1 NO TAG2018071

StackExchange Database Administrators Q#211434, answer score: 4