RMAN Recovery Window retention policy tips

   Assume the following retention policy illustrated in Figure rw01. 

   The retention policy has the following aspects:

   
   ■ The recovery window is 7 days.
   ■ Database backups are scheduled every two weeks on these days:
   – January 1
   – January 15
   – January 29
   – February 12

   ■ The database runs in ARCHIVELOGmode, and archived logs are saved on disk

 

   

 

   As illustrated in Figure rw01
   the current time is January 23 and the point of recoverability is January 16
   Hence, the January 14 backup is needed for recovery,
   and so are the archived logs from log sequence 500 through 850
   The logs before 500 and the January 1 backup are obsolete
   because they are not needed for recovery to a point within the window
   
   Assume the same scenario a week later, as depicted in rw02

 

   

 

   In this scenario
   the current time is January 30 and the point of recoverability is January 23
   Note how the January 14 backup is not obsolete 
   even though a more recent backup (January 28) exists in the recovery window
   This situation occurs 
   because restoring the January 28 backup does not enable you to recover to the earliest time in the window, January 23
   To ensure recoverability to any point within the window
   you must save the January 14 backup as well as all archived redo logs from log sequence 500 to 1150
   
   
   Thus、if the recovery window is 7
   then there must always exist one backup of each datafile that satisfies the following condition:
   
   SYSDATE - BACKUP CHECKPOINT TIME（POR） >= 7
   
   All backups older than the most recent backup that satisfied this condition are obsolete.
   
   The Recovery Window retention policy doesn't just keep the last N days of the backup
   It calculates the point-of-recovery (POR) value
   accounting the current date and the number of the days mentioned in the retention policy.
   The POR is more than just the backup of the last N days
   In addition, you would also need the backup of all those which come under the calculation of POR
   For example, assume today is 20th of April and you decide to go with the Recovery Window of 2 days 
   
   In the case of non-incremental backup, this is how it would look:
   
   Current_Date        Status         POR            
   20-April           Available       20-2=18th April
   21-April           Available       21-2= 19th April
   22-April           Available       22-2=20th April
   23-April  Backup of 20th Obsoleted 23-2=21st April
   
   So in the case of 23rd, the backup would be marked as obsolete