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Monday, June 27, 2016

Converting Physical Standby Database into Snapshot Standby Database


There are three restrictions where we cannot convert to a snapshot.

a) If it is the only standby of a Maximum Protection configuration.
b) If it is the target of a current switchover operation.
c) If it is the designated failover database for Fast Start Failover (FSFO).

If this is your only standby database, if you have to failover to it, the failover is going to take longer - because the standby has to be flashed back, and caught up, before the transition can complete, be aware that worst case scenario. It’s always recommended considering a Data Guard configuration that consists of two physical standby databases (at least), one is always kept physical standby database, and the other can be used/tested as a snapshot standby.

Perform the following steps to convert a physical standby database into a snapshot standby database:

Step 1: Check if Flashback is enabled or not.
This conversion is achieved using flashback database, but the physical standby database does not need to have flashback database explicitly enabled. Even if the flashback is not enabled explicitly the conversion will work.

i) SQL> select DB_UNIQUE_NAME, OPEN_MODE, DATABASE_ROLE from v$database;
NAME OPEN_MODE DATABASE_ROLE
---------------------------------------- ---------- ----------------
PHYSTD2 MOUNTED PHYSICAL STANDBY

ii) SQL> Show parameter db_recovery_file_dest
NAME TYPE VALUE
--------------------------------------------------------------- ----------- ------------------------------
db_recovery_file_dest string /u01/oradata/phystd2/FRA/
db_recovery_file_dest_size big integer 5G

iii) SQL> select flashback_on from v$database;
FLASHBACK
---------
YES

Step 2: Stop Redo Apply, if it is active.
SQL> ALTER DATABASE RECOVER MANAGED STANDBY DATABASE CANCEL;

Step 3: If the standby database is a RAC setup then we need to shutdown all the instances except one on which we will be using the conversion commands.

Step 4: Ensure that the database is mounted.

Step 5: Perform the conversion from physical standby to snapshot standby database:
The conversion from physical standby to snapshot standby database and vice versa can be done in three ways.

I). Manual method. This can be done by issuing the below SQL command in the physical standby database. Once it is converted the database can be opened in read/write mode.
SQL> ALTER DATABASE CONVERT TO SNAPSHOT STANDBY;

II). Data Guard Broker
A physical standby database that is managed by the Data Guard broker can be converted into a snapshot standby database using either DGMGRL or Oracle Enterprise Manager.

DGMGRL> show configuration;

   Configuration - fsfo_config

   Protection Mode:     MaxAvailability

   Databases:
     prima - Primary database
     phystd1 - Physical standby database
     phystd2 - Physical standby database

   Fast-Start Failover: DISABLED

   Current status:
    SUCCESS

The DGMGRL command that converts the database is:
DGMGRL> CONVERT DATABASE standby_unique_name TO SNAPSHOT STANDBY

DGMGRL> CONVERT DATABASE phystd2 TO SNAPSHOT STANDBY
DGMGRL> show configuration;

Configuration - fsfo_config

Protection Mode:     MaxAvailability

Databases:
 prima - Primary database
 phystd1 - Physical standby database
 phystd2 - Snapshot standby database

Fast-Start Failover: DISABLED

Current status:
SUCCESS

III). Enterprise Manager (If the physical standby is managed by DGMGRL)

Step 6: The database is dismounted during this conversion and we have to restart the database, which will be the snapshot standby database.
SQL> startup

Step 7: Check for database role
SQL> Select DB_UNIQUE_NAME, OPEN_MODE, DATABASE_ROLE from v$database;
NAME OPEN_MODE DATABASE_ROLE
-------------------------------------- ---------- -------------------------------
PHYSTD2 READ WRITE SNAPSHOT STANDBY

Meanwhile it will be receiving the redo from the primary, which will be automatically applied when snapshot standby database is converted back to physical standby database.

When we convert a physical standby database to snapshot standby database, then 
  • Redo gap detection and resolution will work as before.
  • A snapshot standby database cannot be the target of a switchover or failover. A snapshot standby database must first be converted back into a physical standby database before performing a role transition to it.
  • In case of role transition of one of the standby database (like failover or switchover), the snapshot standby database continues to receive the redo from the new primary.
  • Even if the primary database has a new incarnation (because of Flashback database or Open resetlogs), the snapshot database continues to receive the redo from the primary.

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