Snapshot standby database can be used to perform both
READ and WRITE activities. Most importantly, a
snapshot standby database keeps receiving the redo data from the primary database but does not apply them. These redo data received from the primary database would be applied only when the snapshot standby database is converted back to the Physical standby mode. There by the snapshot standby database provides data protection on primary database.
A snapshot standby database will allow you to make use of the data available on the physical standby database (which is a mirrored copy of the primary database). This allows the users to test the application on a standby database which has the primary data before implementing it in the Real production environment. When a physical standby database is converted to a snapshot standby database, a
guaranteed restore point is automatically created.flash back is automaticlly created a Once when the updateable transactions are completed for testing purposes on the snapshot standby database and when you are converting back the snapshot standby to physical standby, oracle flashbacks to the restore point that was created earlier which means that the transactions that were made when the standby database was opened in READ WRITE mode will be flushed out.
The only requirement to have the snapshot standby is that FRA (Flash Recovery Area) must be configured on physical standby databas
e. It is not necessary to have flashback enabled. Below are the steps on how to convert a physical standby database to a snapshot standby database and viceversa.
Primary database: SRPRIM
SQL> select status,instance_name,database_role,open_mode from v$database,v$Instance;
STATUS INSTANCE_NAME DATABASE_ROLE OPEN_MODE
------------ ---------------- ---------------- --------------------
OPEN primary PRIMARY READ WRITE
SQL> select thread#,max(sequence#) from v$archived_log group by thread#;
THREAD# MAX(SEQUENCE#)
---------- --------------
1 206
Standby database name: standby
SQL> select status,instance_name,database_role,open_mode from v$database,v$Instance;
STATUS INSTANCE_NAME DATABASE_ROLE OPEN_MODE
------------ ---------------- ---------------- ----------------
OPEN standby PHYSICAL STANDBY READ ONLY WITH APPLY
SQL> select thread#,max(sequence#) from v$archived_log where applied='YES' group by thread#;
THREAD# MAX(SEQUENCE#)
---------- --------------
1 206
SQL> select flashback_on from v$database;
FLASHBACK_ON
------------------
NO
You can observe that the standby database is in sync with the primary database. Below outcome shows that the Flash Recovery Area is configured on the physical standby database.
SQL> show parameter db_recovery_file_dest
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
db_recovery_file_dest string /u01/app/oracle/flash_recovery
_area
db_recovery_file_dest_size big integer 3882M
SQL>
Step 1: Cancel the Managed Recovery Process (MRP) on the physical standby database, shut it down and place it in Mount mode.
SQL> alter database recover managed standby database cancel;
Database altered.
SQL> shut immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup mount
ORA-32004: obsolete or deprecated parameter(s) specified for RDBMS instance
ORACLE instance started.
Total System Global Area 1269366784 bytes
Fixed Size 2227984 bytes
Variable Size 805306608 bytes
Database Buffers 452984832 bytes
Redo Buffers 8847360 bytes
Database mounted.
Step 2: Once the standby database is mounted, convert the Physical standby database to snapshot standby database.
SQL> alter database convert to snapshot standby;
Database altered.
Step 3: You can now open the snapshot standby database and check its mode.
SQL> alter database open;
Database altered.
SQL> select status,instance_name,database_role,open_mode from v$database,v$Instance;
STATUS INSTANCE_NAME DATABASE_ROLE OPEN_MODE
------------ ---------------- ---------------- --------------------
OPEN standby SNAPSHOT STANDBY READ WRITE
Database altered.
Small Test on the snapshot standby database.
1. Create a user called “SNAPTEST”
2. Create a table called “TEST” whose owner is “SNAPTEST” and insert some records in it. You can also update some of the records as well.
SQL> create user snaptest identified by oracle;
User created.
SQL> grant connect,resource to snaptest;
Grant succeeded.
SQL> conn snaptest/oracle@standby
Connected.
SQL>
SQL> create table test(code number, name char(20));
Table created.
SQL> insert into test values (100,'AMIN');
1 row created.
SQL> insert into test values(200,'SYED');
1 row created.
SQL> commit;
Commit complete.
SQL> select * from test;
CODE NAME
---------- --------------------
100 AMIN
200 SYED
SQL> update snaptest.test set code=500 where name='AMIN';
1 row updated.
SQL> commit;
Commit complete.
SQL> select * from snaptest.test;
CODE NAME
---------- --------------------
500 ARUN
200 SHIVU
In the mean time, you can also see that the redo data from the primary database is received by the snapshot standby database but would not be applied.
On primary database the latest sequence generated is 208 and that on the standby database, the RFS process is idle for sequence 209.
Primary
SQL> select thread#,max(sequence#) from v$archived_log group by thread#;
THREAD# MAX(SEQUENCE#)
---------- --------------
1 208
Standby:
SQL> select process,status,sequence# from v$managed_standby;
PROCESS STATUS SEQUENCE#
--------- ------------ ----------
ARCH CLOSING 1
ARCH CONNECTED 0
ARCH CONNECTED 0
ARCH CONNECTED 0
RFS IDLE 0
RFS IDLE 209
RFS IDLE 0
7 rows selected.
Steps on converting back a snapshot standby database to physical standby database.
Step 1: Shut down the snapshot standby database and open it in Mount mode.
SQL>
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup mount
ORA-32004: obsolete or deprecated parameter(s) specified for RDBMS instance
ORACLE instance started.
Total System Global Area 1269366784 bytes
Fixed Size 2227984 bytes
Variable Size 805306608 bytes
Database Buffers 452984832 bytes
Redo Buffers 8847360 bytes
Database mounted.
SQL>
Step 2: Convert the snapshot standby database to physical standby database.
SQL> alter database convert to physical standby;
Database altered.
Step 3: Once done, bounce the physical standby database and start the Managed Recovery Process (MRP) on it.
SQL> shut immediate
ORA-01507: database not mounted
ORACLE instance shut down.
SQL> startup
ORA-32004: obsolete or deprecated parameter(s) specified for RDBMS instance
ORACLE instance started.
Total System Global Area 1269366784 bytes
Fixed Size 2227984 bytes
Variable Size 805306608 bytes
Database Buffers 452984832 bytes
Redo Buffers 8847360 bytes
Database mounted.
Database opened.
SQL> select status,instance_name,database_role,open_mode from v$database,v$Instance;
STATUS INSTANCE_NAME DATABASE_ROLE OPEN_MODE
------------ ---------------- ---------------- --------------------
MOUNTED standby PHYSICAL STANDBY MOUNTED
SQL> alter database recover managed standby database disconnect from session;
Database altered.
SQL> select process,status,sequence# from v$managed_standby;
PROCESS STATUS SEQUENCE#
--------- ------------ ----------
ARCH CONNECTED 0
ARCH CONNECTED 0
ARCH CONNECTED 0
ARCH CONNECTED 0
RFS IDLE 0
RFS IDLE 0
RFS IDLE 0
MRP0 WAIT_FOR_LOG 213
8 rows selected.
On Primary database:
SQL> select thread#,max(sequence#) from v$archived_log group by thread#;
THREAD# MAX(SEQUENCE#)
---------- --------------
1 212
On Standby database:
SQL> select thread#,max(sequence#) from v$archived_log where applied='YES' group by thread#;
THREAD# MAX(SEQUENCE#)
---------- --------------
1 212
===============================================================
If you have a physical standby database that you would like to convert to a snapshot standby database, use the DGMGRL CONVERT DATABASE command. Redo data will continue to be received by the database while it is operating as a snapshot standby database, but it will not be applied until the snapshot standby is converted back into a physical standby database.
A physical standby database must be configured with a fast recovery area to convert it to a snapshot standby database. This is because a guaranteed restore point is created during the conversion process, and guaranteed restore points require a fast recovery area.
When you are ready to revert the database back to a physical standby database, use the DGMGRL CONVERT DATABASE command again as follows. Any updates made to the database while it was operating as a snapshot standby database will be discarded. All accumulated redo data will be applied by Redo Apply services after the database is converted back to a physical standby database.
Limitation of Snapshot database;
========================
i1)f the standby database is in maximum protection mode it cannot be converted in to snapshot
2)Target of switchover
3) Target of fast start failover
