Oracle Dataguard:

1. What are different protection modes in dataguard?

There are three modes.

a. MAXIMUM PROTECTION

This mode provides maximum protection. It guarantees zero data loss. In this mode the redo/transaction data must be written to both primary redo log and standby redo log. For any reason(mostly N/W issue) if it is unable to write to standby, Then primary will get shutdown.

b. MAXIMUM AVAILABILITY:

It provides the highest level of data protection that is possible without affecting the availability of the primary database.Transactions are not allowed to commit until all redo data are written to the online redo logs and propagated to at least one synchronized secondary database. If for any reason, the primary database cannot propagate its redo stream to one secondary database, the primary will NOT shutdown and operates as it it were in maximum performance mode until issues are fixed.

c. MAXIMUM PERFORMANCE:(default one)

This is the default protection mode. With this protection mode, a transaction is committed as soon as the redo data needed to recover the transaction is written to the local (online) redo log.

2. What are different types of dataguard and their difference?

There are two types of dataguard setups. PHYSICAL and LOGICAL.

PHYSICAL STANDBY:

A physical standby database is an exact, block-for-block copy of a primary database. A physical standby is maintained as an exact copy through a process called REDO APPLY, in which redo data received from a primary database is continuously applied to a physical standby database using the database recovery mechanisms. So it will be always in sync with primary.

This Standby database can be opened in read only mode( knows as ACTIVE DATA GUARD), for reporting purpose. Most of the corporations use physical standby for dataguard configuration.

LOGICAL STANDBY:

The logical standby database is kept synchronized with the primary database through SQL APPLY, which transforms the data in the redo received  from the primary database into SQL statements and then executes the SQL statements on the standby database. So it contains same logical information as that of production , but physical structure of data can be different.

3. What is the process to apply a psu patch in dataguard setup.

• Make sure lag between primary and standby is zero.

• Cancel the recovery (MRP) on standby.

• Shutdown standby db and listener.

• Apply patch to binary using opatch apply command.

• Once patch applied to binary , startup the listener and standby in mount stage or OPEN(if active dataguard).

• Now shutdown primary db and listener.

• Apply patch to binary using opatch apply command.

• Once patch applied to binary , startup the listener and prim db in mount OPEN(if active dataguard).

• Start the MRP recovery process on standby.

4. What is active dataguard. Does it needs additional licensing?

Active dataguard means, the standby database is open with read only mode, when redo logs are getting applied in real time.
Below are the benefit of using active dataguard.
Reporting queries can be offloaded to standby database.
Physical block corruptions are repaired automatically either at primary or physical standby database.
RMAN backups can be initiated from standby , instead  of primary which will reduce cpu load from primary.

NOTE – To use active dataguard, you need additional license from oracle.