Data Guard Complete Understanding

Data Guard Complete Understanding

Ø  Data Guard Basics

Ø  Types of Standby Databases.

What is Data Guard?

Oracle Data Guard ensures high availability, data protection, and disaster recovery for enterprise data.

Ø  Data Guard provides a set of services that create, maintain, manage, and monitor one or more standby databases

Ø  Data Guard maintains these standby databases as transactionally consistent copies of the production database

Ø  Data Guard can switch any standby database to the production role

Without Data Guard:

With Data Guard:

Standby Database Types

Ø    Physical Standby Databases

Ø    Logical Standby Databases

Ø    Snapshot Standby Databases

 

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.

A logical standby database is initially created as an identical copy of the primary database, but it later can be altered to have a different structure. The logical standby database is updated by executing SQL statements. This allows users to access the standby database for queries and reporting at any time. Thus, the logical standby database can be used concurrently for data protection and reporting operations.

A snapshot standby database is a type of updatable standby database that provides full data protection for a primary database. A snapshot standby database receives and archives, but does not apply, redo data from its primary database. Redo data received from the primary database is applied when a snapshot standby database is converted back into a physical standby database, after discarding all local updates to the snapshot standby database.

Physical Standby

•       Standby is identical copy of primary database

•       Redo changes

–      transported from primary to standby

–      applied on standby (Redo Apply)

•       Can switch operations to standby

–      Planned (switchover / switchback)

–      Unplanned (failover)

Logical Standby

•       Redo copied from primary to standby

•       Changes converted into logical change records (LCR)

•       Logical change records applied on standby (SQL Apply)

•       Standby database can be opened for updates

–      Can modify propagated objects

–      Can create new indexes for propagated objects

•       May need larger system for logical standby

–      LCR apply can be less efficient than redo apply

–      Array updates on primary become single row updates on standby

A standby database is a transactionally consistent copy of an Oracle production database that is initially created from a backup copy of the primary database. Once the standby database is created and configured, Data Guard automatically maintains the standby database by transmitting primary database redo data to the standby system, where the redo data is applied to the standby database. 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. 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. A snapshot standby database is a type of updatable standby database that provides full data protection for a primary database.

Redo Log Shipping

•       ARCH background process

–      Copies completed redo log files to standby

•       LGWR background process - modes are:

–      ASYNC - asynchronous

–      redo written by LGWR to local disk

–      read from disk by LNSn background process

–      SYNC - synchronous

           –   Redo written to standby by LGWR - modes are:

                 –      AFFIRM - wait for confirmation redo written to disk

                 –      NOAFFIRM - do not wait

ARCH Redo Transmission

LGWR Redo (ASYNC) Transmission

LGWR Redo (SYNC) Transmission

Regards,

Mallik