在前面Oracle 10g DataGuard学习（二）中解释了DG的redo传输及应用的原理，为了进一步理解DG，同时便于后续从实战层面演示DG配置、创建、切换等过程，该篇会解释DG三种数据保护模式及如何设置这三种保护模式。

DG共有三种运行模式：最大保护模式、最大可用性模式、最大性能模式。在DG实际生产中环境中，可以根据不同的需求选择不同的运行模式，以下为官档对三种数据保护模式的说明：
1.1 Maximum Protection Mode
This protection mode ensures that no data loss will occur if the primary database fails. To provide this level of protection, the redo data needed to recover each transaction must be written to both the local online redo log and to the standby redo log on at least one standby database before the transaction commits. To ensure data loss cannot occur, the primary database shuts down if a fault prevents it from writing its redo stream to at least one remote standby redo log. For multiple-instance RAC databases, Data Guard shuts down the primary database if it is unable to write the redo records to at least one properly configured database instance. The maximum protection mode requires that at least one standby instance has a standby redo log and the LGWR, SYNC, and AFFIRM attributes be used on the LOG_ARCHIVE_DEST_n parameter for this destination.
1.2 Maximum Availability Mode
This protection mode provides the highest level of data protection that is possible without compromising the availability of the primary database. Like maximum protection mode, a transaction will not commit until the redo needed to recover that transaction is written to the local online redo log and to at least one remote standby redo log. Unlike maximum protection mode, the primary database does not shut down if a fault prevents it from writing its redo stream to a remote standby redo log. Instead, the primary database operates in maximum performance mode until the fault is corrected and all gaps in redo log files are resolved. When all gaps are resolved, the primary database automatically resumes operating in maximum availability mode.
This mode ensures that no data loss will occur if the primary database fails, but only if a second fault does not prevent a complete set of redo data from being sent from the primary database to at least one standby database.
Like maximum protection mode, the maximum availability mode requires that you:
Configure standby redo log files on at least one standby database.
Set the SYNC, LGWR, and AFFIRM attributes of the LOG_ARCHIVE_DEST_n parameter for at least 1 standby database.
1.3 Maximum Performance Mode
This protection mode (the default) provides the highest level of data protection that is possible without affecting the performance of the primary database. This is accomplished by allowing a transaction to commit as soon as the redo data needed to recover that transaction is written to the local online redo log. The primary database’s redo data stream is also written to at least one standby database, but that redo stream is written asynchronously with respect to the commitment of the transactions that create the redo data.
When network links with sufficient bandwidth are used, this mode provides a level of data protection that approaches that of maximum availability mode with minimal impact on primary database performance.
The maximum performance mode enables you to either set the LGWR and ASYNC attributes, or set the ARCH attribute on the LOG_ARCHIVE_DEST_n parameter for the standby database destination. If the primary database fails, you can reduce the amount of data that is not received on the standby destination by setting the LGWR and ASYNC attributes.

在了解DG以上三种数据库保护模式后，下面说明一下如何设置这三种数据保护模式：
Step 1   Configure the LOG_ARCHIVE_DEST_n parameters on the primary database
On the primary database, configure the LOG_ARCHIVE_DEST_n parameter attributes appropriately. Each of the Data Guard data protection modes requires that at least one standby database in the configuration meet the minimum set of requirements listed in Table 5-2.
Table 5-2 Minimum Requirements for Data Protection Modes

Note:
Oracle recommends that a Data Guard configuration that is running in maximum protection mode contains at least two standby databases that meet the requirements listed in Table 5-2. That way, the primary database can continue processing if one of the standby databases cannot receive redo data from the primary database.
The following example shows how to configure the maximum availability mode:

SQL> ALTER SYSTEM SET LOG_ARCHIVE_DEST_2='SERVICE=PROD
  2> OPTIONAL LGWR SYNC AFFIRM
  3> VALID_FOR=(ONLINE_LOGFILES,PRIMARY_ROLE)
  4> DB_UNIQUE_NAME=PROD';

If they are not already specified in the SPFILE, you should also specify unique names with the DB_UNIQUE_NAME initialization parameter and list all databases on the LOG_ARCHIVE_CONFIG parameter with the DG_CONFIG attribute. For example:

SQL> ALTER SYSTEM SET LOG_ARCHIVE_CONFIG='DG_CONFIG=(PROD,SBDB)';

This will enable the dynamic addition of a standby database to a Data Guard configuration that has a Real Application Clusters primary database running in either maximum protection or maximum availability mode.
Step 1 If you are upgrading the protection mode, perform this step.
Perform this step only if you are upgrading the protection mode (for example, from maximum performance to maximum availability mode). Otherwise, go to Step 3.
Assume this example is upgrading the Data Guard configuration from the maximum performance mode to the maximum availability mode. Shut down the primary database and restart it in mounted mode:

SQL> SHUTDOWN IMMEDIATE;
SQL> STARTUP MOUNT;

For a Real Application Clusters database, shut down all of the primary instances but start and mount only one primary instance.
Step 2 Set the data protection mode.
To specify a data protection mode, issue the SQL ALTER DATABASE SET STANDBY DATABASE TO MAXIMIZE {PROTECTION | AVAILABILITY | PERFORMANCE} statement on the primary database. For example, the following statement specifies the maximum availability mode:

SQL> ALTER DATABASE SET STANDBY DATABASE TO MAXIMIZE AVAILABILITY;

Step 3 Open the primary database.
If you performed Step 1 to upgrade the protection mode, open the database:

SQL> ALTER DATABASE OPEN;

If you are downgrading the protection mode, the database will already be open.
Step 4 Configure the LOG_ARCHIVE_DEST_n parameters on standby databases.
On the standby databases, configure the LOG_ARCHIVE_DEST_n parameter attributes so the configuration can continue to operate in the new protection mode after a switchover. For example:

SQL> ALTER SYSTEM SET LOG_ARCHIVE_DEST_2='SERVICE=boston
  2> OPTIONAL LGWR SYNC AFFIRM
  3> VALID_FOR=(ONLINE_LOGFILES,PRIMARY_ROLE)
  4> DB_UNIQUE_NAME=boston';

Step 5 Confirm the configuration is operating in the new protection mode.
Query the V$DATABASE view to confirm the Data Guard configuration is operating in the new protection mode. For example:

SQL> SELECT PROTECTION_MODE, PROTECTION_LEVEL FROM V$DATABASE;
PROTECTION_MODE                   PROTECTION_LEVEL
---------------------             ---------------------
MAXIMUM AVAILABILITY              MAXIMUM AVAILABILITY

以上结合官档简要说明了DG的数据保护模式，后期会继续更新相关内容，敬请期待！