Data Guard模式切换

概念
a、最大性能(maximize performance):
    这是Data Guard默认的保护模式。primay上的事务commit前不需要从standby上收到反馈信息，该模式在primary故障时可能丢失数据，但standby对primary的性能影响最小。
b、最大可用(maximize availability):
    在正常情况下，最大可用模式和最大保护模式一样；在standby不可用时，最大可用模式会自动降低成最大性能模式，所以standby故障不会导致primay不可用。只要至少有一个standby可用的情况下

，即使primary down机，也能保证不丢失数据。
c、最大保护(maximize protection):
    最高级别的保护模式。primay上的事务在commit前必须确认redo已经传递到至少一个standby上，如果所有standby不可用，则primary会挂起。该模式能保证零数据丢失, 一般为保证主库在某个从库

故障后仍能正常运行,最好配置两个及以上的从库。

摘自官方文档的描述：
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.

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.

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