Data Guard Protection Modes（dg保护模式）

Home / Database / Oracle
Database Online Documentation 11g Release 2 (11.2) / High
Availability

Data Guard Concepts and Administration

1.4 Data
Guard Protection Modes dg保护模式

In some situations, a business cannot afford to lose data regardless of the circumstances. In other situations, the availability of the database may be more important than any potential data loss in the unlikely event of a multiple failure. Finally, some applications
require maximum database performance at all times, and can therefore tolerate a small amount of data loss if any component should fail. The following descriptions summarize the three distinct modes of data protection.

在一些环境中，业务无论如何也不能丢数据；另一些环境则是，业务的可用性要比存在潜在的数据丢失要重要；还有的环境，应用程序需要数据库跑出最大的性能，并且可以接受数据的少量丢失。下边的介绍总结了这三种不同的数据保护模式。

Maximum
availability 最大可用性This protection mode provides the highest level of data protection that is possible without compromising the availability of a primary database. Transactions do not commit
until all redo data needed to recover those transactions has been written to the online redo log and to the standby redo log on at least one synchronized standby database. If the primary database cannot write its redo stream to at least one synchronized standby
database, it operates as if it were in maximum performance mode to preserve primary database availability until it is again able to write its redo stream to a synchronized standby database.

这种保护模式在不影响主库可用性的同时，可以（但不保证）提供最高级别的数据保护。事物 直到与其相关的 redo日志 写到 在线重做日志 和 至少一个备库的
standby redo日志 才会被提交。如果主库不能将redo日志写到至少一个备库中，则数据库将会运行在 最大性能 模式下直到 主库可以写redo日志到至少一个备库（意思就是说 最大可用性 模式 就是 最大保护 和 最大性能 的结合体，当有至少一个备库正常时，此时主库可以将redo传输到备库，主库 运行在 最大保护
模式；当所有的备库都不正常时，主库 的可用性也不会受到影响，即数据库还是可以正常使用，只是这时的保护模式 切换成 最大性能）。

This protection mode ensures zero data loss except in the case of certain double faults, such as failure of a primary database after failure of the standby database.

只要不是主库、备库同时挂掉，这种 保护模式 可以保证0数据丢失。

Maximum
performance 最大性能This is the default protection mode. It provides the highest level of data protection that is possible without affecting the performance of a primary database. This is accomplished
by allowing transactions to commit as soon as all redo data generated by those transactions has been written to the online log. Redo data is also written to one or more standby databases, but this is done asynchronously with respect to transaction commitment,
so primary database performance is unaffected by delays in writing redo data to the standby database(s).

这一种是默认的保护模式。这种 保护模式 在不影响主库性能的同时，可以（但不保证）提供最高级别的数据保护。它允许事物在相关的日志写到在线重做日志后尽快的提交。redo也会被写到一个或多个备库，但是是以异步的形式，redo写到备库会有延时，主库的性能不会因为redo写到备库而受影响（意思就是说 事物提交要等
在线重做日志 写盘，而不管备库的standby redo是否写盘，只要把数据发送过就不管了，不关心成功或是失败）。

This protection mode offers slightly less data protection than maximum availability mode and has minimal impact on primary database performance.

此保护模式提供的数据保护能力要比 最大可用性 稍微小些，同时对主库的性能影响要最小。

Maximum
protection 最大保护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 a transaction
must be written to both the online redo log and to the standby redo log on at least one synchronized standby database before the transaction commits. To ensure that data loss cannot occur, the primary database will shut down, rather than continue processing
transactions, if it cannot write its redo stream to at least one synchronized standby database.

这种保护模式保证在主库挂掉的时候不丢数据。提供这种级别的保护，事物在提交前需要 把相关的 redo日志 写到 在线重做日志 和至少 一个 备库的standby redo中。为了不丢数据，如果主库不能把redo写到 至少一个备库的standby redo中，则主库会关闭（意思就是说 如果所有备库都不能正常的接收redo日志，则主库就会主动挂掉）。

All three protection modes require that specific redo transport options be used to send redo data to at least one standby database.

所有的三种保护模式都需要redo transport把日志传输到至少一个备库。

See Also:

Chapter 5, "Data Guard Protection
Modes" for more detailed descriptions of these modes and for information about setting the protection mode of a primary database.