DataGuard的安全保护模式
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primary and standby不一定在同一个局域网上,可以分开的,可能会在一个广域网中,那么primary和standby之间的redo data传输因为网络问题难免会有丢失问题.

针对现实中的三种情况
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1.A bussiness cannot afford to lose data regardless of the circumstances.

2.The availability of the database may be more important than any potential data loss in the unlikely event of a multiple failure.

3.Performance is the maximum requirement and can therefore tolerate a small amount of data loss.

就有了三种保护模式:
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1.Maximum availability
这是最高级别的数据保护模式,除非主/备库都坏掉了,数据是不会丢失的,Transactions DO NOT commit until  all redo data needed to recover those transactions has been written to the online redo log and to AT LEAST one standby database.如果发现primary可能因为网络或其他原因不能把可以recover transaction的redo data传输到最少一个standby database的时候,他会effectively切换到maximum performance的安全保护模式下面,那么直到,可以传输到至少一个standby database,才会切换回来.

2.Maximum performance(默认模式)
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.
只要事务日志写到online redo log file,但是redo data是以异步(Asyn)方式传输到standby database.因此用户的transactions是可以提交的,即使网络有问题,primary可能临时不能把所产生的redo log传到standby那边(过会儿再传),transactions也是可以提交的,注意这个和maximum availability的模式有些不同.

3.Maximum protection
和Maximum availability差不多,只不过出于某些原因,比如network,primary database不能将redo data传输到standby database,那么to ensure that data loss cannot occur ,the primary database will shutdown,rather than continue processing transactions.所以在这种情况下和maximum availability切换到maximum performance下的方式完全不同.