此时，如果发现Primary和Standby中的块SCN不一致，说明较小的块存在写丢失，会主动抛出错误。
可以通过设置DB_ULTRA_SAFE，来同时隐含设置DB_BLOCK_CHECKING, DB_BLOCK_CHECKSUM, DB_LOST_WRITE_PROTECT参数。
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Values:
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OFF
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When any of DB_BLOCK_CHECKING, DB_BLOCK_CHECKSUM, or DB_LOST_WRITE_PROTECT are explicitly set, no changes are made.
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DATA_ONLY
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DB_BLOCK_CHECKING will be set to MEDIUM.
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DB_LOST_WRITE_PROTECT will be set to TYPICAL.
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DB_BLOCK_CHECKSUM will be set to FULL.
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DATA_AND_INDEX
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DB_BLOCK_CHECKING will be set to FULL.
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DB_LOST_WRITE_PROTECT will be set to TYPICAL.
- DB_BLOCK_CHECKSUM will be set to FULL.
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- alter system set db_ultra_safe='DATA_ONLY' scope=spfile;