2点，以下内容摘录之官网，第一点，我们必须先disable circular logging 然后才能创建DATABASE COPIES， 第二点解释了在DAG环境中circular logging的原理和不同之处。
1. If circular logging is enabled, you must first disable it. After the mailbox database copy has been added, circular logging can be enabled
2. To prevent a buildup of log files, you enable circular logging for your replicated databases. When you combine circular logging with continuous replication, you have a new type of circular logging called continuous replication circular logging (CRCL), which is different from ESE circular logging. Whereas ESE circular logging is performed and managed by the Microsoft Exchange Information Store service, CRCL is performed and managed by the Microsoft Exchange Replication Service. When enabled, ESE circular logging does not generate additional log files and instead overwrites the current log file when needed. However, in a continuous replication environment, log files are needed for log shipping and replay. As a result, when you enable CRCL, the current log file is not overwritten and closed log files are generated for the log shipping and replay process.
Specifically, the Microsoft Exchange Replication Service manages CRCL so that log continuity is maintained and logs are not deleted if they are still needed for replication. The Microsoft Exchange Replication Service and the Microsoft Exchange Information Store service communicate by using remote procedure calls (RPCs) regarding which log files can be deleted.
