参考1:
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.
中文解释：
Exchange 2010Help文档说明
如果将循环日志记录和连续复制组合使用，即产生被称为连续复制循环日志记录 (CRCL) 的新型循环日志记录，它与 ESE 循环日志记录不同。ESE 循环日志记录由 Microsoft Exchange 信息存储服务执行和管理，而 CRCL 由 Microsoft Exchange 复制服务执行和管理。启用 ESE 循环日志记录时，并不会另外生成日志文件，而会视需要覆盖当前日志文件。但是，在连续复制环境中，需要使用日志文件进行日志传送和重播。因此，启用 CRCL 时，不会覆盖当前的日志文件，并且会生成用于日志传送和传重播过程的已关闭的日志文件。具体而言，Microsoft Exchange 复制服务管理 CRCL 以维持日志连续性，当仍然需要日志以进行复制时，日志删除程序不会删除这些日志。
参考2:
1.可以启用循环日志，并且不会影响数据库之间的复制，DAG依然有效。
它的原则是这样的，当一个事务日志被写入主动的数据库中，并且被复制到所有数据库副本的节点上之后，才可以被新的事务日志所覆盖。因此不会导致数据库不一致的情况发生
2.DAG数据库之间的复制就是通过事务日志来进行的。数据库副本从主动节点上覆制最新产生的事务日志文件，继而写入数据库副本，以确保一致性