參考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.
中文解釋：
Exchange 2010Help文檔說明
如果將循環日誌記錄和連續複製組合使用，即產生被稱爲連續複製循環日誌記錄 (CRCL) 的新型循環日誌記錄，它與 ESE 循環日誌記錄不同。ESE 循環日誌記錄由 Microsoft Exchange 信息存儲服務執行和管理，而 CRCL 由 Microsoft Exchange 複製服務執行和管理。啓用 ESE 循環日誌記錄時，並不會另外生成日誌文件，而會視需要覆蓋當前日誌文件。但是，在連續複製環境中，需要使用日誌文件進行日誌傳送和重播。因此，啓用 CRCL 時，不會覆蓋當前的日誌文件，並且會生成用於日誌傳送和傳重播過程的已關閉的日誌文件。具體而言，Microsoft Exchange 複製服務管理 CRCL 以維持日誌連續性，當仍然需要日誌以進行復制時，日誌刪除程序不會刪除這些日誌。
參考2:
1.可以啓用循環日誌，並且不會影響數據庫之間的複製，DAG依然有效。
它的原則是這樣的，當一個事務日誌被寫入主動的數據庫中，並且被複制到所有數據庫副本的節點上之後，纔可以被新的事務日誌所覆蓋。因此不會導致數據庫不一致的情況發生
2.DAG數據庫之間的複製就是通過事務日誌來進行的。數據庫副本從主動節點上覆制最新產生的事務日誌文件，繼而寫入數據庫副本，以確保一致性