In synchronous mode, committed transactions are guaranteed to be recorded on the mirror server. Should a failure occur on the primary server, no committed transactions are lost when the mirror server takes over. Using synchronous mode provides transaction safety because the operational servers are in a synchronized state, and changes sent to the mirror must be acknowledged before the primary can proceed.

In asynchronous mode, committed transactions are not guaranteed to be recorded on the mirror server. In this mode, the primary server sends transaction log pages to the mirror when a transaction is committed. It does not wait for an acknowledgement from the mirror before replying to the application that the COMMIT has completed. Should a failure occur on the primary server, it is possible that some committed transactions may be lost when the mirror server takes over.