A database mirroring session runs with either synchronous or asynchronous operation. Under asynchronous operation, the transactions commit without waiting for the mirror server to write the log to disk, which maximizes performance. Under synchronous operation, a transaction is committed on both partners, but at the cost of increased transaction latency.
There are two mirroring operating modes. One of them, high-safety mode supports synchronous operation. Under high-safety mode, when a session starts, the mirror server synchronizes the mirror database together with the principal database as quickly as possible. As soon as the databases are synchronized, a transaction is committed on both partners, at the cost of increased transaction latency.
The second operating mode, high-performance mode, runs asynchronously. The mirror server tries to keep up with the log records sent by the principal server. The mirror database might lag somewhat behind the principal database. However, typically, the gap between the databases is small. However, the gap can become significant if the principal server is under a heavy work load or the system of the mirror server is overloaded.
In high-performance mode, as soon as the principal server sends a log record to the mirror server, the principal server sends a confirmation to the client. It does not wait for an acknowledgement from the mirror server. This means that transactions commit without waiting for the mirror server to write the log to disk. Such asynchronous operation enables the principal server to run with minimum transaction latency, at the potential risk of some data loss.