A transaction log mirror is an identical copy of the transaction log. If a database has a transaction log mirror, every database change is written to both the transaction log and the transaction log mirror.
A transaction log mirror is required for production systems for extra protection of high-volume or critical application data. It enables complete data recovery if a media failure on the transaction log occurs, thus ensuring the durability of committed transactions. Without a transaction log mirror, recovering back to a point in time may require all clients to recreate their databases and re-synchronize (and they may lose non-uploaded changes). To ensure maximum availability, you should also mirror data (non-transaction log) devices.
By default, databases on the data tier do not have transaction log mirrors enabled. Mirroring might degrade performance, depending on the nature and volume of database traffic and on the physical configuration of the database and logs.
To avoid losing both the log and the database file in the event of failure, SAP suggests that you keep the mirror on a different physical disk.