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.
SAP recommends that you enable transaction log mirror for extra protection of high-volume or critical application data. It enables complete data recovery if a media failure on the transaction log occurs.
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.