A transaction log mirror is an identical copy of the transaction log, maintained at the same time as 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. By default, databases do not have transaction log mirrors.
A transaction log mirror provides extra protection for critical data. It enables complete data recovery if a media failure on the transaction log occurs. A transaction log mirror also enables a database server to perform automatic validation of the transaction log on database startup.
It is recommended that you use a transaction log mirror when running high-volume or critical applications. For example, at a consolidated database in a SQL Remote setup, replication relies on the transaction log, and if the transaction log is damaged or becomes corrupt, data replication can fail.
If you are using a transaction log mirror, and an error occurs while trying to write to one of the logs (for example, if the disk is full), the database server stops. The purpose of a transaction log mirror is to ensure complete recoverability if a media failure occurs on either log device; this purpose would be lost if the server continued with a single transaction log.
You can specify the -fc option when starting the database server to implement a callback function when the database server encounters a file system full condition.
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