When a database using a transaction log mirror starts up, the database server performs a series of checks and automatic recovery operations to confirm that the transaction log and its mirror are not corrupt, and to correct some problems if corruption is detected.
On startup, the server checks that the transaction log and its mirror are identical by performing a full comparison of the two files; if they are identical, the database starts as usual. The comparison of log and mirror adds to database startup time.
If the database stopped because of a system failure, it is possible that some operations were written into the transaction log but not into the mirror. If the server finds that the transaction log and the mirror are identical up to the end of the shorter of the two files, the remainder of the longer file is copied into the shorter file. This produces an identical log and mirror. After this automatic recovery step, the server starts as usual.
If the check finds that the log and the mirror are different in the body of the shorter of the two, one of the two files is corrupt. In this case, the database does not start, and an error message is generated saying that the transaction log or its mirror is invalid.