"unless mirroring gets suspended, then log grows because log records can't be discarded."

Hum.....

Can not be discarted?

Suppose this:

I have an instance of SQL Server with a database named test1, this database is in other instance as mirror.

Daily backups to log are happening have 10 min. When the backup from the log is made, the information is not discarded as it happens with normal full reconvery model?