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If your transaction log size continues to grow despite issuing BACKUP LOG or CHECKPOINT statements, check for a rogue transaction. Begin your research by sorting the Last Batch column on the SQL Processes grid into ascending mode such that the processes containing the oldest batches appear first. Scroll down the list until you find the first user process with a non-zero number of transactions in its Trans column, and then kill this process. Once this is done, perform a CHECKPOINT or BACKUP LOG to reduce the Log File Used Size.