1. A system for backing up cache memory in a double backup server structure, said system comprises, a first backup server having at least a first SAS controller, said first SAS controller having a sending end initiator defining a mirror command for mirroring cache memory and a flush command for flushing cache memory, a first channel for accepting write commands and write data from a host system, a first cache memory, and at least a first hard drive; a second backup server having at least a second SAS controller, said second SAS controller having a sending end initiator defining a mirror command for mirroring cache memory and a flush command for flushing cache memory, and a second cache memory; a SAS channel for connecting said first SAS controllers and said second SAS controller; wherein in response to a write command and write data received from said host system through said first channel, said first backup server writes said data into said first cache memory, and said first SAS controller establishes a first CDB with an operation code of a cache mirror command as well as a first data header for said data, said first CDB and data header are subsequently transmitted through said SAS channel to said second SAS controller and said second backup server writes said data into said second cache memory, in response to said data in said first cache memory being completely stored in said first hard drive, said first SAS controller establishes a second CDB with an operation code of a flush command as well as a second data header for said data, said second CDB and data header are subsequently transmitted through said SAS channel to said second SAS controller and said second SAS controller flushes said data stored in said second cache memory in response to said second data header.