An algorithm for storage device selection and file assignment

Newly available write-once/read-many (WORM) optical storage devices provide the opportunity for storing massive amounts of data online at very low cost. However, the slow random access time of the WORM and its write-once limitation has, in general, restricted its application to archival storage and static files. In this paper the authors analyse a hybrid storage system which uses a combination of conventional magnetic disks and optical devices for management of files which have moderate volatility. The hybrid system appears to be particularly attractive for databases used for decision support. Cost models and solution algorithms are developed which determine a near-optimum database ‘storage plan’. A storage plan specifies the selection of device types and the assignment of files to devices. The solution approach uses a dynamic programming-based heuristic to obtain an initial solution followed by a set-covering algorithm which employs column generation to both search for an improved solution and to provide a tight lower bound. Computational results indicate that the method produces solutions with an average suboptimality of less than 1 percent. More importantly, this research demonstrates that a storage plan which uses a mix of device types can provide costs savings of up to 70% over a storage plan which is limited to conventional magnetic devices.