Redundant Optical Storage System Using DVD-RAM Library

A Digital virtual Disk (DVD) random access memory (RAM) Redundant Array of Inexpensive Libraries (RAIL) optical storage system has been developed and tested at NTT Integrated Information & Energy Systems Laboratories. The RAIL storage system incorporates multiple DVD libraries that consist of dual DVD-RAM drives. Each DVD library utilizes a single mechanical robot picker for media loading and unloading. The current capacity of the single sided and single layered DVD optical media used in that system is 2.6 gigabytes. To increase the reliability of stored data and at the same time to eliminate the need for read after write verification, a process that can double the recording time, a RAID 4 algorithm has been implemented in the control unit of the RAIL storage system. Data sent by the host are transferred to a control unit, that stripes data over five data groups plus one parity unit. The striped and parity data are sent to individual libraries and written to DVD media. This system writes and retrieves storage data with a transfer rate of approximate 6 MB/sec, using write and read control methods that minimize data striping overhead. Other performance factors that affect the transfer rates are the striping size and the number of drives used in the RAIL system. Experimental results indicate that stripe sizes of 32 to 64 KB are sufficient to achieve high throughput. In addition, the transfer rates showed no further increase when the number of drives exceeded eight. This RAIL optical storage system which offers data redundancy can be used for networked multimedia applications.