The next factor to consider in estimating libraries is the number of libraries needed to satisfy the maximum retrieval rate for objects on optical storage. The application work load (as estimated in Table 1) has a maximum retrieval rate of 100 objects per hour. Even if each retrieval results in mounting a different disk, one library is sufficient to satisfy this requirement. If you are making backup copies of objects, you need to plan for additional optical disks to accommodate them. The megabytes of data written per day is related to objects assigned to a management class that specifies an auto backup value of yes.