The terabyte-level (10¹² bytes) database systems such as digital libraries need to use tape library as an online device to store and retrieve their massive data. Since the access of a tape library is often random and the tape library has low random access performance, thus it is critical to study the random I/O scheduling strategies and algorithms in order to improve the performance of tape library. In this paper we study a cost-benefit-based scheduling algorithm, and as well as give an effective estimating method of cost-benefit weight ratio.This algorithm improves the performance of tape library system under different workloads by dynamically tuning the cost-benefit weight ratio of scheduling policies according to workloads.This algorithm particularly overcomes the problem of workload-sensitive of existing scheduling algorithms,and is significantly effrctive under heavy workload.