Continuous Data Management on Tape-Based Tertiary Storage Systems

Video-on-Demand (VOD) servers are becoming feasible. These servers have voluminous data to store and manage. If only disk-based secondary storage systems are used to store and manage this huge amount of data the system cost would be extensively high. A tape-based tertiary storage system seems to be a reasonable solution to lowering the cost of storage and management of this continuous data. However, the usage of a tertiary storage system to store large continuous data introduces several issues. These are mainly the replacement policy on disks, the decomposition and the placement of continuous data chunks on tapes, and the scheduling of multiple requests for materializing objects from tapes to disks. In this paper we address these issues and we propose solutions based on some heuristics we experimented in a simulator. We first extend a replacement policy that has been proposed for a single user environment to a multi-user one with several servicing streams. We then study different policies for continuous object decomposition and chunks placement on tapes under different characteristics of the tertiary storage drives. Finally, we propose a scheduling algorithm for object materialization; this algorithm guarantees the materialization on disks of all chunks of an object at their service deadlines in a pipelined service. We present the results of some simulations we made to measure the impacts of our proposed algorithms on the average latency time of the system.