I. Introduction
Due to the rapid development of internet and multimedia data bases, the need for a big storage system becomes more and more important. The advanced storage system must provide rapid data processing ability and stable storage environment in order to avoid data loss. Presently, take hard disk with IDE interface and 7200 RPM as an example, size of hard-disk in mainstream market is usually 120 GB ~ 200 GB per drive. The size is big enough for personal computer. However, the size is too small for a server used as a database. No matter in capacity or reliability, it is deficient. In order to overcome the problems about limitations of single disk capacity and data processing speed and reliability of data storage, there are many distributed systems and fault-tolerant methods being proposed. RAID system is mostly appreciated.