With a large number of internal disks and the rapid growth of disk capacity, storage systems become more susceptible to double disk failures. Thus, the need for such reliable storage systems as RAID6 is expected to gain in importance. However RAID6 architectures such as RM2, P+Q, EVEN-ODD, and DATUM traditionally suffer from a low write I/O performance caused by updating two distinctive parity data associated with user data. To overcome such a low write I/O performance, we propose an enhanced RM2 architecture which combines RM2, one of the well-known RAID6 architectures, with a Lazy Parity Update (LPU) technique. Extensive performance evaluations reveal that the write I/O performance of the proposed architecture is about two times higher than that of RM2 under various I/O workloads with little degradation in reliability.

Under the condition that the communication time is relatively shorter than the computation time for a given task, the task duplication-based scheduling (TDS) algorithm proposed by S. Darbha and D.P. Agrawal (1998) generates an optimal schedule. In this paper, we propose an extended TDS algorithm whose optimality condition is less restricted and where the length of the generated schedule is shorter than the original TDS algorithm.

The task scheduling problem in distributed memory machines is to allocate the tasks of an application into processors in order to minimize the total execution time. This is known as an NP-complete problem. Under the condition where the communication time is relatively shorter than the computation time for every task, the task duplication based scheduling (TDS) algorithm proposed by Darbha and Agrawal (1998) generates an optimal schedule. We propose an extended TDS algorithm whose optimality condition is less restricted than the TDS algorithm. Given a DAG where the condition is met, our algorithm has the time complexity of O(|V|/sup 2/d/sup 2/) where |V| represents the number of tasks, and d represents the maximum degree of tasks.

There is a growing demand in high reliability beyond what current RAID can provide and there are various levels of user demand for data reliability. An efficient data placement scheme called RM2 has been proposed in [11], which makes a disk array system tolerable against double disk failures. In this paper, we consider how to choose an optimal striping unit for RM2 particularly when no workload information is available except read/write ratio. A disk array simulator for RM2 has been developed for experimental works. It is shown that RM2 has an optimal striping unit of two and half tracks in the case of disk read operations, and one third of a single track if any disk write operations are involved. Key words : data placement, disk array, performance, reliability, striping 1 Introduction In the past few years, the performance of processors has been growing steadily, doubling approximately every two years [6], and with the advance of parallel processing technology, the processi...