High-end enterprise storage has traditionally consisted of monolithic systems with customized hardware, multiple redundant components and paths, and no single point of failure. Distributed storage systems realized through networked storage nodes offer several advantages over monolithic systems such as lower cost and increased scalability. In order to achieve reliability goals associated with enterprise-class storage systems, redundancy will have to be distributed across the collection of nodes to tolerate both node and drive failures. In this paper, we present alternatives for distributing this redundancy, and models to determine the reliability of such systems. We specify a reliability target and determine the configurations that meet this target. Further, we perform sensitivity analyses, where selected parameters are varied to observe their effect on reliability.

Despite the reliability of modern disks, recent studies have made it clear that a new class of faults, UndetectedDisk Errors (UDEs) also known as silent data corruption events, become a real challenge as storage capacity scales. While RAID systems have proven effective in protecting data from traditional disk failures, silent data corruption events remain a significant problem unaddressed by RAID. We present a fault model for UDEs, and a hybrid framework for simulating UDEs in large-scale systems. The framework combines a multi-resolution discrete event simulator with numerical solvers. Our implementation enables us to model arbitrary storage systems and workloads and estimate the rate of undetected data corruptions. We present results for several systems and workloads, from gigascale to petascale. These results indicate that corruption from UDEs is a significant problem in the absence of protection schemes and that such schemes dramatically decrease the rate of undetected data corruption.

Today's data storage systems are increasingly adopting low-cost disk drives that have higher capacity but lower reliability, leading to more frequent rebuilds and to a higher risk of unrecoverable media errors. We propose an efficient intradisk redundancy scheme to enhance the reliability of RAID systems. This scheme introduces an additional level of redundancy inside each disk, on top of the RAID redundancy across multiple disks. The RAID parity provides protection against disk failures, whereas the proposed scheme aims to protect against media-related unrecoverable errors. In particular, we consider an intradisk redundancy architecture that is based on an interleaved parity-check coding scheme, which incurs only negligible I/O performance degradation. A comparison between this coding scheme and schemes based on traditional Reed--Solomon codes and single-parity-check codes is conducted by analytical means. A new model is developed to capture the effect of correlated unrecoverable sector errors. The probability of an unrecoverable failure associated with these schemes is derived for the new correlated model, as well as for the simpler independent error model. We also derive closed-form expressions for the mean time to data loss of RAID-5 and RAID-6 systems in the presence of unrecoverable errors and disk failures. We then combine these results to characterize the reliability of RAID systems that incorporate the intradisk redundancy scheme. Our results show that in the practical case of correlated errors, the interleaved parity-check scheme provides the same reliability as the optimum, albeit more complex, Reed--Solomon coding scheme. Finally, the I/O and throughput performances are evaluated by means of analysis and event-driven simulation.

High-end enterprise storage has traditionally consisted of monolithic systems with customized hardware, multiple redundant components and paths, and no single point of failure. Distributed storage systems realized through networked storage nodes offer several advantages over monolithic systems such as lower cost and increased scalability. In order to achieve reliability goals associated with enterprise-class storage systems, redundancy has to be distributed across the collection of nodes to tolerate both node and drive failures. In this paper, we present alternatives for distributing this redundancy, and models to determine the reliability of such systems. We specify a reliability target and determine the configurations that meet this target. Further, we perform sensitivity analyses where selected parameters are varied to observe their effect on reliability

The paper presents a method for detecting linear features in a satellite image. For enhancing these features, a local operation based algorithm is proposed which uses a certain multiplicative score rather than the commonly used additive score. The main contribution is in segmenting the linear features in the enhanced image where an adaptive segmentation technique is proposed on the basis of statistical models. A robust parameter estimation is done for this purpose.