Solid-state drives (SSDs) have been widely deployed in desktops and data centers. However, SSDs suffer from bit errors, and the bit error rate is time dependent since it increases as an SSD wears down. Traditional storage systems mainly use parity-based RAID to provide reliability guarantees by striping redundancy across multiple devices, but the effectiveness of RAID in SSDs remains debatable as parity updates aggravate the wearing and bit error rates of SSDs. In particular, an open problem is that how different parity distributions over multiple devices, such as the even distribution suggested by conventional wisdom, or uneven distributions proposed in recent RAID schemes for SSDs, may influence the reliability of an SSD RAID array. To address this fundamental problem, we propose the first analytical model to quantify the reliability dynamics of an SSD RAID array. Specifically, we develop a "non-homogeneous" continuous time Markov chain model, and derive the transient reliability solution. We validate our model via trace-driven simulations and conduct numerical analysis to provide insights into the reliability dynamics of SSD RAID arrays under different parity distributions and subject to different bit error rates and array configurations. Designers can use our model to decide the appropriate parity distribution based on their reliability requirements.
RAID was first introduced in @cite_0 and has been widely used in many storage systems. Performance and reliability analysis on RAID in the context of hard disk drives has been extensively studied (e.g., see @cite_27 @cite_23 @cite_13 @cite_22 @cite_6 ). On the other hand, SSDs have a distinct property that their error rates increase as they wear down, so a new model is necessary to characterize the reliability of SSD RAID.
@cite_22 @cite_6 @cite_0 @cite_27 @cite_23 @cite_13
1965724639 2100923816 2147504831 1546442581 2149509970 1829547464
Abstract A reliability analysis of various disk array architectures (different levels of RAID) is performed. The dependence of reliability and mean time to data loss on various parameters of a disk array is characterized. A study of these characteristics reveals the impact of several design choices for a disk array on its reliability. Issues such as scalability of disk arrays, imperfect coverage of disk failures, cold versus hot spares, effect of predictive disk failures, and dependence of disk array reliability on data reconstruction time are studied. We present an analytic model to study the reliability of some important disk array organizations that have been proposed by others in the literature. These organizations are based on the combination of two options for the data layout, regular RAID-5 and block designs, and three alternatives for sparing: hot sparing, distributed sparing and parity sparing. Uncorrectable bit errors have big effects on reliability but are ignored in traditional reliability analysis of disk arrays. We consider both disk failures and uncorrectable bit errors in the model. The reliability of disk arrays is measured in terms of MTTDL (Mean Time To Data Loss). A unified formula of MTTDL has been derived for these disk array organizations. The MTTDLs of these disk array organizations are also compared using the analytic model. Increasing performance of CPUs and memories will be squandered if not matched by a similar performance increase in I O. While the capacity of Single Large Expensive Disks (SLED) has grown rapidly, the performance improvement of SLED has been modest. Redundant Arrays of Inexpensive Disks (RAID), based on the magnetic disk technology developed for personal computers, offers an attractive alternative to SLED, promising improvements of an order of magnitude in performance, reliability, power consumption, and scalability. This paper introduces five levels of RAIDs, giving their relative cost performance, and compares RAID to an IBM 3380 and a Fujitsu Super Eagle. Disk arrays (RAID) have been proposed as a possible approach to solving the emerging I O bottleneck problem. The performance of a RAID system when all disks are operational and the MTTF,,, (mean time to system failure) have been well studied. However, the performance of disk arrays in the presence of failed disks has not received much attention. The same techniques that provide the storage efficient redundancy of a RAID system can also result in a significant performance hit when a single disk fails. This is of importance since single disk failures are expected to be relatively frequent in a system with a large number of disks. In this paper we propose a new variation of the RAID organization that has significant advantages in both reducing the magnitude of the performance degradation when there is a single failure and can also reduce the MTTF,,,. We also discuss several strategies that can be implemented to speed the rebuild of the failed disk and thus increase the MTTF,,,. The efficacy of these strategies is shown to require the improved properties of the new RAID organization. An analysis is carried out to quantify the tradeoffs. In today's computer systems, the disk I O subsystem is often identified as the major bottleneck to system performance. One proposed solution is the so called redundant array of inexpensive disks (RAID). We examine the performance of two of the most promising RAID architectures, the mirrored array and the rotated parity array. First, we propose several scheduling policies for the mirrored array and a new data layout, group-rotate declustering, and compare their performance with each other and in combination with other data layout schemes. We observe that a policy that routes reads to the disk with the smallest number of requests provides the best performance, especially when the load on the I O system is high. Second, through a combination of simulation and analysis, we compare the performance of this mirrored array architecture to the rotated parity array architecture. This latter study shows that: 1) given the same storage capacity (approximately double the number of disks), the mirrored array considerably outperforms the rotated parity array; and 2) given the same number of disks, the mirrored array still outperforms the rotated parity array in most cases, even for applications where I O requests are for large amounts of data. The only exception occurs when the I O size is very large; most of the requests are writes, and most of these writes perform full stripe write operations. Fault tolerance requirements for near term disk array storage systems are analyzed. The excellent reliability provided by RAID Level five data organization is seen to be insufficient for these systems. The authors consider various alternatives - improved MTBF and MTTR times as well as smaller reliability groups and increased numbers of check disks per group - to obtain the necessary improved reliability. Two data organization schemes based on maximum distance separable error correcting codes are introduced. Several figures of merit are calculated using a standard Markov failure and repair model for these organizations. Based on these results, the multiple check disk approach to improved reliability is an excellent option.