Disk Scrubbing Research Articles

Achieving high data reliability at low scrubbing cost via failure-aware scrubbing

Latent Sector Errors (LSEs) happen at a significant frequency in the field and can impose a huge risk to data reliability. Disk scrubbing is a background process that reads disks periodically to detect LSEs timely, thus shortening the window of vulnerability to data loss. Nowadays, proactive error prediction, using machine learning techniques, has been proposed to improve storage system reliability by increasing the scrubbing rate for disks with higher error rates. Unfortunately, the majority of works incur non-trivial scrubbing costs and overlook the relationship between complete disk failures and LSEs.In this paper, we attempt to maintain or improve data reliability at reduced scrubbing costs. In particular, we design a novel adaptive approach that enforces a lower scrubbing rate for healthy disks and a higher scrubbing rate for disks which are subject to LSEs. Besides LSEs that are specific to partial disk failures, we also adjust scrubbing rates according to complete disk failure rates, because disks typically develop LSEs before they finally fail. Moreover, a voting-based method that exploits the periodic characteristic of scrubbing is proposed to ensure prediction accuracy. Experimental results on a real-world field dataset have demonstrated the effectiveness of our proposed approach. Specifically, the results show that we can achieve the same level of reliability, in terms of Mean-Time-To-Detection (MTTD), as the traditional fixed-rate scrubbing scheme with almost 49% less scrubbing costs or we can improve the reliability by a factor of 2.4X without extra scrubbing costs. Compared with the state-of-the-art approaches, our method can achieve the same level of reliability with nearly 32% less scrubbing costs.

Disk Scrubbing Versus Intradisk Redundancy for RAID Storage Systems

Two schemes proposed to cope with unrecoverable or latent media errors and enhance the reliability of RAID systems are examined. The first scheme is the established, widely used, disk scrubbing scheme, which operates by periodically accessing disk drives to detect media-related unrecoverable errors. These errors are subsequently corrected by rebuilding the sectors affected. The second scheme is the recently proposed intradisk redundancy scheme, which uses a further level of redundancy inside each disk, in addition to the RAID redundancy across multiple disks. A new model is developed to evaluate the extent to which disk scrubbing reduces the unrecoverable sector errors. The probability of encountering unrecoverable sector errors is derived analytically under very general conditions regarding the characteristics of the read/write process of uniformly distributed random workloads and for a broad spectrum of disk scrubbing schemes, which includes the deterministic and random scrubbing schemes. We show that the deterministic scrubbing scheme is the most efficient one. We also derive closed-form expressions for the percentage of unrecoverable sector errors that the scrubbing scheme detects and corrects, the throughput performance, and the minimum scrubbing period achievable under operation with random, uniformly distributed I/O requests. Our results demonstrate that the reliability improvement due to disk scrubbing depends on the scrubbing frequency and the load of the system, and, for heavy-write workloads, may not reach the reliability level achieved by a simple interleaved parity-check (IPC)-based intradisk redundancy scheme, which is insensitive to the load. In fact, for small unrecoverable sector error probabilities, the IPC-based intradisk redundancy scheme achieves essentially the same reliability as that of a system operating without unrecoverable sector errors. For heavy loads, the reliability achieved by the scrubbing scheme can be orders of magnitude less than that of the intradisk redundancy scheme. Finally, the I/O and throughput performances are evaluated by means of analysis and event-driven simulation.

Modeling the Impact of Disk Scrubbing on Storage System

One of characteristics of the rising cloud storage technology is low-cost and high reliability. A distinct benefit of disk scanning or scrubbing operation is identifying the potential failure sectors as early as possible, thus providing high reliability. Obviously, the higher the scrubbing frequency is, the higher the system reliability is. However, it may take a few hours for a scanning process to check the whole disk. In other words, the scrubbing process may result in a downtime or a lower system performance. Furthermore, the scrubbing process consumes energy. In order to reduce the impact of disk scrubbing on disk performance and energy consumption, system designers choose to scan the disk in a low frequency, which results in a lower reliability. Thus it is essential to design a good scrubbing scheme in a large scale storage system over long time horizons. In this paper, we present a novel scrubbing scheme to solve the challenge. In this scheme, an optimum scrubbing cycle is decided by keeping a balance between data loss cost, scrubbing cost, and disk failure rate. Our research shows how the data price and the scrubbing cost affect scrubbing frequency, and the scrubbing scheme is applicable for storage with inexpensive data. Our experiment shows that our scheme outperforms routine method 73.3% in cost and 40% in reliability.

Higher reliability redundant disk arrays

Parity is a popular form of data protection in redundant arrays of inexpensive/independent disks (RAID) . RAID5 dedicates one out of N disks to parity to mask single disk failures, that is, the contents of a block on a failed disk can be reconstructed by exclusive-ORing the corresponding blocks on surviving disks. RAID5 can mask a single disk failure, and it is vulnerable to data loss if a second disk failure occurs. The RAID5 rebuild process systematically reconstructs the contents of a failed disk on a spare disk, returning the system to its original state, but the rebuild process may be unsuccessful due to unreadable sectors. This has led to two disk failure tolerant arrays (2DFTs) , such as RAID6 based on Reed-Solomon (RS) codes. EVENODD, RDP (Row-Diagonal-Parity), the X-code, and RM2 (Row-Matrix) are 2DFTs with parity coding. RM2 incurs a higher level of redundancy than two disks, while the X-code is limited to a prime number of disks. RDP is optimal with respect to the number of XOR operations at the encoding, but not for short write operations. For small symbol sizes EVENODD and RDP have the same disk access pattern as RAID6, while RM2 and the X-code incur a high recovery cost with two failed disks. We describe variations to RAID5 and RAID6 organizations, including clustered RAID, different methods to update parities, rebuild processing, disk scrubbing to eliminate sector errors, and the intra-disk redundancy (IDR) method to deal with sector errors. We summarize the results of recent studies of failures in hard disk drives. We describe Markov chain reliability models to estimate RAID mean time to data loss (MTTDL) taking into account sector errors and the effect of disk scrubbing. Numerical results show that RAID5 plus IDR attains the same MTTDL level as RAID6, while incurring a lower performance penalty. We conclude with a survey of analytic and simulation studies of RAID performance and tools and benchmarks for RAID performance evaluation.

Disk scrubbing versus intra-disk redundancy for high-reliability raid storage systems

Two schemes proposed to cope with unrecoverable or latent media errors and enhance the reliability of RAID systems are examined. The first scheme is the established, widely used disk scrubbing scheme, which operates by periodically accessing disk drives to detect media-related unrecoverable errors. These errors are subsequently corrected by rebuilding the sectors affected. The second scheme is the recently proposed intradisk redundancy scheme which uses a further level of redundancy inside each disk, in addition to the RAID redundancy across multiple disks. Analytic results are obtained assuming Poisson arrivals of random I/O requests. Our results demonstrate that the reliability improvement due to disk scrubbing depends on the scrubbing frequency and the workload of the system, and may not reach the reliability level achieved by a simple IPC-based intra-disk redundancy scheme, which is insensitive to the workload. In fact, the IPC-based intra-disk redundancy scheme achieves essentially the same reliability as that of a system operating without unrecoverable sector errors. For heavy workloads, the reliability achieved by the scrubbing scheme can be orders of magnitude less than that of the intra-disk redundancy scheme.