Studies of disk arrays tolerating two disk failures and a proposal for a heterogeneous disk array

There has been an explosion in the amount of generated data in the past decade. Online access to these data is made possible by large disk arrays, especially in the RAID (Redundant Array of Independent Disks) paradigm. According to the RAID level a disk array can tolerate one or more disk failures, so that the storage subsystem can continue operating with disk failure(s). RAID 5 is a single disk failure tolerant array which dedicates the capacity of one disk to parity information. The content on the failed disk can be reconstructed on demand and written onto a spare disk. However, RAID5 does not provide enough protection for data since the data loss may occur when there is a media failure (unreadable sectors) or a second disk failure during the rebuild process. Due to the high cost of downtime in many applications, two disk failure tolerant arrays, such as RAID6 and EVENODD, have become popular. These schemes use 2/N of the capacity of the array for redundant information in order to tolerate two disk failures. RM2 is another scheme that can tolerate two disk failures, with slightly higher redundancy ratio. However, the performance of these two disk failure tolerant RAID schemes is impaired, since there are two check disks to be updated for each write request. Therefore, their performance, especially when there are disk failure(s), is of interest. In the first part of the dissertation, the operations for the RAID5, RAID6, EVENODD and RM2 schemes are described. A cost model is developed for these RAID schemes by analyzing the operations in various operating modes. This cost model offers a measure of the volume of data being transmitted, and provides adevice-independent comparison of the efficiency of these RAID schemes. Based on this cost model, the maximum throughput of a RAID scheme can be obtained given detailed disk characteristic and RAID configuration. Utilizing M/G/1 queuing model and other favorable modeling assumptions, a queuing analysis to obtain the mean read response time is described. Simulation is used to validate analytic results, as well as to evaluate the RAID systems in analytically intractable cases. The second part of this dissertation describes a new disk array architecture, namely Heterogeneous Disk Array (HDA). The HDA is motivated by a few observations of the trends in storage technology. The HDA architecture allows a disk array to have two forms of heterogeneity: (1) device heterogeneity, i.e., disks of different types can be incorporated in a single HDA; and (2) RAID level heterogeneity, i.e., various RAID schemes can coexist in the same array. The goal of this architecture is (1) utilizing the extra resource (i.e. bandwidth and capacity) introduced by new disk drives in an automated and efficient way; and (2) using appropriate RAID levels to meet the varying availability requirements for different applications. In HDA, each new object is associated with an appropriate RAID level and the allocation is carried out in a way to keep disk bandwidth and capacity utilizations balanced. Design considerations for the data structures of HDA metadata are described, followed by the actual design of the data structures and flowcharts for the most frequent operations. Then a data allocation algorithm is described in detail. Finally, the HDA architecture is prototyped based on the DASim simulation toolkit developed at NJIT and simulation results of an HDA with two RAID levels (RAID 1 and RAIDS) are presented

Rebuild Strategies for Clustered Redundant Disk Arrays

RAID5 tolerates single disk failures by recreating lost data blocks on demand, but this results in the doubling of the load of surviving disks for pure read workload. This increase may be unacceptable if the original load was high. Clustered RAID (CRAID) with parity group size G smaller than the number of disks (G < N) was proposed so that the increase in load is α = (G − 1)/(N − 1) < 1, but this is at the cost of higher parity overhead 1/G. There have been two implementation of CRAID (i) the balanced incomplete block design – BIBD and (ii) the nearly random permutation layout – NRP data layouts. In this study we consider the latter implementation, since it provides more flexibility in varying α for a fixed N. Rebuild is a systematic reconstruction of the contents of the failed disk on a spare disk, which involves the reading of the rebuild units (say tracks) from a subset of surviving disks. We compare the effect of processing rebuild requests using the vacationing server model – VSM and the permanent customer model – PCM, which process rebuild requests at a lower or the same priority as user requests, respectively. We also investigate the effect of a control policy to ensure the progress of the rebuild process, since the spare disk may become a bottleneck in this case. The effect of various parameters on the completion time of rebuild processing and mean disk response time are also explored

Rebuild Strategies for Redundant Disk Arrays

RAID5 performance is critical while rebuild is in progress, since in addition to the increased load to recreate lost data on demand, there is interference caused by rebuild requests. We report on simulation results, which show that processing user requests at a higher, rather than the same priority as rebuild requests, results in a lower response time for user requests, as well as reduced rebuild time. Several other parameters related to rebuild processing are also explored.

Design of a Liquid‐Driven Laser Scanner with Low Voltage Based on Liquid‐Infused Membrane

Laser energy is commonly used in tissue ablation, wound suturing, and other precise manipulations during surgery. However, currently available laser scanners require further improvements in terms of miniaturization, driving voltage, and stability to steer the laser beam accurately within a constrained environment. Herein, the development of a liquid‐driven laser scanner installed on the end effector of a continuum endoscope to perform fast and reliable laser steering is proposed. The developed laser scanner is 7 mm in diameter and 7 mm in length, and it is actuated with a voltage lower than 15 V due to the liquid‐infused membrane. The miniature size and low driving voltage of the proposed laser scanner facilitate safe laser‐assisted surgery in confined spaces. A theoretical model is established to predict laser spot position quantitatively, and laser steering ability is also tested experimentally. The fiber‐delivered laser beam can be steered for 21.2° (±10.6°) with a standard deviation of 0.3° in 1000 cycles, demonstrating excellent stability. A laser steering speed of up to 27.3 mm s−1 and a reflection loss of less than 3.1% are achieved