Clustered RAID Arrays and Their Access Costs

Thomasian, Alexander

doi:10.1093/comjnl/bxh108

Cited by 24 publications

(7 citation statements)

References 24 publications

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“…BIBD has the shortcoming that it is not available for all values of N and α [33]. RAID5 and RAID6 disk utilizations for combinations of read and write requests with CRAID are given in [30], which served as a starting point for computing their performance [31].…”

Section: Methods To Improve Rebuild Performancementioning

confidence: 99%

“…When all disk requests are reads then when a fraction 1 − f of the data on a failed disk is materialized then the increase in the disk load is (1 + f )ρ, where ρ is the initial disk utilization in normal mode. There is a reduction in the load for write requests as well, which is quantified in [27,29,30,31].…”

Section: Raid5 Performancementioning

confidence: 99%

“…Requests processed in a RAID5 processing accesses to small blocks are classified as: Single Read (SR), Single Write (SW), and Read-Modify-Write (RMW) [27,29,30,31]. We shorten our discussion by assuming that all disk requests are SRs with service time x disk , which is the sum of seek time, rotational latency, and transfer time.…”

Section: Queueing Modeling Of Individual Disksmentioning

confidence: 99%

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Rebuild processing in RAID5 with emphasis on the supplementary parity augmentation method[37]

Thomasian¹

2012

SIGARCH Comput. Archit. News

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The rotated parity RAID5 disk array tolerates single disk failures by continuing operation by on-demand reconstruction of data blocks of the failed disk, until the systematic reconstruction of the contents of the failed disk is completed by the rebuild process on a spare disk. Supplementary Parity Augmentation (SPA), unlike the pyramid code, which has two parities covering half of the arrays disks each, extends RAID5's P parity with an additional S parity, which covers half of the disks. The extra load with respect to RAID5 of updating the S parity by one half of the disks is compensated by the more efficient on demand reconstructtion and rebuild processing when a disk fails. Although SPA has the same disk space redundancy level as RAID6, unlike RAID6 it can only deal with roughly half of all possible double disk failure cases for eight disks. For rebuild processing SPA reads half of the disks required by RAID5 and this leads to a higher Mean Time to Data Loss than RAID5, since fewer Latent Sector Errors are encountered. We review performance and reliability modeling of RAID5 arrays to provide insights into SPA's performance and reliability, which cannot be gained from numerical results alone. SPA is outperformed by the Intra-Disk Redundancy schemes combined with RAID5, which results in RAID6's reliability and RAID5 performance.,

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Section: Methods To Improve Rebuild Performancementioning

confidence: 99%

Section: Raid5 Performancementioning

confidence: 99%

Section: Queueing Modeling Of Individual Disksmentioning

confidence: 99%

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Rebuild processing in RAID5 with emphasis on the supplementary parity augmentation method[37]

Thomasian¹

2012

SIGARCH Comput. Archit. News

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“…The load in clustered RAID5 and RAID6 is obtained using decision trees in Thomasian 2005b [228]. 213 Let x• denotes read, write, and Read-Modify-Write -RMW mean disk service times.…”

Section: Raid5 Operation In Degraded Modementioning

confidence: 99%

Reliability and Performance of Mirrored Disk Organizations

Thomasian

2008

The Computer Journal

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This is a followup to the 1994 tutorial by Berkeley RAID researchers whose 1988 RAID paper foresaw a revolutionary change in storage industry based on advances in magnetic disk technology, i.e., replacement of large capacity expensive disks with arrays of small capacity inexpensive disks. NAND flash SSDs which use less power, incur very low latency, provide high bandwidth, and are more reliable than HDDs are expected to replace HDDs as their prices drop. Replication in the form of mirrored disks and erasure coding via parity and Reed-Solomon codes are two methods to achieve higher reliability through redundancy in disk arrays. RAID(4+k), k=1,2,... arrays utilizing k check strips makes them k-disk-failure-tolerant with maximum distance separable coding with minimum redundancy. Clustered RAID, local recovery codes, partial MDS, and multilevel RAID are proposals to improve RAID reliability and performance. We discuss RAID5 performance and reliability analysis in conjunction with HDDs w/o and with latent sector errors -LSEs, which can be dealt with by intradisk redundancy and disk scrubbing, the latter enhanced with machine learning algorithms. Undetected disk errors causing silent data corruption are propagated by rebuild. We utilize the M/G/1 queueing model for RAID5 performance evaluation, present approximations for fork/join response time in degraded mode analysis, and the vacationing server model for rebuild analysis. Methods and tools for reliability evaluation with Markov chain modeling and simulation are discussed. Queueing and reliability analysis are based on probability theory and stochastic processes so that the two topics can be studied together. Their application is presented here in the context of RAID arrays in a tutorial manner.

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“…With K + 1 disks and a parity group size G, the declustering ratio α = (G − 1)/K is a measure of increased read load, which is 100% for G = K + 1. The load increase for a mixture of read and update requests is given in [7]. The Balanced Incomplete Block Designs (BIBD) clustered RAID implementation is one method to equalize the number of parity strips per disk [9].…”

Section: Introductionmentioning

confidence: 99%

Balancing disk access times in RAID5 disk arrays in degraded mode by conditionally prioritizing fork/join requests

Thomasian¹,

Liu²,

Deng³

2014

SIGARCH Comput. Archit. News

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RAID5 disk arrays with rotated parities can tolerate single disk failures by reconstructing missing blocks on demand by XORing the contents of corresponding K blocks on surviving disks by a K-way Fork/Join (F/J) request, which is considered completed after the K disks are accessed. F/J accesses in RAID5 are processed concurrently with interfering disk accesses. The mean response time of F/J and independent/interfering requests: R F/J K and R Ind and the mean delay from the completion of the first to the last F/J task, known as task dispersion time: T disp K , are performance metrics of interest. Given R F/J K > R Ind with FCFS scheduling, it is desirable to equalize disk access times, but giving a higher nonpreemptive priority to disk accesses due to F/J requests with respect to interfering disk accesses results in R Ind > R F/J K . We propose a continuum of conditional priority methods based on the fraction F of F/J accesses completed with FCFS scheduling. F = ∞ stands for FCFS and F = 0 stands for unconditional priorities. Simulation shows that F = 1/8 with K = 8 yields R F/J K ≈ R Ind for three distributions of disk requests and in the range of F/J and independent disk requests considered. F can be varied adaptively based on measurement results to balance disk access times.

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Clustered RAID Arrays and Their Access Costs

Cited by 24 publications

References 24 publications

Rebuild processing in RAID5 with emphasis on the supplementary parity augmentation method[37]

Rebuild processing in RAID5 with emphasis on the supplementary parity augmentation method[37]

Reliability and Performance of Mirrored Disk Organizations

Balancing disk access times in RAID5 disk arrays in degraded mode by conditionally prioritizing fork/join requests

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