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Wang, MingyangImproving Performance And Reliability Of Flash Memory Based Solid State Storage Systems
PhD, University of Cincinnati, 2016, Engineering and Applied Science: Computer Science and Engineering
Flash memory based Solid State Disk systems (SSDs) are becoming increasingly popular in enterprise applications where high performance and high reliability are paramount. While SSDs outperform traditional Hard Disk Drives (HDDs) in read and write operations, they pose some unique and serious challenges to I/O and file system designers. The performance of an SSD has been found to be sensitive to access patterns. Specifically read operations perform much faster than write ones, and sequential accesses deliver much higher performance than random accesses. The unique properties of SSDs, together with the asymmetric overheads of different operations, imply that many traditional solutions tailored for HDDs may not work well for SSDs. The close relation between performance overhead and access patterns motivates us to design a series of novel algorithms for I/O scheduler and buffer cache management. By exploiting refined access patterns such as sequential, page clustering, block clustering in a per-process per-file manner, a series of innovative algorithms on I/O scheduler and buffer cache can deliver higher performance of the file system and SSD devices. Other than the performance issues, SSDs also face some unique reliability challenges due to the natural properties of flash memory. Along with the well-known write-endurance, flash memory also suffers from read-disturb and write-disturb. Even repeatedly reading from an SSD may cause data corruption because the read voltage may stress neighboring memory cells. As the density of flash memory keeps increasing, the disturbing problems are becoming even more severe for memory cells to store data reliably. One of the structural merits of an SSD is its internal parallelism. Such parallelism of flash memory chips could be exploited to support data redundancy in a similar fashion to traditional HDD RAID. Recently an emerging non-volatile memory (NVM) such as PCM is receiving increasing research interest, as it outperforms flash memory by providing in-place update and better performance and reliability. Hybrid solutions, which combine both flash memory and NVM to balance performance and cost, are under special investigation to address the reliability and performance issues of flash memory based storage systems. To address the reliability concerns, we present a novel storage architecture called i-RAID (internal RAID) that introduces RAID-like parity-based redundancy while avoiding many of its problems. What make i-RAID so unique like no other are its deferred parity maintenance, selective RAID protection and dynamic RAID organization. It solves traditional RAID’s small update problem and avoids SSD RAID pitfalls. Unlike traditional disk drives, SSDs cannot perform in-place updates. We view this unique characteristic as an opportunity instead of a hurdle. The out-of-place update feature means that old data will not be over-written by the new data, which enables us to design some fundamentally new algorithms that defer the computing and updating of parity blocks until the garbage collection time, thereby significantly reducing the overhead and possibly increasing the life-time of SSDs. Our algorithms also dynamically and selectively construct parity stripes only on aged, error-prone blocks, and utilize the internal parallelism of SSDs to further improve performance.

Committee:

Yiming Hu, Ph.D. (Committee Chair); Kenneth Berman, Ph.D. (Committee Member); Karen Davis, Ph.D. (Committee Member); Wen-Ben Jone, Ph.D. (Committee Member); Carla Purdy, Ph.D. (Committee Member)

Subjects:

Computer Engineering

Keywords:

Flash Memory;RAID;Solid State Disk;Non Volatile Memory;Write Endurance;Read Write Disturb

Jones, Gregory RThey Fought the War Together: Southeastern Ohio's Soldiers and Their Families During the Civil War
PHD, Kent State University, 2013, College of Arts and Sciences / Department of History
Soldiers from southeastern Ohio and their families fought the Civil War (1861-1865) in a reciprocal relationship, sustaining one another throughout the course of the conflict. The soldiers needed support from their families at home. The families, likewise, relied upon the constant contact via letters for assurance that the soldiers were surviving and doing well in the ranks. This dissertation qualitatively examines the correspondence between soldiers and their families in southeastern Ohio, developing six major themes of analysis including early war patriotism, war at the front, war at home, political unrest at home, common religion, and the shared cost of the war. The source base for the project included over one thousand letters and over two hundred and fifty newspaper articles, all of which contribute to a sense of the mood of southeastern Ohioans as they struggled to fight the war together. The conclusions of the dissertation show that soldiers and their families developed a cooperative relationship throughout the war. This dissertation helps to provide a corrective to the overly romantic perspective on the Civil War that it was fought between divided families. Rather, Civil War soldiers and their families fought the war in shared suffering and in support of one another.

Committee:

Leonne Hudson (Advisor); Bradley Keefer (Committee Member); John Jameson (Committee Member); David Purcell (Committee Member); Willie Harrell (Committee Member)

Subjects:

American History; History; Military History

Keywords:

Civil War; Ohio; Southeastern Ohio; soldiers; Civil War soldiers; Union soldiers; civilians; northern civilians; northern home front; John Hunt Morgan; Morgans Raid; Athens Ohio; Marietta Ohio; Chillicothe, Ohio; Ohio Volunteer Infantry;

Kolla, Purushotham Pothu RajuParallel Garbage Collection in Solid State Drives
MS, University of Cincinnati, 2012, Engineering and Applied Science: Computer Engineering

Flash memories are making their way into both desktop and server environments. Over the years, the major limitation to the wide-adoption of flash memories has been their cost. However, with the advancements in the semiconductor industry, the price per gigabyte (GB) gap between the conventional disk drives and flash memories is getting closer. As such, flash memories can replace disks, where disk utilization is less and extra spindles are added just to increase performance. Though they ventured into the storage architecture as cache and as a hybrid counterpart with Hard Disk Drives (HDD), slowly they are expected to replace the disk drives in servers and super computers [1]. The other major drawback with flash memory is its inability to sustain unlimited erase cycles, which directly limits their lifetime [2]. In order to improve reliability, it is proposed to create redundancy [3].

Creating a Redundant Array of Independent Disks (RAID) is a conventional way of providing redundancy in hard disk drives (HDD) [4]. The same idea is adopted in Solid State Drives (SSD). In addition to the conventional RAID techniques that are implemented at the device level (external RAID), redundancy can be created in an SSD at a much lower level (internal RAID) [3]. The scope of this work is limited to internal RAID.

This work uses i-RAID [3]; an architecture and simulator for internal RAID as background and proposes two improvements. The first contribution is to improve the dynamic stripe formation using access patterns. Another enhancement is to utilize the idle domains when i-RAID is not active by invoking parallel instances of garbage collection.

This thesis describes how these methods can affect the performance of the device and explains how the internal parallelization of an SSD can be better exploited. Both the methods are evaluated individually and the findings are presented. Though both the methods have a great potential to improve the performance of the device, the earlier work (on which the current work is based) is done in such a way that exploiting access patterns during stripe formation could not provide much improvement.

Committee:

Yiming Hu, PhD (Committee Chair); Wen Ben Jone, PhD (Committee Member); Carla Purdy, PhD (Committee Member)

Subjects:

Computer Engineering

Keywords:

Solid State Drives;SSD;Garbage Collection;RAID;Flash Memory;Access Patterns;