RAIDX: RAID EXTENDED FOR HETEROGENEOUS ARRAYS

The computer hard drive market has diversified with the establishment of solid state disks (SSDs) as an alternative to magnetic hard disks (HDDs). Each hard drive technology has its advantages: the SSDs are faster than HDDs but the HDDs are cheaper. Our goal is to construct a parallel storage system with HDDs and SSDs such that the parallel system is as fast as the SSDs. Achieving this goal is challenging since the slow HDDs store more data and become bottlenecks, while the SSDs remain idle. RAIDX is a parallel storage system designed for disks of different speeds, capacities and technologies. The RAIDX hardware consists of an array of disks; the RAIDX software consists of data structures and algorithms that allow the disks to be viewed as a single storage unit that has capacity equal to the sum of the capacities of its disks, failure rate lower than the failure rate of its individual disks, and speeds close to that of its faster disks. RAIDX achieves its performance goals with the aid of its novel parallel data organization technique that allows storage data to be moved on the fly without impacting the upper level file system. We show that storage data accesses satisfy the locality of reference principle, whereby only a small fraction of storage data are accessed frequently. RAIDX has a monitoring program that identifies frequently accessed blocks and a migration program that moves frequently accessed blocks to faster disks. The faster disks are caches that store the solo copy of frequently accessed data. Experimental evaluation has shown that a HDD+SSD RAIDX array is as fast as an all-SSD array when the workload shows locality of reference

Stochastic Analysis on RAID Reliability for Solid-State Drives

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.Comment: 12 page

On the Ground in Afghanistan

The book includes 15 vignettes about different units from the U.S. Marines, U.S. Army, and U.S. Army Special Forces; the British army and marines; the Dutch army and marines; and the Canadian army. The case studies cover 10 provinces in the south and east of Afghanistan. They describe the diverse conditions the units faced in these provinces, how they responded to these conditions, what worked and what did not, and the successes involved in these operations

Beyond Respectability: Dismantling the Harms of Illegality

Current pro-immigrant reform efforts focus on legalization. Proposals seek to place as many of the eleven million undocumented people in the United States as possible on a “path to earned citizenship.” However, these reform efforts suffer from a significant and underappreciated blind spot: the strategies used to advocate legalization harm those to whom the path to citizenship is barred—such as those with prior deportation orders, prior criminal convictions, and those who have yet to arrive. The problem begins with rhetoric: in making the push for legalization, immigrant rights groups have deployed imagery of the undocumented as law-abiding, hard-working, and family-oriented—the ideal respectable candidates for an invitation into the protected sphere of citizenship. The flaw in this approach is evident in the comprehensive immigration reform bill passed by the Senate in 2013. While the bill would have provided additional safeguards for those who qualify for the path to legalization, it would have simultaneously rendered more vulnerable the millions of immigrants who do not qualify. For that latter group, the bill would have meant further criminalization of employment, increased border enforcement and deaths, and a cemented pipeline between local law enforcement, detention, and deportation. This Article proposes that the push for legalization is responsible for the legislative bait-and-switch, which appears to fix a broken system by offering legalization to some, but in fact makes the system worse for many. To avoid that result, advocates should avoid prioritizing legalization, and instead address the systemic harms related to the category of “illegality.” Pro-immigrant advocacy and scholarship should be guided by the question, “Will this intervention increase or decrease the harms related to living without lawful status?” Such a strategy would move the focus away from an individual’s eligibility for citizenship and towards issues that confront the most vulnerable among the undocumented. By addressing those most harmed by “illegality,” new opportunities emerge for crafting reforms that dismantle immigrant vulnerability

Foreign Intervention and Warfare in Civil Wars: The effect of exogenous resources on the course and nature of the Angolan and Afghan conflicts

This dissertation asks how foreign assistance to one or both sides in a civil war affects the dynamics of the conflict. This overarching question is subsequently divided into two further questions: 1) how does foreign intervention affect the capabilities of the recipient, and 2) how does this affect the nature of the warfare. The puzzle for the first is that the impact of foreign intervention on combat effectiveness frequently varies significantly between recipients. This variation is explained by recipients’ different abilities to convert the inputs of foreign intervention into the outputs of fighting capability. The nature of the warfare in civil war will change in line with the balance of military capabilities between the belligerents. The balance of capabilities will be responsible for the form of warfare at a particular place and time whether it be conventional, irregular or guerrilla/counter-guerrilla. The argument is then illustrated with two extensive case studies, of civil wars in Angola and Afghanistan, where temporal and spatial variation in the type of warfare is shown to correlate with the type, degree, and direction of foreign intervention

Taliban Networks in Afghanistan

Dr. Antonio Giustozzi relies on his extensive experience as a researcher in Afghanistan to create an insightful analysis of a wide range of topics including assessments of the Taliban’s strengths and weaknesses, their ability to reassess and adapt, and their operational and strategic successes and failures. He has presented a balanced treatment of the subject matter; however, balance does not mean that the case study will be uncontroversial. In fact, Giustozzi’s analysis contains some rather blunt appraisals of many of the major actors in this conflict; including both ISAF and the Taliban.https://digital-commons.usnwc.edu/ciwag-case-studies/1006/thumbnail.jp

Reliability of SSD Storage Systems

Solid-state drives (SSDs) are attractive storage components due to their many attractive properties, however, concerns about their reliability still remain and this delays the wider deployment of the SSDs. Many protection schemes have been proposed to improve the reliability of SSDs. For example, some techniques like error correction codes (ECC), log-like writing of ash translation layer (FTL), garbage collection and wear leveling improve the reliability of SSD at the device level. Composing an array of SSDs and employing system level parity protection is one of the popular protection schemes at the system level. Enterprise class (high-end) SSDs are faster and more resilient than client class (low-end) SSDs but they are expensive to be deployed in large scale storage systems. It is an attractive and practical alternative to exploit the high-end SSDs as a cache and low-end SSDs as main storage. The high-end SSD cache equipped on a low-end SSD array enhances both latency and reduces write count of the SSD storage system at the same time. This work analyzes the effectiveness of protection schemes originally designed for HDDs but applied to SSD storage systems. We find that different characteristics of HDDs and SSDs make integration of those solutions in SSD storage systems not so straight-forward. This work, at first, analyzes the effectiveness of the device level protection schemes such as ECC and scrubbing. A Markov model based analysis of the protection schemes is presented. Our model considers time varying nature of the reliability of ash memory as well as write amplification of various device level protection schemes. Our study shows that write amplification from these various sources can significantly affect the benefits of protection schemes in improving the lifetime. Based on the results from our analysis, we propose that bit errors within an SSD page be left uncorrected until a threshold of errors are accumulated. We show that such an approach can significantly improve lifetimes by up to 40%. This work also analyzes the effectiveness of parity protection over SSD arrays, a widely used protection scheme for SSD arrays at system level. The parity protection is typically employed to compose reliable storage systems. However, careful consideration is required when SSD based systems employ parity protection. Additional writes are required for parity updates. Also, parity consumes space on the device, which results in write amplification from less efficient garbage collection at higher space utilization. We present a Markov model to estimate the lifetime of SSD based RAID systems in different environments. In a small array, our results show that parity protection provides benefit only with considerably low space utilizations and low data access rates. However, in a large system, RAID improves data lifetime even when we take write amplification into account. This work explores how to optimize a mixed SSD array in terms of performance and lifetime. We show that simple integration of different classes of SSDs in traditional caching policies results in poor reliability. We also reveal that caching policies with static workload classifiers are not always efficient. We propose a sampling based adaptive approach that achieves fair workload distribution across the cache and the storage. The proposed algorithm enables fine-grained control of the workload distribution which minimizes latency over lifetime of mixed SSD arrays. We show that our adaptive algorithm is very effective in improving the latency over lifetime metric, on an average, by up to 2.36 times over LRU, across a number of workloads

Policing the Project: Crime, Carcerality, and Chicago Public Housing

This project examines how Chicago\u27s public housing was policed from 1937 to 2000, when the city announced plans to redevelop public housing into privately-owned mixed-income communities under the Plan for Transformation. Drawing upon interviews, historical newspapers, and archival records, it centrally argues that policing contributed to making public housing into a carceral space: one that resembled the prison in design and management and also funneled residents into the criminal-legal system. Writing against popular narratives of public housing as an inherent site of crime and violence, this project instead positions the police—and, by extension, the state—as a central contributor to violence in these spaces. That is, far from mitigating crime, policing often spurred it. Ultimately, the inability of police to effect law and order in public housing provided a justification for its ultimate demolition, resulting in the privatization of subsidized housing and the gentrification of former public housing neighborhoods. Thus, through detailed historical-geographical reconstruction of both everyday practices and extraordinary events, this study shows how policing, through violence enacted upon the housed urban poor, contributes to upholding and reproducing racial capitalist property relations in the contemporary U.S. city