17941466
Predicting when not to predict
File prefetching based on previous file access patterns has been shown to be an effective means of reducing file system latency by implicitly loading caches with files that are likely to be needed in the near future. Mistaken prefetching requests can be very costly in terms of added performance overheads, including increased latency and bandwidth consumption. Such costs of mispredictions are easily overlooked when considering access prediction algorithms only in terms of their accuracy; we describe a novel algorithm that uses machine learning not only to improve overall prediction accuracy, but also as a means to avoid those costly mispredictions. Our algorithm is fully adaptive to changing workloads, and is fully automated in its ability to refrain from offering predictions when they are likely to be mistaken. Our trace-based simulations show that our algorithm produces prediction accuracies of up to 98%. While this appears to be at the expense of a very slight reduction in cache hit ratios, application of this algorithm actually results in substantial reductions in unnecessary (and costly) I/O operations.
5159428
Ceph: a scalable, high-performance distributed file system
We have developed Ceph, a distributed file system that provides excellent performance, reliability, and scalability. Ceph maximizes the separation between data and metadata management by replacing allocation tables with a pseudo-random data distribution function (CRUSH) designed for heterogeneous and dynamic clusters of unreliable object storage devices (OSDs). We leverage device intelligence by distributing data replication, failure detection and recovery to semi-autonomous OSDs running a specialized local object file system. A dynamic distributed metadata cluster provides extremely efficient metadata management and seamlessly adapts to a wide range of general purpose and scientific computing file system workloads. Performance measurements under a variety of workloads show that Ceph has excellent I/O performance and scalable metadata management, supporting more than 250,000 metadata operations per second.
6779072
Strong Security for Network-Attached Storage
We have developed a scheme to secure network-attached storage systems against many types of attacks. Our system uses strong cryptography to hide data from unauthorized users; someone gaining complete access to a disk cannot obtain any useful data from the system, and backups can be done without allowing the super-user access to cleartext. While insider denial-of-service attacks cannot be prevented (an insider can physically destroy the storage devices), our system detects attempts to forge data. The system was developed using a raw disk, and can be integrated into common file systems. 
 
All of this security can be achieved with little penalty to performance. Our experiments show that, using a relatively inexpensive commodity CPU attached to a disk, our system can store and retrieve data with virtually no penalty for random disk requests and only a 15-20% performance loss over raw transfer rates for sequential disk requests. With such a minor performance penalty, there is no longer any reason not to include strong encryption and authentication in network file systems.
1463201
Reliability of MEMS-based storage enclosures
MEMS-based storage is a new, non-volatile storage technology currently under development. It promises fast data access, high throughput, high storage density, small physical size, low power consumption, and low entry costs. These properties make MEMS-based storage into a serious alternative to disk drives, in particular for mobile applications. The first generation of MEMS will only offer a fraction of the storage capacity of disks; therefore, we propose to integrate multiple MEMS devices into a MEMS storage enclosure, organizing them as a RAID Level 5 with multiple spares, to be used as the basic storage building block. The paper investigates the reliability of such an enclosure. We find that mean-time-to-failure is an inappropriate reliability metric for MEMS enclosures. We show that the reliability of the enclosures is appropriate for their economic lifetime if users choose not to replace failed MEMS storage components. In addition, we investigate the benefits of occasional, preventive maintenance of enclosures.
2306737
Managing flash crowds on the Internet
A flash crowd is a surge in traffic to a particular Web site that causes the site to be virtually unreachable. We present a model of flash crowd events and evaluate the performance of various multilevel caching techniques suitable for managing these events. By using well-dispersed caches and with judicious choice of replacement algorithms we show reductions in client response times by as much as a factor of 25. We also show that these caches eliminate the server and network hot spots by distributing the load over the entire network.
8965925
Power conservation strategies for MEMS-based storage devices
Power dissipation by storage systems in mobile computers accounts for a large percentage of the power consumed by the entire system. Reducing the power used by the storage device is crucial for reducing overall power consumption. A new class of secondary storage devices based on microelectromechanical systems (MEMS) promises to consume an order of magnitude less power with 10-20 times shorter latency and 10 times greater storage densities. We describe three strategies to reduce power consumption: aggressive spin-down; sequential request merging; sub-sector accesses. We show that aggressive spin-down can save up to 50% of the total energy consumed by the device at the cost of increased response time. Merging of sequential requests can save up to 18% of the servicing energy and reduce response time by about 20%. Transferring less data for small requests such as those for metadata can save 40% of the servicing energy. Finally, we show that by applying all three power management strategies simultaneously the total power consumption of MEMS-based storage devices can be reduced by about 54% with no impact on I/O performance.
2816085
Maximum entropy image restoration revisited
This paper presents a new non-iterative, closed-form approximation to the maximum entropy (ME) image restoration method. A fast frequency domain implementation of this closed form approach is developed for the case of circular convolutional blur. This result dramatically reduces the computational demands compared to conventional iterative ME algorithms such as MART. Some limitations and advantages of ME restoration are investigated, including its dismal performance for high resolution restoration of decimated or randomly sampled blurred observations.
2217946
In-place reconstruction of delta compressed files
results in high latency and low bandwidth to web-enabled clients and prevents the timely delivery of software. We present an algorithm for modifying delta compressed files so that the compressed versions may be reconstructed without scratch space. This allows network clients with limited resources to efficiently update software by retrieving delta compressed versions over a network. Delta compression for binary files, compactly encoding a version of data with only the changed bytes from a previous version, may be used to efficiently distribute software over low bandwidth channels, such as the Internet. Traditional methods for rebuilding these delta files require memory or storage space on the target machine for both the old and new version of the file to be reconstructed. With the advent of network computing and Internet-enabled devices, many of these network attached target machines have limited additional scratch space. We present an algorithm for modifying a delta compressed version file so that it may rebuild the new me version in the space that the current version occupies. Differential or delta compression [5, 11, compactly encoding a new version of a file using only the changed bytes from a previous version, can be used to reduce the size of the file to be transmitted and consequently the time to perform software update. Currently, decompressing delta encoded files requires scratch space, additional disk or memory storage, used to hold a required second copy of the file. Two copies of the compressed file must be concurrently available, as the delta file contains directives to read data from the old file version while the new file version is being materialized in another region of storage. This presents a problem. Network attached devices often have limited memory resources and no disks and therefore are not capable of storing two file versions at the same time. Furthermore, adding storage to network attached devices is not viable, as keeping these devices simple limits their production costs.
5757820
Persuasion as a Form of Inter-Agent Negotiation
Agents in a multi-agent environment must often cooperate to achieve their objectives. In this paper an agent, B, cooperates with another agent, A, if B adopts a goal that furthers A's objectives in the environment. If agents are independent and motivated by their own interests, cooperation cannot be relied upon and it may be necessary for A to persuade B to adopt a cooperative goal. This paper is concerned with the organisation and construction of persuasive argument, and examines how a rational agent comes to hold a belief, and thus, how new beliefs might be engendered and existing beliefs altered, through the process of argumentation. Argument represents an opportunity for an agent to convince a possibly sceptical or resistant audience of the veracity of its own beliefs. This ability is a vital component of rich communication, facilitating explanation, instruction, cooperation and conflict resolution. An architecture is described in which a hierarchical planner is used to develop discourse plans which can be realised in natural language using the LOLITA system. Planning is concerned with the intentional, contextual and pragmatic aspects of discourse structure as well as with the logical form of the argument and its stylistic organisation. In this paper attention is restricted to the planning of persuasive discourse, or monologue.