NASA Scientific and Technical Aerospace Reports

holdings. A study released by the School of Information Management and Systems at the University of California, Berkeley,
estimates that over 5 exabytes of data was created in 2002. Almost 99 percent of this information originally appeared on
magnetic media. The theme for MSST2004 is therefore both timely and appropriate. There have been many discussions about
rapid technological obsolescence, incompatible formats and inadequate attention to the permanent preservation of knowledge
committed to digital storage. Tutorial sessions at MSST2004 detail some of these concerns, and steps being taken to alleviate
them. Over 30 papers deal with topics as diverse as performance, file systems, and stewardship and preservation. A number
of short papers, extemporaneous presentations, and works in progress will detail current and relevant research on the
MSST2004 theme.
Author
Information Management; Computer Networks; Metadata; Computer Storage Devices; Data Storage
20040121031 Houston Univ., TX, USA
Identifying Stable File Access Patterns
Shah, Purvi; Paris, Jehan-Francois; Amer, Ahmed; Long, Darrell D. E.; NASA/IEEE MSST 2004 Twelfth NASA Goddard
Conference on Mass Storage Systems and Technologies in cooperation with the Twenty-First IEEE Conference on Mass
Storage Systems and Technologies; April 2004, pp. 159-163; In English; See also 20040121020
Contract(s)/Grant(s): NSF CCR-99-88390; NSF ANI-03-25353; NSF CCR-02-04358; No Copyright; Avail: CASI; A01,
Hardcopy
Disk access times have not kept pace with the evolution of disk capacities, CPU speeds and main memory sizes. They
have only improved by a factor of 3 to 4 in the last 25 years whereas other system components have almost doubled their
performance every other year. As a result, disk latency has an increasingly negative impact on the overall performance of many
computer applications. Two main techniques can be used to mitigate this problem, namely caching and prefetching. Caching
keeps in memory the data that are the most likely to be used again while prefetching attempts to bring data in memory before
they are needed. Both techniques are widely implemented at the data block level. More recent work has focused on caching
and prefetching entire files. There are two ways to implement file prefetching. Predictive prefetching attempts to predict which
files are likely to be accessed next in order to read them before they are needed. While being conceptually simple, the approach
has two important shortcomings. First, the prefetching workload will get in the way of the regular disk workload. Second, it
is difficult to predict file accesses sufficiently ahead of time to ensure that the predicted files can be brought into main memory
before they are needed. A more promising alternative is to group together on the disk drive files that are often accessed at the
same time. This technique is known as implicit prefetching and suffers none of the shortcomings of predictive prefetching
because each cluster of files can now be brought into main memory in a single I/O operation. The sole drawback of this new
approach is the need to identify stable file access patterns in order to build long-lived clusters of related files. We present here
a new file predictor that identifies stable access patterns and can predict between 50 and 70 percent of next file accesses over
a period of one year. Our First Stable Successor keeps track of the successor of each individual file. Once it has detected m
successive accesses to file Y, each immediately following an access to file X, it predicts that file Y will always be the successor
of file X and never alters this prediction. The remainder of this paper is organized as follows. Section 2 reviews previous work
on file access prediction. Section 3 introduces our First Stable Successor predictor and Section 4 discusses its performance.
Finally, Section 5 states our conclusions.
Author (revised)
File Maintenance (Computers); Memory (Computers); Data Retrieval; Predictions
20040121032 Toronto Univ., Ontario, Canada
Clotho: Transparent Data Versioning at the Block I/O Level
Flouris, Michail D.; Bilas, Angelos; NASA/IEEE MSST 2004 Twelfth NASA Goddard Conference on Mass Storage Systems
and Technologies in cooperation with the Twenty-First IEEE Conference on Mass Storage Systems and Technologies; April
2004, pp. 315-328; In English; See also 20040121020; No Copyright; Avail: CASI; A03, Hardcopy
Recently storage management has emerged as one of the main problems in building cost effective storage infrastructures.
One of the issues that contribute to management complexity of storage systems is maintaining previous versions of data. Up
till now such functionality has been implemented by high-level applications or at the filesystem level. However, many modern
systems aim at higher scalability and do not employ such management entities as filesystems. In this paper we propose pushing
the versioning functionality closer to the disk by taking advantage of modern, block-level storage devices. We present Clotho,
a storage block abstraction layer that allows transparent and automatic data versioning at the block level. Clotho provides a
set of mechanisms that can be used to build flexible higher-level version management policies that range from keeping all data
modifications to version capturing triggered by timers or other system events. Overall, we find that our approach is promising
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