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Practical techniques for purging deleted data using liveness information

Published:01 July 2008Publication History
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Abstract

The layered design of the Linux operating system hides the liveness of file system data from the underlying block layers. This lack of liveness information prevents the storage system from discarding blocks deleted by the file system, often resulting in poor utilization, security problems, inefficient caching, and migration overheads. In this paper, we define a generic "purge" operation that can be used by a file system to pass liveness information to the block layer with minimal changes in the layer interfaces, allowing the storage system to discard deleted data. We present three approaches for implementing such a purge operation: direct call, zero blocks, and flagged writes, each of which differs in their architectural complexity and potential performance overhead. We evaluate the feasibility of these techniques through a reference implementation of a dynamically resizable copy on write (COW) data store in User Mode Linux (UML). Performance results obtained from this reference implementation show that all these techniques can achieve significant storage savings with a reasonable execution time overhead. At the same time, our results indicate that while the direct call approach has the best performance, the zero block approach provides the best compromise in terms of performance overhead and its semantic and architectural simplicity. Overall, our results demonstrate that passing liveness information across the file system-block layer interface with minimal changes is not only feasible but practical.

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            cover image ACM SIGOPS Operating Systems Review
            ACM SIGOPS Operating Systems Review  Volume 42, Issue 5
            Research and developments in the Linux kernel
            July 2008
            113 pages
            ISSN:0163-5980
            DOI:10.1145/1400097
            Issue’s Table of Contents

            Copyright © 2008 Authors

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            Association for Computing Machinery

            New York, NY, United States

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            • Published: 1 July 2008

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