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Enhancing Storage Protocols: Bridging the Information Gap in File Systems

This paper discusses bridging the gap in knowledge between file systems and storage protocols to address issues such as performance degradation, inefficient write operations, and lack of redundancy. It introduces E×RAID and Informed LFS, aiming to enhance storage interfaces by exposing critical performance and failure information. Key features like online expansion, dynamic parallelism, and flexible redundancy are explored as methods to improve manageability and functionality. Theoretical insights are backed by experimental performance evaluations on diverse hardware setups.

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Enhancing Storage Protocols: Bridging the Information Gap in File Systems

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  1. Bridging the Information Gapin Storage Protocol Stacks Timothy E. Denehy, Andrea C. Arpaci-Dusseau, and Remzi H. Arpaci-Dusseau University of Wisconsin, Madison

  2. State of Affairs File System Storage System Namespace, Files, Metadata, Layout, Free Space Block Based, Read/Write Parallelism, Redundancy Interface

  3. Problem • Information gap may cause problems • Poor performance • Partial stripe write operations • Duplicated functionality • Logging in file system and storage system • Reduced functionality • Storage system lacks knowledge of files • Time to re-examine the division of labor

  4. Informed LFS Exposed RAID Our Approach • Enhance the storage interface • Expose performance and failure information • Use information to provide new functionality • On-line expansion • Dynamic parallelism • Flexible redundancy

  5. Outline • ERAID Overview • I·LFS Overview • Functionality and Evaluation • On-line expansion • Dynamic parallelism • Flexible redundancy • Lazy redundancy • Conclusion

  6. ERAID Goals • Backwards compatibility • Block-based interface • Linear, concatenated address space • Expose information to the file system above • Regions • Performance • Failure • Allow file system to utilize semantic knowledge

  7. ERAID Regions • Region • Contiguous portion of the address space • Regions can be added to expand the address space • Region composition • RAID: One region for all disks • Exposed: Separate regions for each disk • Hybrid ERAID

  8. ERAID Performance Information • Exposed on a per-region basis • Queue length and throughput • Reveals • Static disk heterogeneity • Dynamic performance and load fluctuations ERAID

  9. ERAID Failure Information • Exposed on a per-region basis • Number of tolerable failures • Reveals • Static differences in failure characteristics • Dynamic failures to file system above ERAID X RAID1

  10. Outline • ERAID Overview • I·LFS Overview • Functionality and Evaluation • On-line expansion • Dynamic parallelism • Flexible redundancy • Lazy redundancy • Conclusion

  11. I·LFS Overview • Log-structured file system • Transforms all writes into large sequential writes • All data and metadata is written to a log • Log is a collection of segments • Segment table describes each segment • Cleaner process produces empty segments • Why use LFS for an informed file system? • Write-anywhere design provides flexibility • Ideas applicable to other file systems

  12. I·LFS Overview • Goals • Improve performance, functionality, and manageability • Minimize system complexity • Exploits ERAID information to provide • On-line expansion • Dynamic parallelism • Flexible redundancy • Lazy redundancy

  13. I·LFS Experimental Platform • NetBSD 1.5 • 1 GHz Intel Pentium III Xeon • 128 MB RAM • Four fast disks • Seagate Cheetah 36XL, 21.6 MB/s • Four slow disks • Seagate Barracuda 4XL, 7.5 MB/s

  14. I·LFS Baseline Performance • Four slow disks: 30 MB/s • Four fast disks: 80 MB/s

  15. Outline • ERAID Overview • I·LFS Overview • Functionality and Evaluation • On-line expansion • Dynamic parallelism • Flexible redundancy • Lazy redundancy • Conclusion

  16. I·LFS On-line Expansion • Goal: Expand storage incrementally • Capacity • Performance • Ideal: Instant disk addition • Minimize downtime • Simplify administration • I·LFS supports on-line addition of new disks

  17. I·LFS On-line Expansion Details • ERAID: Expandable address space • Expansion is equivalent to adding empty segments • Start with an oversized segment table • Activate new portion of segment table

  18. I·LFS On-line Expansion Experiment • I·LFS immediately takes advantage of each extra disk

  19. I·LFS Dynamic Parallelism • Goal: Perform well on heterogeneous storage • Static performance differences • Dynamic performance fluctuations • Ideal: Maximize throughput of the storage system • I·LFS writes data proportionate to performance

  20. I·LFS Dynamic Parallelism Details • ERAID: Dynamic performance information • Most file system routines are not changed • Aware of only the ERAID linear address space • Reduces file system complexity • Segment selection routine • Aware of ERAID regions and performance • Chooses next segment based on current performance

  21. I·LFS Static Parallelism Experiment • Simple striping limited by the rate of the slowest disk • I·LFS provides the full throughput of the system

  22. I·LFS Dynamic Parallelism Experiment • I·LFS adjusts to the performance fluctuation

  23. I·LFS Flexible Redundancy • Goal: Offer new redundancy options to users • Ideal: Range of mechanisms and granularities • I·LFS provides mirroredper-file redundancy

  24. I·LFS Flexible Redundancy Details • ERAID: Region failure characteristics • Use separate files for redundancy • Even inode N for original files • Odd inode N+1 for redundant files • Original and redundant data in different sets of regions • Flexible data placement within the regions • Use recursive vnode operations for redundant files • Leverage existing routines to reduce complexity

  25. I·LFS Flexible Redundancy Experiment • I·LFS provides a throughput and reliability tradeoff

  26. I·LFS Lazy Redundancy • Goal: Avoid replication performance penalty • Ideal: Replicate data immediately before failure • I·LFS offers redundancy with delayed replication • Avoids replication penalty for short-lived files

  27. I·LFS Lazy Redundancy • ERAID: Region failure characteristics • Segments needing replication are flagged • Cleaner acts as replicator • Locates flagged segments • Checks data liveness and lifetime • Generates redundant copies of files

  28. I·LFS Lazy Redundancy Experiment • I·LFS avoids performance penalty for short-lived files

  29. Outline • ERAID Overview • I·LFS Overview • Functionality and Evaluation • On-line expansion • Dynamic parallelism • Flexible redundancy • Lazy redundancy • Conclusion

  30. Comparison with Traditional Systems • On-line expansion • Yes • Dynamic parallelism (heterogeneous storage) • Yes, but with duplicated functionality • Flexible redundancy • No, the storage system is not aware of file composition • Lazy redundancy • No, the storage system is not aware of file deletions

  31. Conclusion • Introduced ERAID and I·LFS • Extra information enables new functionality • Difficult or impossible in traditional systems • Minimal complexity • 19% increase in code size • Time to re-examine the division of labor

  32. Questions? http://www.cs.wisc.edu/wind/

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