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X-RAY : A Non-Invasive Exclusive Caching Mechanism for RAIDs

This paper introduces X-RAY, a non-invasive exclusive caching mechanism for RAIDs that improves performance by predicting and identifying exclusive blocks in the file system cache. The X-RAY model observes disk traffic and uses semantic knowledge to build a nearly exclusive cache without interface changes. The results show that X-RAY outperforms other caching mechanisms in terms of cache hit ratio and latency reduction.

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X-RAY : A Non-Invasive Exclusive Caching Mechanism for RAIDs

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  1. Lakshmi N. Bairavasundaram Muthian Sivathanu Andrea C. Arpaci-Dusseau Remzi H. Arpaci-Dusseau X-RAY: A Non-Invasive Exclusive Caching Mechanism for RAIDs ADvanced Systems Laboratory Computer Sciences Department University of Wisconsin – Madison

  2. Host Application File system cache RAID RAID cache ....... Introduction • Caching in modern systems • Multiple levels • Storage: 2-level hierarchy • Level 1: File system (FS) cache • Software-managed • Main memory of host/client • LRU-like cache replacement • Level 2: RAID cache • Firmware-managed • Memory inside RAID system • Usually LRU replacement

  3. Read Block no. 10 Read Block no. 10 23 …….. 45 23 …….. 45 10 Introduction – contd. • LRU • Replace LRU block • Cache placement on read LRU 39 MRU

  4. Read Block no. 10 Read Block no. 10 Read Block no. 10 LRU …. 10 11 12 MRU LRU …. 10 11 12 MRU Introduction – contd. • LRU • Replace LRU block • Cache placement on read • 2 levels of LRU • Redundant contents FS Cache LRU …….. 10 MRU RAID Cache …….. MRU 10 LRU

  5. Introduction – contd. • LRU • Cache placement on read • Replace LRU block • 2 levels of LRU • Redundant contents • Goal: • Exclusive caching FS Cache LRU …. 10 11 12 MRU RAID Cache LRU …. 10 11 12 MRU

  6. Improved RAID Caching Multi-Queue (Zhou et al. 2001) Add frequency component to cache policy Not strictly exclusive! • DEMOTE (Wong and Wilkes 2002) • Change interface to disk • File system issues “cache place” command • Has perfect information and hence perfectly exclusive caches • Interface changes – difficult to deploy

  7. Ideal RAID Cache • Exclusive caching • File system and RAID caches should have different contents • Global LRU • Known to work well • RAID cache should be a victim cache • No interface changes FS Cache …. MRU Victim Block RAID Cache Block Read …… LRU

  8. X-RAY • Observes disk traffic • Reads and writes to data and metadata • Builds a model of the FS cache • Uses semantic knowledge • Predicts size and contents of FS cache • Identifies set of exclusive blocks • Recent victims of the FS cache • Reads blocks from disk into cache • Result • A nearly exclusive cache without interface changes Host File system cache RAID X-RAY Model of FS cache RAID cache

  9. Talk Outline Introduction File Systems Information and Inferences X-RAY Cache Design Results Conclusion

  10. File System Operation • Applications perform file reads and writes • File system (Unix) • Translates file accesses to disk block requests • Metadata • To maintain application data on disk and manage disk blocks • Periodically written to disk • Examples: inodes, bitmap blocks

  11. File System Operation • Inode • Pointers to data blocks • File access information Latest access time File Inode Pointers to data blocks Data Blocks

  12. File System Operation • File access • Use inode to obtain pointers to disk data blocks • Read corresponding blocks from disk if they are not in FS cache • Update the access time information in inode • Metadata updates • Periodically check for “dirty” inodes and write to disk

  13. The Problem • To observe disk traffic and infer the contents of FS cache • Why difficult? • FS cache size changes over time • Shares main memory with virtual memory system

  14. The Problem • To observe disk traffic and infer the contents of FS cache • Why difficult? • FS cache size changes over time • Disk cannot observe all FS-level accesses 12 10 11 Read block: FS Cache LRU 12 11 10 MRU Disk Read RAID FS Cache Model 11 12 10 MRU LRU

  15. The Problem • To observe disk traffic and infer the contents of FS cache • Why difficult? • FS cache size changes over time • Disk cannot observe all FS-level accesses 13 10 Read block: FS Cache LRU 10 11 12 MRU Disk Read RAID FS Cache Model 10 11 12 MRU LRU

  16. The Problem • To observe disk traffic and infer the contents of FS cache • Why difficult? • FS cache size changes over time • Disk cannot observe all FS-level accesses Read block: FS Cache LRU 12 10 13 MRU RAID FS Cache Model 11 12 13 MRU LRU

  17. The Problem • To observe disk traffic and infer the contents of FS cache • Why difficult? • FS cache size changes over time • Disk cannot observe all FS-level accesses • Key observation • We need information about accesses that hit in FS cache • File system maintains access information in inodes Read block: FS Cache LRU 12 10 13 MRU RAID FS Cache Model 11 12 13 MRU LRU

  18. Talk Outline • Introduction • File Systems • Information and Inferences • X-RAY Cache Design • Results • Conclusion

  19. Information Obtain information from observing disk traffic Knowledge of file system structures and operations File system maintains time of last access in inodes Periodic inode writes Assuming whole file access, all blocks are in FS cache Assume file system cache policy is LRU

  20. Inferences Read for data block Block will be placed in file system cache (MRU block) • Read for previously read data block • Block became victim in file system cache • Blocks with an earlier access time should also be victims • Inode write: new access time , no disk read observed • All blocks belonging to file are in FS cache • Other blocks with later access time should also be present

  21. Talk Outline • Introduction • File Systems • Information and Inferences • X-RAY Cache Design • Results • Conclusion

  22. Design Recency list (R-list) List of data blocks ordered by access time Cache Begin (CB) pointer Divides R-list into inclusive and exclusive regions RAID Cache contents Subset of blocks in exclusive region B, 1 C, 2 D, 3 E, 3 F, 5 Block number Access time MRU A, 1 LRU Inclusive region Exclusive region CB Blocks the RAID should cache Blocks expected to be in FS cache

  23. Disk Read Read Block ‘D’ ; time = 6 LRU MRU A , 1 B , 1 C , 2 D , 3 E , 3 F , 4 Inclusive region Exclusive region CB

  24. Disk Read Read Block ‘D’ ; time = 6 LRU MRU A , 1 B , 1 C , 2 D , 3 E , 3 F , 4 Exclusive region Inclusive region CB

  25. Disk Read Read Block ‘D’ ; time = 6 LRU MRU A , 1 B , 1 C , 2 D , 6 E , 3 F , 4 Exclusive region Inclusive region CB

  26. Inode Write – Access time change Inode “23” : access time = 6 Semantic knowledge Inode “23” == blocks D & E Blocks D, E : access time = 6 LRU MRU A , 1 B , 1 C , 2 D , 3 E , 4 F , 5 G , 7 Exclusive region Inclusive region CB

  27. Inode Write – Access time change Inode “23” : access time = 6 Blocks D, E : access time = 6 LRU MRU A , 1 B , 1 C , 2 D , 3 E , 4 F , 5 G , 7 Exclusive region Inclusive region CB

  28. Inode Write – Access time change Inode “23” : access time = 6 Blocks D, E : access time = 6 D , 6 E , 6 LRU MRU A , 1 B , 1 C , 2 F , 5 G , 7 Exclusive region Inclusive region CB

  29. X-RAY Cache RAID Cache (size = 2 blocks) LRU MRU A , 1 B , 1 C , 2 F , 5 D , 6 E , 6 G , 7 Exclusive region Inclusive region CB • Keep track of additions to window in exclusive region

  30. X-RAY Cache RAID Cache (size = 2 blocks) LRU MRU A , 1 B , 1 C , 2 F , 5 D , 6 E , 6 G , 7 Exclusive region Inclusive region CB • Read newly-added blocks from disk • Replace blocks no longer in the window • Additional disk bandwidth • Idle time, extra internal bandwidth, freeblock scheduling

  31. Talk Outline • Introduction • File Systems • Information and Inferences • X-RAY Cache Design • Results • Tracking FS Cache Contents • RAID Cache Performance • Conclusion

  32. Results – Tracking • Accurate size and content prediction • Highly responsive to FS cache size changes • Tolerates changes in inode write interval • Partial file reads • X-RAY performs well if percentage of partially accessed files is < 40% (typical traces have less than 30%)

  33. Results – Cache Performance • Performs better than LRU and Multi-Queue • Close to DEMOTE, in spite of imperfect information • Hit rate advantage translates to lower read latency

  34. Additional Results • File system cache policy is not LRU • Clock, 2Q • X-RAY performs nearly as well as before • It performs better than both LRU and Multi-Queue • Idle time requirements • X-RAY reads blocks into cache only during idle time • It performs well if idle time is greater than one-third of actual idle time observed in the trace • More in the paper …

  35. Conclusion Easy deployment is an important goal in developing technology Avoid interface changes – use non-invasive mechanisms Higher-level systems maintain various pieces of information about data they manage Provide low-level systems with basic semantic knowledge Semantic intelligence for managing RAID caches Use access information in metadata to track file system cache contents and cache exclusive blocks In spite of imperfect information, X-RAY performs nearly as well as changing the interface Semantically-smart Disk Systems Availability, security and performance improvements

  36. ADvanced Systems Laboratory (ADSL) Computer Sciences, University of Wisconsin-Madison http://www.cs.wisc.edu/adsl Questions ?

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