1 / 28

Journal-guided Resynchronization for Software RAID

Journal-guided Resynchronization for Software RAID. Timothy E. Denehy, Andrea C. Arpaci-Dusseau, and Remzi H. Arpaci-Dusseau University of Wisconsin, Madison. RAID Consistent Update Problem. RAID task is to maintain consistency Challenging in the face of crashes

raysalazar
Download Presentation

Journal-guided Resynchronization for Software RAID

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Journal-guided Resynchronizationfor Software RAID Timothy E. Denehy, Andrea C. Arpaci-Dusseau, and Remzi H. Arpaci-Dusseau University of Wisconsin, Madison

  2. RAID Consistent Update Problem • RAID task is to maintain consistency • Challenging in the face of crashes • Updates must be applied to more than one disk • Inconsistency means window of vulnerability • Disk failure may lead to data loss P P P P P P P P P P P P P P P P

  3. High-end RAID Solution • Consistent update with non-volatile memory • Logs writes in NVRAM until they reach disk • Performance – logging to NVRAM is fast • Reliability – data is safe in NVRAM • Availability – recovery is fast • But, enterprise systems are expensive

  4. Software RAID Solutions • Consistent update is challenging • Performance versus reliability trade-off • Performance: resynchronization after crash • Scan entire volume to fix inconsistencies • Extremely slow, hours for 100s of GBs to days for TBs • Reliability: lengthens window of vulnerability • Availability: consumes array bandwidth • Reliability: log intentions to a bitmap • Performance: extra writes to maintain bitmap

  5. Cooperative Software RAID Solution • Journaling file systems perform logging • Maintain file system data structure consistency • ext3, ReiserFS, JFS, NTFS • Journal-guided resynchronization • New ext3 mode: declared mode • New software RAID interface: verify read • Achieves performance, reliability, availability

  6. Journal-guided Resync Overview • Crash: What writes were outstanding? • Narrow the range of possible inconsistencies • Obtain information from journal (declared mode) • Restart: journal-guided resynchronization • Use journal to identify outstanding writes • Communicate locations to RAID (verify read) • Check redundancy and repair inconsistencies • Greatly reduce the time for resynchronization

  7. Outline • Problem • ext3 Background and Analysis • ext3 Declared Mode and RAID Verify Read • Journal-guided Resynchronization • Evaluation • Conclusion

  8. ext3 Modes • Data-journaling mode • All data and metadata is written to the journal • Ordered mode (default) • Only metadata is written to the journal • Strict ordering between data and metadata • Writeback mode • Only metadata is written to the journal • No ordering between data and metadata

  9. ext3 Transactions • Updates are grouped into transactions • Transaction states • Running – collect updates in memory • Commit – write updates to journal • Checkpoint – write updates to home locations

  10. ext3 Journal Structures • Journal superblock • Head and tail pointers into journal file • Transaction sequence number • Descriptor block • List of home locations for upcoming blocks • Commit block • Marks the end of a transaction

  11. Data-journaling Write Analysis Running Checkpointing Committing Commit: write desc, meta, data to journal, wait (bounded) write commit to journal, wait (bounded) Checkpoint: write journaled blocks to home, wait (known) update superblock (known) Running: collect file system updates in memory META DATA DATA DATA DATA DESC 11 META DATA DATA DATA DATA COMM 11 Journal Super P P P P P P P P P P P P P P P P

  12. Data-journaling Summary • Provides a record of all outstanding writes • Suitable for journal-guided resynchronization • Offers poor performance

  13. Ordered Write Analysis Running Committing Commit: write data to home, wait (unknown) write desc and meta to journal, wait (bounded) write commit to journal, wait (bounded) Running: collect file system updates in memory pdflush may write data to home (unknown) META DATA DATA DATA DATA DESC 11 META DATA COMM 11 Journal Super P P P P P P P P P P P P P P P P

  14. Ordered Summary • Does not provide outstanding write record • Unsuitable for journal-guided resynchronization

  15. Outline • Problem • ext3 Background and Analysis • ext3 Declared Mode and RAID Verify Read • Journal-guided Resynchronization • Evaluation • Conclusion

  16. Declared Mode • Variation of ordered mode • Only metadata is journaled, strict ordering • Declares its intent to write to home locations • New journal structure: declare block • List of home data locations for the transaction • Space and performance overheads

  17. Declared Write Analysis Running Committing Commit: write declare to journal, wait (bounded) write data to home, wait (known) write desc and meta to journal, wait (bounded) write commit to journal, wait (bounded) Running: collect file system updates in memory pdflush may write data to home (unknown) META DATA DATA DATA DATA DECL 11 DESC 11 META DATA COMM 11 Journal Super P P P P P P P P P P P P P P P P

  18. Software RAID Verify Read • File system must communicate possible inconsistencies to the software RAID layer • New interface: verify read request • Read block and verify its redundant information • Repair redundant information if inconsistent xor xor = ? P P P P P P P P P P P P P P P P P

  19. Outline • Problem • ext3 Background and Analysis • ext3 Declared Mode and RAID Verify Read • Journal-guided Resynchronization • Evaluation • Conclusion

  20. Journal-guided Resynchronization Recovery and Resynchronization: superblock write: verify read for superblock checkpointing: verify reads for descriptor home locations committing: verify reads for head of the journal home data writes: verify reads for declared home locations checkpoint committed transactions DECL 11 DESC 11 META DATA COMM 11 DECL 12 Journal Super P P P P P P P P P P P P P P P P

  21. Outline • Problem • ext3 Background and Analysis • ext3 Declared Mode and RAID Verify Read • Journal-guided Resynchronization • Evaluation • Conclusion

  22. Declared Mode Evaluation • Microbenchmarks (versus ordered mode) • Random write (3% slowdown) • Sequential write (5% slowdown) • Sprite create, read, unlink (4% slowdown) • Macrobenchmarks • ssh Benchmark (3% speedup for unpack) • Postmark (40% speedup - 5% slowdown) • Speedup from globally sorted write order • TPC-B (20% - 5% slowdown) • Small transaction size increases declare overhead

  23. Implementation Complexity • Cooperative approach reduces complexity

  24. Resynchronization Experiment • Five disk, 1 GB RAID-5 array • Foreground process reading a set of files • After 30 seconds, crash and restart machine • Resynchronization begins • Foreground process restarts • Monitor foreground bandwidth and resync

  25. Resynchronization Results • Availability: foreground BW from 29.6 to 34.1 MB/s • Reliability: vulnerability from 254 to 0.21 seconds • Reduced from O(array size) to O(journal size)

  26. Outline • Problem • ext3 Background and Analysis • ext3 Declared Mode and RAID Verify Read • Journal-guided Resynchronization • Evaluation • Conclusion

  27. Conclusion • RAID consistent updates are challenging • Analyzed ext3 journaling, declared mode • Identifies outstanding writes after a crash • Software RAID verify read interface • Journal-guided Resynchronization • Leverages functionality, reducing complexity • Provides performance, reliability, and availability • Cooperation between layers is the key

  28. Questions? http://www.cs.wisc.edu/adsl/

More Related