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Filesytems and file access

Filesytems and file access. Wahid Bhimji University of Edinburgh, Sam Skipsey , Chris Walker …. This session:. 2 Themes: What are the demands made on storage by LHC data analysis – how might they change What solutions ( filesystems or other) help us to meet such demands.

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Filesytems and file access

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  1. Filesytems and file access Wahid Bhimji University of Edinburgh, Sam Skipsey, Chris Walker …. Wahid Bhimji – Files access

  2. This session: 2 Themes: • What are the demands made on storage by LHC data analysis – how might they change • What solutions (filesystems or other) help us to meet such demands Wahid Bhimji – Files access

  3. Demands - Data access • LHC data analysis at Tier2s involves large files read from servers over the network. • Significant load and bottlenecks on storage. Tuning is affected by : • File access pattern • File size (e.g. whether can be cached in RAM) • Access method (direct local access, copy to WN etc.) • And more… • Problems can be “fixed” by the VO or site. Wahid Bhimji – Files access

  4. Examples – rfio access A reminder of STEP last year (Sam/ Glasgow) • “Rfio” access • Large data files. • “random” file access • Large rfio buffers –> Disk server Memory exhausted, network saturated Solution Employed: • Smaller RFIO buffers and copy to WN But ….. Wahid Bhimji – Files access

  5. copy to WN – moves the problem .. • More recent tests using FILESTAGER • Disk seek count hits max for drive Solution: • SSDs (see Sams talk)? • More WN disks? Or …: Smaller files/ Sequential access ?.... Wahid Bhimji – Files access

  6. Heart of the matter - ROOT I/O • Fundamental to most LHC analysis… • Need not be the bad guy. Root Data structure – “Tree” has multiple “Entries” – (physics “events”) and “Branches” (Objects – e.g. a Track). Wahid Bhimji – Files access

  7. Problems • Order that elements of Tree written into file • Ordering by branches –compresses well - good for extracting a single branch • BUT: in experiments data, there are lots of branches, branches are uneven sized, buffers are fixed (and small) so • Branches for the same event are scattered in the file Wahid Bhimji – Files access

  8. Aside on testing • “ROOT Test” - Read through the Tree with Tree->GetEntry, observe pattern with TTreePerfStats • Athena / CMSSW - Real analysis jobs • Hammercloud tests - Multiple continuous athena jobs Wahid Bhimji – Files access

  9. “Random” access – ROOT test on Atlas AOD file • RFIO access (128k buffer) • Scatter in read • Poor cpu Efficiency (~ 15 % ) Wahid Bhimji – Files access

  10. Tuning / Filesystem can make a difference • GPFS: 3 x faster than rfio access (in this test for this file) • Buffering Wahid Bhimji – Files access

  11. CMS – similar picture Brian Bockelman https://twiki.cern.ch/twiki/bin/view/Sandbox/CmsIOWork Wahid Bhimji – Files access

  12. Reordering helps • Reordered ATLAS AOD Tree using CloneTree(-1,"SortBasketsByEntry"); • Factor 5 quicker in reading through Entries (or rfio) Wahid Bhimji – Files access

  13. “new” ATLAS files • Recentlyboth CMS and ATLAS have been optimising no. of branches and writing baskets in event order. • Files are clearly reordered - CPU eff = ~90% (for this ROOT test) Wahid Bhimji – Files access

  14. HammercloudLocal Access CPU eff / event rate Indicative results – many things can effect these – further testing needed http://gangarobot.cern.ch/hc/1243/test/ http://gangarobot.cern.ch/hc/1246/test/ Wahid Bhimji – Files access

  15. Copy to WN with reordered files • HDD < 120 Seeks/s Much Happier! Wahid Bhimji – Files access

  16. TTree Caching • Group into a buffer all blocks from used branches, sort in ascending order and merge so read sequentially. • Learn from the first few events what branches are of interest. • Claims to reduce “typically by a factor 10000 the number of transactions with the disk and in particular the network with servers like xrootdor dCache.” (vector reads) • Configuration still under investigation • Size of cache / training • Behavior with rfio – inc. possible segfaults. • Not yet used “by default” in ATLAS – config likely to be site specific Wahid Bhimji – Files access

  17. Shows further improvements e.g. Dcache- GuenterDuckeck Wahid Bhimji – Files access

  18. Conclusions • I/O pattern of software can cost money for site • Much of this is closely coupled to storage choices and configuration • Close feedback from sites to VOscan help access method (and site tuning) to become (and stay) optimal • Hotch-Potch of evidence – could do with systematic study Wahid Bhimji – Files access

  19. Memory <--> TreeEach Node is a branch in the Tree Memory T.GetEntry(6) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 T.Fill() 18 T Rene Brun - Sinaia Wahid Bhimji – Files access tr

  20. ROOT I/O -- Split/ClusterTree version Tree entries Streamer Branches Tree in memory File Wahid Bhimji – Files access

  21. Andrew Lahiff – RAL - CMS Reconstruction (4000 events) Skimming (4000 events) Wahid Bhimji – Files access

  22. Wahid Bhimji – Files access

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