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CVMFS: Software Access Anywhere

CVMFS (CERN Virtual File System) is a network filesystem that offers a read-only POSIX interface and is designed for high-performance data access. It utilizes HTTP to fetch files, ensuring data integrity through aggressive caching and local disk usage. This guide covers installation, configuration, and usage of CVMFS and its integration with the Parrot virtual filesystem for non-root users. It also discusses the management of repositories, performance considerations, and integration with HTCondor to improve job scheduling and prevent failures due to CVMFS issues.

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CVMFS: Software Access Anywhere

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  1. CVMFS:Software Access Anywhere Dan Bradley dan@hep.wisc.edu Any data, Any time, Anywhere Project

  2. What is CVMFS? • Network filesystem • Read-only POSIX interface • FUSE mounted • Fetches files via http • Verifies data integrity • Aggressive caching • Local disk • Web proxies /cvmfs/MyRepo httpd0 httpd1 httpd2 SQUID SQUID SQUID SQUID SQUID SQUID local cache CVMFS FUSE client Dan Bradley

  3. Using Client Only • Install rpm • Tell it which http proxies to use • Allocate cache space • Enabledesired repositories See http://cernvm.cern.ch/portal/filesystem Dan Bradley

  4. What if I am not root? • Parrot Virtual Filesystem • No root privileges required • Works as job wrapper parrot_run /cvmfs/repo/MyProgram … See http://www.nd.edu/~ccl/software/parrot/ Dan Bradley

  5. Parrot Performance Cost • Experience in CMS: • For typical CMS user jobs, running under Parrot is not much slower • Your mileage may vary • Assume 5% performance hit until proven otherwise

  6. A glideinWMS Job Wrapper • If job requires CVMFS • Wraps job in parrot • Uses site squid, if possible • Otherwise, need a friendly squid at home • May limit scalability • Access control? • See https://github.com/dcbradley/parrot_glidein_wrapper Dan Bradley

  7. CVMFS Server • Needed to create your own repository • Lightweight service • Kernel module to detect updates • Program to prepare published files • httpd to serve files • Most I/O done by proxies • May also want a mirror server • httpd + periodic sync of repository files Dan Bradley

  8. Managing the Repository • Simple case: one software maintainer (cvmfs user) • Updates software tree • Triggers cvmfs publication step • New files show up on clients an hour later (or less) Dan Bradley

  9. Managing the Repository • More complicated scenario: implementing OSG_APP with CVMFS • There are many software maintainers • We don’t want them to have to trigger publication • Tried periodically running publication • Caused long delays and/or write errors to software maintainers operating at time of publication • Instead, using periodic rsync from user-maintained tree into cvmfs software tree • Then publish to cvmfs • Software maintainers are never blocked Dan Bradley

  10. Integrating with HTCondor • Problem: job runs and fails on machine with broken CVMFS • How to avoid such black holes: • startdcron job tests for working cvmfs • Publishes MyRepo_CVMFS_Exists = True • Actual expression: ifThenElse(isUndefined(LastHeardFrom),CurrentTime,LastHeardFrom) - 1352866188 < 3600 • True until test expires in 1 hour • Job requiresTARGET.MyRepo_CVMFS_Exists == True Dan Bradley

  11. Integration with HTCondor • Problem: job runs and fails on machine that does not yet see latest update • How to avoid this race condition: • startdcron job publishes catalog version:MyRepo_CVMFS_Revision = 4162 • Job should require execute node revision >= submit node revision • For OSG jobs,we dothis in condor.pm Dan Bradley

  12. Coming Soon in OSG? • OASIS project • Proposed OSG-maintained CVMFS server for VOs to use • Simplify software distribution across OSG sites • Install once, run everywhere • Could it be used by OSG-related campus grids? • I don’t see why not, but we should clarify • Would have to live within OASIS quota Dan Bradley

  13. Links • CVMFS website:http://cernvm.cern.ch/portal/filesystem • Parrot website:http://www.nd.edu/~ccl/software/parrot/ • Parrot CVMFS job wrapper for glideinWMShttps://github.com/dcbradley/parrot_glidein_wrapper • Scripts used to implement OSG_APP with CVMFS:http://www.hep.wisc.edu/cms/comp/ops/cvmfs.html Dan Bradley

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