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High Energy Physics Data Management Richard P. Mount Stanford Linear Accelerator Center DOE Office of Science Data Management Workshop, SLAC March 16 -18, 2004
The Science (1) • Understand the nature of the Universe (experimental cosmology? ) – Ba. Bar at SLAC (1999 on): measuring the matter-antimatter asymmetry – CMS and Atlas at CERN (2007 on): understanding the origin of mass and other cosmic problems
The Science (2) From the Fermilab Web • Research at Fermilab will address the grand questions of particle physics today. – Why do particles have mass? – Does neutrino mass come from a different source? – What is the true nature of quarks and leptons? Why are there three generations of elementary particles? – What are the truly fundamental forces? – How do we incorporate quantum gravity into particle physics? – What are the differences between matter and antimatter? – What are the dark particles that bind the universe together? – What is the dark energy that drives the universe apart? – Are there hidden dimensions beyond the ones we know? – Are we part of a multidimensional megaverse? – What is the universe made of? – How does the universe work?
Experimental HENP • • Large (500 – 2000 physicist) international collaborations 5 – 10 years accelerator and detector construction 10 – 20 years data-taking and analysis Countable number of experiments: – Alice, Atlas, Ba. Bar, Belle, CDF, CLEO, CMS, D 0, LHCb, PHENIX, STAR … • Ba. Bar at SLAC – – Measuring matter-antimatter asymmetry (why we exist? ) 500 Physicists Data taking since 1999 More data than any other experiment (but likely to overtaken by CDF, D 0 and STAR soon and will be overtaken by Alice, Atlas and CMS later)
Hydrogen Bubble Chamber Photograph 1970 CERN Photo
UA 1 Experiment, CERN 1982: Discovery of the W Boson (Nobel Prize 1983) CERN Photo
Ba. Bar Experiment at SLAC Taking data since 1999. Now at 1 TB/day rising rapidly Over 1 PB in total. Matter-antimatter asymmetry Understanding the origins of our universe
CMS Experiment: “Find the Higgs” ~10 PB/year by 2010
Characteristics of HENP Experiments 1980 – present God does play dice Large, (approaching worldwide) collaborations: 500 – 2000 physicists Large, complex detectors Long (10 – 20 year) timescales High statistics (large volumes of data) needed for precise physics Typical data volumes: 10000 n tapes (1 n 20)
HEP Data Models • HEP data models are complex! Event – Typically hundreds of structure types (classes) – Many relations between them Tracker – Different access patterns • Most experiments now rely on OO technology Track. List – OO applications deal with networks of objects – Pointers (or references) are used to describe relations Calor. Hit. List Track Track Hit Hit Hit Dirk Düllmann/CERN
Today’s HENP Data Management Challenges • Sparse access to objects in petabyte databases: – Natural object size 100 bytes – 10 kbytes – Disk (and tape) non-streaming performance dominated by latency – Approach - current: • Instantiate richer database subsets for each analysis application – Approaches – possible • Abandon tapes (use tapes only for backup, not for data-access) • Hash data over physical disks • Queue and reorder all disk access requests • Keep the hottest objects in (tens of terabytes of) memory • etc.
Today’s HENP Data Management Challenges • Millions of Real or Virtual Datasets: – Ba. Bar has a petabyte database and over 60 million “collections”. (lists of objects in the database that somebody found relevant) – Analysis groups or individuals create new collections of new and/or old objects – It is nearly impossible to make optimal use of existing collections and objects
Latency and Speed – Random Access
Latency and Speed – Random Access
Storage Characteristics – Cost Storage Hosted on Network Cost per PB ($M) net after RAID, hot spares etc. Cost per GB/s ($M) Streaming Random access to typically accessed objects Cost per GB/s ($M) Object Size Good Memory * 750 0. 001 0. 018 4 bytes Cheap Memory 250 0. 0004 0. 006 4 bytes Enterprise SAN maxed out 40 0. 4 8 5 kbytes High-quality fibrechannel disk * 10 0. 1 2 5 kbytes Tolerable IDE disk 5 0. 05 1 5 kbytes Robotic tape (STK 9480 C) 1 2 25 500 Mbytes 0. 4 2 50 500 Mbytes Robotic tape (STK 9940 B) * * Current SLAC choice
Storage-Cost Notes • Memory costs per TB are calculated: Cost of memory + host system • Memory costs per GB/s are calculated: (Cost of typical memory + host system)/(GB/s of memory in this system) • Disk costs per TB are calculated: Cost of disk + server system • Disk costs per GB/s are calculated: (Cost of typical disk + server system)/(GB/s of this system) • Tape costs per TB are calculated: Cost of media only • Tape costs per GB/s are calculated: (Cost of typical server+drives+robotics only)/(GB/s of this server+drives+robotics)
Storage Issues • Tapes: – Still cheaper than disk for low I/O rates – Disk becomes cheaper at, for example, 300 MB/s per petabyte for randomaccessed 500 MB files – Will SLAC every buy new tape silos?
Storage Issues • Disks: – Random access performance is lousy, independent of cost unless objects are megabytes or more – Google people say: “If you were as smart as us you could have fun building reliable storage out of cheap junk” – My Systems Group says: “Accounting for TCO, we are buying the right stuff”
Generic Storage Architecture Client Disk Server Tape Server Client Tape Server Disk Server Client Disk Server Tape Server
SLAC-Ba. Bar Storage Architecture Client Client IP Network (Cisco) Disk Server Tape Server 1500 dual CPU Linux 900 single CPU Sun/Solaris Objectivity/DB object database + HEP-specific ROOT software Disk Server IP Network (Cisco) Tape Server Client Disk Server 120 dual/quad CPU Sun/Solaris 300 TB Sun Fibre. Channel RAID arrays HPSS + SLAC enhancements to Objectivity and ROOT server code Tape Server 25 dual CPU Sun/Solaris 40 STK 9940 B 6 STK 9840 A 6 STK Powderhorn over 1 PB of data
Quantitatively (1) • Volume of data per experiment: – Today: 1 petabyte – 2009: 10 petabytes • Bandwidths: – Today: ~1 Gbyte/s (read) – 2009 (wish): ~1 Tbyte/s (read) • Access patterns: – Sparse iteration, 5 kbyte objects – 2009 (wish): sparse iteration/random, 100 byte objects
Quantitatively (2) • File systems: – Fundamental unit is an object (100 – 5000 bytes) – Files are WORM containers, of arbitrary size, for objects – File systems should be scalable, reliable, secure and standard • Transport and remote replication: – Today: A data volume equivalent to ~100% of all data is replicated, more-or-less painfully, on another continent – 2009 (wish): painless worldwide replication and replica management • Metadata management: – Today: a significant data-management problem (e. g 60 million collections) – 2009 (wish): miracles
Quantitatively (3) • Heterogeneity and data transformation: – Today: not considered an issue … 99. 9% of the data are only accessible to and intelligible by the members of a collaboration – Tomorrow: we live in terror of being forced to make data public (because it is unintelligible and so the user-support costs would be devastating) • Ontology, Annotation, Provenance: – Today: we think we know what provenance means – 2009 (wish): • Know the provenance of every object • Create new objects and collections making optimal use of all pre -existing objects