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Understanding Client-Based T3 Sites in the ATLAS Framework at CERN

This presentation focuses on T3g sites in the ATLAS framework. Unlike T3gs, T3gs have more functionalities but T3gs require less manpower and offer easier setup. T3g sites must manage their data download and monitor processes without automatic data distribution services. Attendees will learn about necessary installations, data management, and how to effectively use the dq2-get command for downloading datasets. Additionally, we will discuss the modifications to dq2-get and its integration with FTS for improved reliability.

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Understanding Client-Based T3 Sites in the ATLAS Framework at CERN

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  1. Grid (data) access Simone Campana CERN IT/ES

  2. Types of ATLAS T3s • The SERVICE- based T3: T3gs • Looks like a T2 but with unpledged resources • Runs a Storage and a Computing element • Is known by the WLCG information system • Is known by the ATLAS DDM system and served by DDM Site Services • The CLIENT-based T3: T3g • Needs a minimal installation of the Grid middleware • Needs a minimal installation of DDM clients • This presentation will focus on the T3g

  3. The pain and the gain • Since this is a client based T3: • There is no service for automatic data distribution • It is up to the site to trigger data download • It is up to the site to monitor the process and handle the failures • Data are not catalogued • Nor in DDM Central Catalogs nor in LFC • So, less functionality in respect of T3gs but a definite gain in manpower • Point (1) involves a bit of work but no expertise • Point (2) reduces a lot the amount of work • … and you do not have to run Grid services • I believe 10% of a PhD student can cope with this

  4. Setting up a T3g • You need to install Grid clients • You do not need the full WLCG UI • Only Data Management and Security • I believe there is a recipe from OSG or even USATLAS • You need to install DDM clients • https://twiki.cern.ch/twiki/bin/view/Atlas/DQ2ClientsHowTo • You need a Grid certificate and to be part of the ATLAS VO • https://www.racf.bnl.gov/docs/howto/grid/voatlas • See next talk “Support in setting up a T3g” for more details

  5. Getting datasets at your site • To “discover” datasets you can • Use the ATLAS Metadata Interface (AMI) • http://ami.in2p3.fr/opencms/opencms/AMI/www/ • Use the Panda Dataset Browsed • http://voatlas48.cern.ch:23080/browser/ • Use the DQ2 clients directly • https://twiki.cern.ch/twiki/bin/view/Atlas/DQ2ClientsHowTo#Querying • To download a dataset at your site you can use dq2-get

  6. Using dq2-get • dq2-get downloads files from a dataset into a local filesystem • If you need to store data into a storage system (like xroot) it is up to you to put the data there • DDM to LOCAL to XROOT • DDM to XROOT via FUSE • dq2-get offer various features • Download a full dataset, one file, N files, a random sample • Choose the source automatically or manually • Documentation and examples in • https://twiki.cern.ch/twiki/bin/view/Atlas/DQ2ClientsHowTo#Retrieving_data

  7. More on dq2-get • dq2-get needs to know the name of your site in DDM • If you are a T3g (and therefore you are not in DDM) use “ROAMING” • dq2-get might fail to download a few (or all) files • Happens when the storage is unavailable • It is up to the user to retry the failed files • dq2-get is not allowed to retrieve files from some given location • For example TAPE systems • If you select a specific source (-s option) dq2-get might refuse to use it • The source might be blacklisted in DDM because is not working

  8. Heavy usage of dq2-get From Hiro dq2-get –L site –T=1,10000 DSN list=Array.new() `dq2-list-files DSN`.split("\n").each do |line| line=line.split(" ") if line.size>3 list.push(Thread.new { `"dq2-get -L site -f #{line[0]} DSN"` }) end end list.each { |t| t.join } Easy! But, don’t do this, please!

  9. Heavy usage of dq2-get • Heavy usage of dq2-get is discouraged • Could cause a lot of load to production sites • And then we are forced to apply restrictive measures • Heavy usage = more than 0.5TB/day • For sites which need more than this we invented dq2-get-on-Steroids • Special option of dq2-get using FTS as transfer mechanism • FTS throttles the access to storage and offers a more reliable transfer service

  10. Status of dq2-get+FTS • Modifications in dq2 code done in current Release Candidate • Usage of FTS implemented as plugin • The plugin itself has been implemented by Hiro • It is basically ready to test, get in tough with Hiro if you are willing to try it out • May be we should start from some expert user • The release candidate will become production based on the feedback

  11. How to use dq2-get + FTS • Bad News: you need to setup a storage element • A simple gridFTP server on top of a filesystem would do • Fairly simple with VDT • If you plan to have a storage larger than i.e. 10TB, you should be considering a real pool manager + SRM • For example dCache or BestMan • You need to inform your US cloud contact about your storage • He needs to setup a FTS channel • You need to be careful about which source you use • Most likely BNL is always the best choice • In other words, the “operator” at the T3g needs to be a bit more advanced

  12. dq2-get + FTS usage • Example: dq2-get -qFTS -ohttps://fts.usatlas.bnl.gov:8443/glite-data-transfer-fts/services/FileTransfer -S gsiftp://atlfs02.phy.duke.edu:2811/srv/data/srm/test2 -L BNL-OSG2_DATADISK step09.20201023000134L.physics_A.recon.ESD.closed • This will return a FTS job id which you can use to see the status of your transfer • Via the glite-transfer-status command • In the FTS monitor

  13. Final Remarks on dq2-get • dq2-get + FTS needs some testing and cleaning. • Despite dq2-get + FTS is more automated and gentle than dq2-get, you should not foresee a heavy usage • If you are thinking about downloading mire than 1TB/day you should consider becoming a T3gs

  14. Uploading data • You might need to upload data INTO DDM • for other people to look at them • Because they become of general interest for a group or the all VO • In this case you can use dq2-put • Uploads data from local file system into DDM • Same logic of dq2-get but the other way around • https://twiki.cern.ch/twiki/bin/view/Atlas/DQ2ClientsHowTo#Creating_data • You need to know the DDM endpoint (scratchdisk) where to upload your data • Any SCRATCHDISK endpoint would do, but better negotiate this with your cloud • REMEMBER: data in SCRATCHDISK have limited (few weeks) lifetime

  15. Conclusions • You should consider how to approach the “obtaining ATLAS data” problem depending on your needs • How big is your site • How much data per day you need • How many people you have • There are basically three solutions • T3gs • T3g • T3g + some “s” (using FTS) • Many details are specific to your cloud, so in any case get in touch with the cloud support

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