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iSGTW Feature - No business like show business

Feature - No business like show business: Final rehearsal for WLCG

Before the show can go on, there must be extensive, full-scale dress rehearsals. For the Worldwide LHC Computing Grid, that means several Common Computing Readiness Challenges.
Stock image courtesy of sxc.chu

Rehearsal, rehearsal

Opening night

Like a theatrical company on the eve of a big new production, CERN's Computing Centre and its partner computing sites are preparing for their opening night.

This summer, the two stars of their show will mark their debuts: The Large Hadron Collider ─ largest particle accelerator built to date ─ will start up, and the Worldwide LHC Computing Grid (WLCG) will collect, move and process the massive amount of data the LHC generates.

In the next few months, final touches will be added to ensure that the WLCG and the people involved with it can put on their best performance, says Jamie Shiers, responsible for overall coordination of the system's practice runs ─ known as 'Common Computing Readiness Challenges.' To prepare, WLCG staff have run these "dress rehearsals" for years, which have become more intense as curtain time nears.

"These final two dress rehearsals are much more realistic," he says. "Before this, the testing was done in isolation; now it is everyone at the same time."

Rehearsal, rehearsal

During each run, the LHC Computing Grid receives data from four main LHC experiments simultaneously, at loads predicted for a fully operational machine. After a quick cache, the Tier-0 site at CERN sends data to Tier-1 sites, which send it to Tier-2 sites. Over one hundred auxiliary Tier-1 and Tier-2 sites around the world also aid in storage and processing.

The first full rehearsal was held in February; the final one will be in May. February's test yielded a middleware bug and a delay in storage configurations, but nothing was crippling, states Shiers.

Preparing for opening night, left to right: Jan van Eldik, Miguel Coelho from CERN CASTOR operations team, Gavin McCance from the FTS operations team.

Images courtesy of Jamie Shiers

"It was quite a success," he says. "We managed to get everyone to work together, all sites and all experiments, and we made it last. Before we started we honestly didn't know if we could support them all at the same time. We could have had to turn the volume down on the machine-and no one wanted to see that happen."

Shiers believes May's tests will probably reveal more glitches. For example, the grid may not be able to keep up with the scaling-up of computing volume. However, Shiers observes, the "LCG team" has grown adept at finding fixes or work-arounds.

Start-up itself will be a time of excitement and relief, Shiers says. "Switching on a machine like this doesn't happen very often," he says. "There is a sense that this whole process is like putting on a theatrical production ─ and we're the actors."
- Danielle Venton

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