Friday, 30 May 2008

Last chance to test, gone

So here we are, consuming the final hours of the last scheduled test of the WLCG service. Next weeks we should be just making final preparations, waiting for the real data to arrive.
Overall, the Tier-1 services have been up and running basically 100% of the time. As the two more relevant issues, I would mention first the CMS skimming jobs brutal I/O. After they were limited (to 100 accoriding to CMS) they were still running until mid-thursday and sustaining a quite high load on the LAN (around 500MB/s). On wednesday 28-May evening, ATLAS launched a battery of reprocessing jobs which very fast filled up more than 400 job slots. Apparently all of these jobs read the detector Conditions data from a big (4GB) file sitting in dCache. This file of course fast became super-hot, since all of these jobs were trying to access it simultaneously. This caused the second issue of the week. The output traffic of the pool in which this file was sitting immediately grew up to 100MB/s, saturating the 1Gbps switch that (sadly, and until we manage to deploy the definitive 2x10Gbps 3Coms) links it to the central router. This network saturation caused the dcache pool-server control connetion to lose packets, which eventually hanged the pool.
At first sight it seems that the Local Area Network has been the big issue at PIC this CCRC08. Let's see what the more detailed post-mortem analysis teaches us.

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