LHCb computing tasks

This document describes the computing tasks of the LHCb computing system. It also describes the logistics of the dataflow between the tasks and the detailed requirements for each task, in particular the data sizes and CPU power requirements. All data sizes are calculated assuming that the LHCb experiment will take data about 107 s per year at a frequency of 200 Hz, which gives 2 \Theta 109 real events per year. The raw event size should not exceed 100 kB (200 TB per year). We will have to generate about 109 MonteCarlo events per year. The current MonteCarlo simulation program based on the GEANT3.21 package requires about 12 s to produce an average event (all CPU times are normalised to a 1000 MIPS processor). The size of an average MonteCarlo event will be about 200 kB (100 TB per year) of simulated data (without the hits). We will start to use the GEANT4 package in 1998. Rejection factors of 8 and 25 are required in the Level-2 and Level-3 triggers respectively, to reduce the frequency of events to 200 Hz. The goal is to achieve processing times of 10 ms per event for Level-2 and 200 ms per event for Level-3. Current reconstruction of an average MonteCarlo event takes about 2 s processing time. The expected size of a real reconstructed event will be about 70 kB (140 TB per year), the size of a MonteCarlo reconstructed event will be about 150 kB (150 TB per year). The total data volume will be about 0.7 PB per year, including about 2 TB analysis object data per year. The total processing power has to be about 2:7 \Theta 106 MIPS, about half of which will be needed in the Level-2 and Level-3 software triggers.1