The three-level event building system for the Belle II experiment

The Belle II experiment is an e
⁺
e
^-
collider for the search physics in the decays of B mesons, and it will start the operation in near future. The trigger rate is expected as 30 kHz and the raw data rate will exceed 30 GB/s. It is unable to record the data on the storage system, thus the online reduction is indispensable. We have developed a three-level event building system using inexpensive box switches and the software for the traffic smoothing to enable the online event processing with adequate cost. The data will be recorded on the storage system about 3 GB/s after the rate reduction.