Abstract—Belle II, an experiment designed to search for physics
beyond the standard model in the decay of mesons, will soon
start operation at the Super KEKB collider. The trigger rate
is expected to be 30 kHz with a margin of 10 kHz, and the raw
data rate will exceed 30 GB/s. Since the system will be unable to
record all data on its storage system, online reduction is required.
We have developed a three-level event building system using inexpensive
switches and software for traffic smoothing, enabling online
event processing at a reasonable cost. The data will be recorded
on the storage system about 3 GB/s after rate reduction. This paper
describes the architecture and the verification result in actual beam
tests.