HERA-B, a B-physics experiment to study CP violation in the neutral B-system at DESY, has entered a new regime with respect to data rates and volumes in HEP. Data handling, in particular event reconstruction, has become a major issue. HERA-B plans for event rates of 20 Hz to archive and data volumes of a few ten TB per year. This requires a DAQ system which is able to process event data as they come along online. As the last part of HERA-B's four level data acquisition and trigger system a large Linux-PC farm was built and put into operation which performs full event reconstruction online. Apart from event reconstruction, which allows for immediate event analysis right after archiving, a sophisticated data quality monitoring system is based on processed events. Moreover, the system provides data for online calibration and alignment which is needed in all trigger levels. The farm exploits 200 processors which are capable of processing event rates of 50 Hz at average processing times of 4 sec per event. It consists of commodity dual-CPU PCs connected by means of a switched Fast-Ethernet network with GigaBit-Ethernet up-links. In- and output data rates up to 10 MB/sec were reached. The contribution to ACAT 2000 covers conceptional aspects of the data handling in the HERA-B environment as well as installation and running experiences of the farm after 6 months of full-time operation. In addition, system aspects such as slow control, reliability, and maintainability will be discussed.

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