Non-POSIX file system for LHCb online event handling

LHCb aims to use its O(20000) CPU cores in the High Level Trigger (HLT) and its 120 TB Online storage system for data reprocessing during LHC shutdown periods. These periods can last a few days for technical maintenance or only a few hours during beam interfill gaps. These jobs run on files which are staged in from tape storage to the local storage buffer. The result are again one or more files. Efficient file writing and reading is essential for the performance of the system. Rather than using a traditional shared file-system such as NFS or CIFS we have implemented a custom, light-weight, non-Posix file-system for the handling of these files. Streaming this file-system for the data-access allows to obtain high performance, while at the same time keep the resource consumption low and add nice features not found in NFS such as high-availability, transparent fail-over of the read and write service. The writing part of this file-system is in successful use for the Online, real-time writing of the data during normal data acquisition operation. The implementation together with performance figures are presented.