The online buffer

Abstract

This is a discussion document regarding the proposed use of the Online Buffer. The Online Buffer is used to store locally the RAW data files created by the Event Builder, before they are uploaded to Castor by the data mover. The files may also be used by the online monitoring and reconstruction activities. At the Trigger-DAQ-Controls Review, the reviewers warned that this three-way activity might saturate the disks, and also that the file uploads to the Grid could conflict with the writing of DAQ data. It was proposed to ameliorate this by splitting the buffer into a set of independent volumes into which the DAQ data would be written on a round-robin basis; outgoing files would meanwhile be read only from one of the other volumes. Further, files being uploaded to the Grid would be staged on the transfer box’ system disk, as the (local) staging process is expected to be more deterministic and easier to control than transfers across the WAN.