Buffer Provisioning for Large-Scale Data-Acquisition Systems

The data acquisition system of the ATLAS experiment, a major experiment of the Large Hadron Collider (LHC) at CERN, will go through a major upgrade in the next decade. The upgrade is driven by experimental physics requirements, calling for increased data rates on the order of 6~TB/s. By contrast, the data rate of the existing system is 160~GB/s. Among the changes in the upgraded system will be a very large buffer with a projected size on the order of 70 PB. The buffer role will be decoupling of data production from on-line data processing, storing data for periods of up to 24~hours until it can be analyzed by the event processing system. The larger buffer will allow a new data recording strategy, providing additional margins to handle variable data rates. At the same time it will provide sensible trade-offs between buffering space and on-line processing capabilities. This compromise between two resources will be possible since the data production cycle includes time periods where the experiment will not produce data. In this paper we analyze the consequences of such trade-offs, and introduce a tool that allows a detailed exploration of different strategies for resource provisioning. It is based on a model of the upgraded data acquisition system, implemented in a simulation framework. From this model it is possible to obtain insight into the dynamics of the running system. Given predefined resource constraints, we provide bounds for the provisioning of buffering space and on-line processing requirements.