  The ReadOut System (ROS) is a central part of the data acquisition (DAQ)
system of the ATLAS Experiment at the CERN Large Hadron Collider (LHC). The
system is responsible for receiving and buffering event data from all detector
subsystems and serving these to the High Level Trigger (HLT) system via a 10
GbE network, discarding or transporting data onward once the trigger has
completed its selection process. The ATLAS ROS was completely replaced during
the 2013-2014 experimental shutdown in order to meet the demanding conditions
expected during LHC Run 2 and Run 3 (2015-2025). The ROS consists of roughly
one hundred Linux-based 2U-high rack-mounted servers equipped with PCIe I/O
cards and 10 GbE interfaces. This paper documents the system requirements for
LHC Runs 2 and 3 and the design choices taken to meet them. The results of
performance measurements and the re-use of ROS technology for the development
of data sources, test platforms for other systems, and another ATLAS DAQ system
component, namely the Region of Interest Builder (RoIB), are also discussed.
Finally performance results for Run 2 operations are presented before looking
at the upgrade for Run 3.
