A Blueprint for a Contemporary Storage Element, building a new WLCG storage system with widely available hardware and software components: Ceph, XRootD, and Prometheus

Abstract

When a new long-term storage facility was needed at the Lancaster WLCG Tier-2 Site, an architecture was chosen involving CephFS as a failure-tolerant back-end volume, and load-balanced XRootD as an endpoint exposing the volume via the HTTPS/DAVS protocols increasingly favoured by the WLCG and other users. This allows operations to continue in the face of disc/node failures with minimal management, and enables good utilization of network connectivity for remote access. We deployed a Prometheus/Loki/Grafana monitoring/alerting stack for timely detection and resolution of failures in such a production environment. Some custom scripts were required to adapt the off-the-shelf functional components with monitoring. With such a monitoring system in place, failures such as disc defects, data corruption and resource exhaustion in long-running processes can be anticipated, and their management planned. We describe the hardware platform and our requirements on it, and detail the software architecture from initial design, through adaptations to face challenges encountered during production, to present condition. Developments and contributions to related projects that help to fully exploit our design decisions are described. We include performance metrics of the system, the lessons learned during production, and our future plans.