ATLAS Event Store and I/O developments in support for Production and Analysis in Run 3

During the long shutdown, ATLAS is preparing several fundamental changes to its offline event processing framework and analysis model. These include moving to multi-threaded reconstruction and simulation and reducing data duplication during derivation analysis by producing a combined mini-xAOD stream. These changes will allow ATLAS to take advantage of the higher luminosity at Run 3 without overstraining processing and storage capabilities. They also require significant changes to the underlying event store and the I/O framework to support them. These changes and their effects are discussed in the presentation: - The Run 2 I/O framework was overhauled to be thread-safe and minimize serial bottlenecks. - For object navigation, new immutable references are deployed, which don't rely on storage container entry number so data can be merged in-memory. - Filter decisions can be used to annotate combined output stream allowing for fast event selection on input. - Compression algorithms and settings were optimized to allow efficient reading of event selections.