# Exploring compression techniques for ROOT IO

## TL;DR

This paper investigates various compression techniques for ROOT IO, aiming to optimize storage and access performance in high-energy physics data management through quantitative analysis and practical guidance.

## Contribution

It introduces new methods for event-level compression and whole-file compression, providing a comprehensive evaluation of tradeoffs in ROOT IO data handling.

## Key findings

- Alternate algorithms can improve read performance.
- Event compression enables efficient random access.
- Different compression strategies suit different use cases.

## Abstract

ROOT provides an flexible format used throughout the HEP community. The number of use cases - from an archival data format to end-stage analysis - has required a number of tradeoffs to be exposed to the user. For example, a high "compression level" in the traditional DEFLATE algorithm will result in a smaller file (saving disk space) at the cost of slower decompression (costing CPU time when read). At the scale of the LHC experiment, poor design choices can result in terabytes of wasted space or wasted CPU time. We explore and attempt to quantify some of these tradeoffs. Specifically, we explore: the use of alternate compressing algorithms to optimize for read performance; an alternate method of compressing individual events to allow efficient random access; and a new approach to whole-file compression. Quantitative results are given, as well as guidance on how to make compression decisions for different use cases.