  The XRootD system is used to transfer, store, and cache large datasets from
high-energy physics (HEP). In this study we focus on its capability as
distributed on-demand storage cache. Through exploring a large set of daily log
files between 2020 and 2021, we seek to understand the data access patterns
that might inform future cache design. Our study begins with a set of summary
statistics regarding file read operations, file lifetimes, and file transfers.
We observe that the number of read operations on each file remains nearly
constant, while the average size of a read operation grows over time.
Furthermore, files tend to have a consistent length of time during which they
remain open and are in use. Based on this comprehensive study of the cache
access statistics, we developed a cache simulator to explore the behavior of
caches of different sizes. Within a certain size range, we find that increasing
the XRootD cache size improves the cache hit rate, yielding faster overall file
access. In particular, we find that increase the cache size from 40TB to 56TB
could increase the hit rate from 0.62 to 0.89, which is a significant increase
in cache effectiveness for modest cost.
