# Rucio Storage Element

A Rucio Storage Element (RSE) is a container for physical files. It is the smallest unit of storage space addressable within Rucio. It has an unique identifier and a set of meta attributes describing properties such as supported protocols, e.g., file, https, srm; host/port address; quality of service; storage type, e.g., disk, tape, …; physical space properties, e.g., used, available, non-pledged; and geographical zone.

Rucio Storage Elements can be grouped in many logical ways, e.g., the UK RSEs, the Tier-1 RSEs, or the `good’ RSEs. One can reference groups of RSEs by metadata attributes or by explicit enumeration of RSEs.

RSE tags are expanded at transfer time to enumerate target sites. Post-facto changes to the sites in an RSE tag list will not affect currently replicated files.

A cache is storage service which keeps additional copies of files to reduce response time and bandwidth usage. In Rucio, a cache is an RSE, tagged as volatile. The control of the cache content is usually handled by an external process or applications (e.g. Panda) and not by Rucio. Thus, as Rucio doesn’t control all file movements on these RSEs, the application populating the cache must register and unregister these file replicas in Rucio. The information about replica location on volatile RSEs can have a lifetime. Replicas registered on volatile RSEs are excluded from the Rucio replica management system (replication rules, quota, replication locks) described in the section Replica management. Explicit transfer requests can be made to Rucio in order to populate the cache.