Physical volume library deadlock avoidance in a striped media environment

Most modern high performance storage systems store data in large repositories of removable media volumes. Management of the removable volumes is performed by a software module known as a physical volume library (PVL). To meet performance and scalability requirements, a PVL can be asked to mount multiple removable media volumes for use by a single client for parallel data transfer. Mounting sets of volumes creates an environment in which it is possible for multiple client requests to deadlock while attempting to gain access to storage resources. Scenarios leading to deadlock in a PVL include multiple client requests that contend for the same cartridge(s), and client requests that vie for a limited set of drive resources. These deadlock scenarios are further complicated by the potential for volumes to be mounted out-of-order (for example, by automatic cartridge loaders or human operators). This paper begins by introducing those PVL requirements which create the possibility of deadlock resolution and how they might be applied in a PVL. This leads to a design for a PVL that addresses deadlock scenarios. Following the design presentation is a discussion of possible design enhancements. We end with a case study of an actual implementation of the PVL design in the high performance storage system (HPSS).