Existential QoS for Storage

In this paper we describe two quality of service (QoS) management models, grammar-based QoS and existential QoS, in the context of storage systems. We compare the two models in terms of generality, implementation complexity, and, most importantly, ease of administration. We describe one implementation of existential QoS, in the Zettabyte File System. We conclude that the existential QoS model provides more flexibility and configurability, eliminates many difficult design problems, and simplifies administration.