In network (e.g., Web) servers, it is often desirable to isolate the performance of different classes of requests from each other. That is, one seeks to achieve that a certain minimal proportion of server resources are available for a class of requests, independent of the load imposed by other requests. Recent work demonstrates how to achieve this performance isolation in servers consisting of a single, centralized node; however, achieving performance isolation in a distributed, cluster based server remains a problem.This paper introduces a new abstraction, the cluster reserve, which represents a resource principal in a cluster based network server. We present a design and evaluate a prototype implementation that extends existing techniques for performance isolation on a single node server to cluster based servers.In our design, the dynamic cluster-wide resource management problem is formulated as a constrained optimization problem, with the resource allocations on individual machines as independent variables, and the desired cluster-wide resource allocations as constraints. Periodically collected resource usages serve as further inputs to the problem.Experimental results show that cluster reserves are effective in providing performance isolation in cluster based servers. We demonstrate that, in a number of different scenarios, cluster reserves are effective in ensuring performance isolation while enabling high utilization of the server resources.