A system for utilizing informed throttling to guarantee quality of service to a plurality of clients includes a server core having a performance analyzer that compares a performance level received by a client to a corresponding contracted service level and determines if the client qualifies as a victim whose received performance level is less than the corresponding contracted service level. The performance analyzer is further configured to identify one or more candidates for throttling in response to an I/O stream receiving insufficient resources by determining if the client qualifies as a candidate whose received performance level is better than the corresponding contracted service level. The server core further includes a scheduler that selectively and dynamically issues a throttling command to the candidate client, and provides a quality of service enforcement point by concurrently monitoring a plurality of I/O streams to candidate clients and concurrently throttling commands to the candidate clients.