input / output operations are issued to a storage system using request queues that are each maintained for a resource targeted by the ios . when an io is requested , the target resource for the io is first identified . if a request queue is maintained for the target resource , the io is added to the request queue and the io is issued to the storage system as the target resource becomes available . the availability of the target resource may be determined through periodic checks or by monitoring completions of ios issued out of the request queue .