Reservation mechanisms have been used to guarantee users' access to cluster resources at the desired time instead of adopting a batch mode processing, where jobs are processed at the nearest time in the future, risking to wait an uncertain amount of time before being run. Allowing the cluster to accept reservations, besides queued jobs, has a major impact on the average response and waiting time of the latter. This paper adopts a technique that allows proper functioning of the system when both types of jobs are supported. The applied technique combines the queue discipline FCFS with a backfilling algorithm that scans the real time queue and allows jobs behind in this priority queue, ordered by the arrival time stamps, to be processed without delaying the head of the queue or an accepted reservation. Further experimental results show that the number of advance reservations accepted by the cluster should fall below a threshold in order to maintain the cluster performance and even so, the scheduling algorithm must be updated accordingly.