When dealing with soft real-time tasks with highly variable??execution times in open systems, an approach that is becoming??popular is to use feedback scheduling techniques to dynamically??adapt the bandwidth reserved to each task.??According to this model,??each task is assigned an adaptive reservation, with a variable??budget and a constant period. The response times of the jobs of the??task are monitored and if different from expected (i.e.??much larger??or much shorter than the task relative deadline), a feedback control??law adjusts the reservation budget accordingly.??However, when the??feedback law algorithm demands an increase of the reservation??budget, the system must run a schedulability test to check if there??is enough spare bandwidth to accommodate such increase.??The??schedulability test must be very efficient, as it may be performed??at each budget update, i.e. potentially at each instance of a task.??In this paper, we tackle the problem of performing an efficient??on-line schedulability test for Resource Reservation systems??implemented through the Sporadic Server on Fixed Priority??scheduling. We propose five different tests with different??complexity and performance. In particular, we propose a novel??on-line test, called Spare Pot algorithm which shows a good??cost/performance ratio.