XFS (in a GRIO daemon) keeps information on the transfer capacity of all real-time subvolumes, as well as the capacity of the controllers and busses to which they are attached. When you request a reservation, XFS tests whether it is possible to transfer data at that rate, from that file, during that time period.
This test considers the capacity of the hardware as well as any other reservations that apply during the same time period to the same subvolume, drives, or controllers. Each reservation consumes some part of the total capacity.
When XFS predicts that the guaranteed rate can be met, it accepts the reservation. Over the reservation period, the available capacity of the subvolume is reduced by the promised rate. Other processes can place reservations against any capacity that remains.
If XFS predicts that the guaranteed rate cannot be met at some time in the reservation period, XFS returns the maximum data rate it could supply. The program can reissue the request for that available rate. However, this is a new request that is evaluated afresh.
During the reservation period, the process can use read() and write() to transfer up to the guaranteed number of bytes in each time unit. XFS raises the priority of requests as needed in order to ensure that the transfers take place. However, a request that would transfer more than the promised number of bytes within a 1-second unit is blocked until the start of the next time unit.