However, in order to achieve the higher rate, the striped volume must be used concurrently by multiple processes, each reading in a different stripe. The maximum rate is reached when as many processes are reading sequentially in stripe-sized units as the subvolume has drives.
When a program requests a VOD guarantee, it must specify a data rate that equals one stripe-width per second. VOD guarantees can be given concurrently to several processes for the same subvolume. As long as all the processes read different stripes, the guaranteed rate can be sustained for each.
When the first VOD guarantee is granted against a striped volume, the XFS system begins VOD-style I/O scheduling for that volume. This establishes a strict cyclic rotation of time intervals during which any disk in the striped volume can be read. In general, a process must be ready for access when its turn in the rotation comes up. If it is not ready, it can be delayed by as many seconds as there are disks in the volume.