Complex sentence: It should be noted that the tunables in other schedulers will only be useful if there is some I/O contention on the device in question, either by other applications or different subsystems in a single application.
Split into simpler sentences: It should be noted that the tunables in other schedulers will only be useful if there is some I/O contention on the device in question. This would be contention either by other applications or different subsystems in a single application.