[$] I/O scheduling for single-queue devices

Block I/O performance can be one of the determining factors for theperformance of a system as a whole, especially on systems with slowerdrives. The need to optimize I/O patterns has led to the development of along series of I/O schedulers over the years; one of the most recent ofthose is BFQ, which was merged during the4.12 development cycle. BFQ incorporates an impressive set of heuristicsdesigned to improve interactive performance, but it has, thus far, seenrelatively little uptake in deployed systems. An attempt to make BFQ thedefault I/O scheduler for some types of storage devices has raised someinteresting questions, though, on how such decisions should be made.