[$] More IOPS with BIO caching

Once upon a time, block storage devices were slow, to the point that theyoften limited the speed of the system as a whole. A great deal of effortwent into carefully ordering requests to get the best performance out ofthe storage device; achieving that goal was well worth expending some CPUtime. But then storage devices got much faster and the equation changed.Fancy I/O-scheduling mechanisms have fallen by the wayside and effort is nowfocused on optimizing code so that the CPU can keep up with its storage. Ablock-layer change that was merged for the 5.15 kernel shows the kinds oftradeoffs that must be made to get the best performance from current hardware.