UNIX systems utilized to use disk-layout optimizations based on the rotation position of disk data however modern implementations including Linux simply optimize for sequential data access. Why do they do therefore? Of what hardware kind does sequential access take advantage? Why is rotational optimization no longer consequently useful?
Answer: The performance quality of disk hardware have changed substantially in recent years. In particular numerous enhancements have been introduced to raise the maximum bandwidth that can be achieved on a disk. In a modern system there is able to be a long pipeline among the operating system and the disk's read-write head. A disk I/O demand has to pass through the computer's local disk controller over bus logic to the disk drive itself as well as then internally to the disk where there is probable to be a complex controller that can cache data accesses as well as potentially optimize the order of I/O requests.
For the reason that of this complexity the time taken for one I/O request to be acknowledged and for the next request to be generated and received by the disk can far exceed the amount of time between one disk sector passing under the read-write head as well as the next sector header arriving.
In order to be able efficiently to read multiple sectors at once, disks will employ a read ahead cache. While one sector is being passed back to the host computer the disk will be busy reading the next sectors in anticipation of a request to read them. If read needs start arriving in an order that breaks this read ahead pipeline performance will drop.
Accordingly performance benefits substantially if the operating system tries to keep I/O requests in strict sequential order. A subsequent feature of modern disks is that their geometry can be extremely complex. The number of sectors per cylinder is able to vary according to the position of the cylinder- more data can be squeeze into the longer tracks nearer the edge of the disk than at the canter of the disk. For an OS (operating system) to optimize the rotational position of data on such disks it would have to have complete understanding of this geometry as well as the timing characteristics of the disk and its controller. In general, only the disk's internal logic can determine the optimal scheduling of I/Os and the disk's geometry is probable to defeat any attempt by the operating system to perform rotational optimizations.