A Practical Introduction to Data Structures and Algorithm Analysis

288 Chap. 8 File Processing and External SortingHow much time is required to read the track? On average, it will requirehalf a rotation to bring the first sector of the track under the I/O head, andthen one complete rotation to read the track.How long will it take to read a file of 1MB divided into 2048 sectorsized(512 byte) records? This file will be stored in 256 clusters, becauseeach cluster holds 8 sectors. The answer to the question depends in largemeasure on how the file is stored on the disk, that is, whether it is all togetheror broken into multiple extents. We will calculate both cases to seehow much difference this makes.If the file is stored so as to fill all of the sectors of eight adjacent tracks,then the cost to read the first sector will be the time to seek to the first track(assuming this requires a random seek), then a wait for the initial rotationaldelay, and then the time to read. This requires9.5 + 11.1 × 1.5 = 26.2 ms.At this point, because we assume that the next seven tracks require only atrack-to-track seek because they are adjacent, each requiresThe total time required is therefore2.2 + 11.1 × 1.5 = 18.9 ms.26.2ms + 7 × 18.9ms = 158.5ms.If the file’s clusters are spread randomly across the disk, then we mustperform a seek for each cluster, followed by the time for rotational delay.Once the first sector of the cluster comes under the I/O head, very little timeis needed to read the cluster because only 8/256 of the track needs to rotateunder the head, for a total time of about 5.9 ms for latency and read time.Thus, the total time required is about256(9.5 + 5.9) ≈ 3942msor close to 4 seconds. This is much longer than the time required when thefile is all together on disk!This example illustrates why it is important to keep disk files from becomingfragmented, and why so-called “disk defragmenters” can speed upfile processing time. File fragmentation happens most commonly when thedisk is nearly full and the file manager must search for free space whenevera file is created or changed.