chap file processing and external sorting how much time is required to read the trackon averageit will require half rotation to bring the first sector of the track under the / headand then one complete rotation to read the track how long will it take to read file of mb divided into sectorsized ( byterecordsthis file will be stored in clustersbecause each cluster holds sectors the answer to the question depends in large measure on how the file is stored on the diskthat iswhether it is all together or broken into multiple extents we will calculate both cases to see how much difference this makes if the file is stored so as to fill all of the sectors of eight adjacent tracksthen the cost to read the first sector will be the time to seek to the first track (assuming this requires random seek)then wait for the initial rotational delayand then the time to read this requires ms at this pointbecause we assume that the next seven tracks require only track-to-track seek because they are adjacenteach requires ms the total time required is therefore ms ms ms if the file' clusters are spread randomly across the diskthen we must perform seek for each clusterfollowed by the time for rotational delay once the first sector of the cluster comes under the / headvery little time is needed to read the cluster because only / of the track needs to rotate under the headfor total time of about ms for latency and read time thusthe total time required is about ( ms or close to seconds this is much longer than the time required when the file is all together on diskthis example illustrates why it is important to keep disk files from becoming fragmentedand why so-called "disk defragmenterscan speed up file processing time file fragmentation happens most commonly when the disk is nearly full and the file manager must search for free space whenever file is created or changed