chap file processing and external sorting growsthere might not be free space physically adjacent thusa file might consist of several extents widely spaced on the disk the fuller the diskand the more that files on the disk changethe worse this file fragmentation (and the resulting seek timebecomes file fragmentation leads to noticeable degradation in performance as additional seeks are required to access data another type of problem arises when the file' logical record size does not match the sector size if the sector size is not multiple of the record size (or vice versa)records will not fit evenly within sector for examplea sector might be bytes longand logical record bytes this leaves room to store records with bytes left over either the extra space is wastedor else records are allowed to cross sector boundaries if record crosses sector boundarytwo disk accesses might be required to read it if the space is left empty insteadsuch wasted space is called internal fragmentation second example of internal fragmentation occurs at cluster boundaries files whose size is not an even multiple of the cluster size must waste some space at the end of the last cluster the worst case will occur when file size modulo cluster size is one (for examplea file of bytes and cluster of bytesthuscluster size is tradeoff between large files processed sequentially (where large cluster size is desirable to minimize seeksand small files (where small clusters are desirable to minimize wasted storageevery disk drive organization requires that some disk space be used to organize the sectorsclustersand so forth the layout of sectors within track is illustrated by figure typical information that must be stored on the disk itself includes the file allocation tablesector headers that contain address marks and information about the condition (whether usable or notfor each sectorand gaps between sectors the sector header also contains error detection codes to help verify that the data have not been corrupted this is why most disk drives have "nominalsize that is greater than the actual amount of user data that can be stored on the drive the difference is the amount of space required to organize the information on the disk additional space will be lost due to fragmentation disk access costs the primary cost when accessing information on disk is normally the seek time this assumes of course that seek is necessary when reading file in sequential order (if the sectors comprising the file are contiguous on disk)little seeking is necessary howeverwhen accessing random disk sectorseek time becomes the dominant cost for the data access while the actual seek time is highly variabledepending on the distance between the track where the / head currently is and