chap file processing and external sorting there are generally two approaches to minimizing disk accesses the first is to arrange information so that if you do access data from secondary memoryyou will get what you need in as few accesses as possibleand preferably on the first access file structure is the term used for data structure that organizes data stored in secondary memory file structures should be organized so as to minimize the required number of disk accesses the other way to minimize disk accesses is to arrange information so that each disk access retrieves additional data that can be used to minimize the need for future accessesthat isto guess accurately what information will be needed later and retrieve it from disk nowif this can be done cheaply as you shall seethere is little or no difference in the time required to read several hundred contiguous bytes from disk as compared to reading one byteso this strategy is indeed practical one way to minimize disk accesses is to compress the information stored on disk section discusses the space/time tradeoff in which space requirements can be reduced if you are willing to sacrifice time howeverthe disk-based space/time tradeoff principle stated that the smaller you can make your disk storage requirementsthe faster your program will run this is because the time to read information from disk is enormous compared to computation timeso almost any amount of additional computation to unpack the data is going to be less than the disk read time saved by reducing the storage requirements this is precisely what happens when files are compressed cpu time is required to uncompress informationbut this time is likely to be much less than the time saved by reducing the number of bytes read from disk current file compression programs are not designed to allow random access to parts of compressed fileso the disk-based space/time tradeoff principle cannot easily be taken advantage of in normal processing using commercial disk compression utilities howeverin the future disk drive controllers might automatically compress and decompress files stored on diskthus taking advantage of the disk-based space/time tradeoff principle to save both space and time many cartridge tape drives (which must process data sequentiallyautomatically compress and decompress information during / disk drives java programmer views random access file stored on disk as contiguous series of byteswith those bytes possibly combining to form data records this is called the logical file the physical file actually stored on disk is usually not contiguous series of bytes it could well be in pieces spread all over the disk the file managera part of the operating systemis responsible for taking requests for data from logical file and mapping those requests to the physical location