A Practical Introduction to Data Structures and Algorithm Analysis

282 Chap. 8 File Processing and External SortingThere are generally two approaches to minimizing disk accesses. The first isto arrange information so that if you do access data from secondary memory, youwill get what you need in as few accesses as possible, and preferably on the firstaccess. File structure is the term used for a data structure that organizes datastored in secondary memory. File structures should be organized so as to minimizethe required number of disk accesses. The other way to minimize disk accesses isto arrange information so that each disk access retrieves additional data that canbe used to minimize the need for future accesses, that is, to guess accurately whatinformation will be needed later and retrieve it from disk now, if this can be donecheaply. As you shall see, there is little or no difference in the time required to readseveral hundred contiguous bytes from disk as compared to reading one byte, sothis strategy is indeed practical.One way to minimize disk accesses is to compress the information stored ondisk. Section 3.9 discusses the space/time tradeoff in which space requirements canbe reduced if you are willing to sacrifice time. However, the disk-based space/timetradeoff principle stated that the smaller you can make your disk storage requirements,the faster your program will run. This is because the time to read informationfrom disk is enormous compared to computation time, so almost any amountof additional computation to unpack the data is going to be less than the disk readtime saved by reducing the storage requirements. This is precisely what happenswhen files are compressed. CPU time is required to uncompress information, butthis time is likely to be much less than the time saved by reducing the number ofbytes read from disk. Current file compression programs are not designed to allowrandom access to parts of a compressed file, so the disk-based space/time tradeoffprinciple cannot easily be taken advantage of in normal processing using commercialdisk compression utilities. However, in the future disk drive controllers mightautomatically compress and decompress files stored on disk, thus taking advantageof the disk-based space/time tradeoff principle to save both space and time.Many cartridge tape drives (which must process data sequentially) automaticallycompress and decompress information during I/O.8.2 Disk DrivesA Java programmer views a random access file stored on disk as a contiguousseries of bytes, with those bytes possibly combining to form data records. Thisis called the logical file. The physical file actually stored on disk is usually nota contiguous series of bytes. It could well be in pieces spread all over the disk.The file manager, a part of the operating system, is responsible for taking requestsfor data from a logical file and mapping those requests to the physical location