Data Structures and Algorithm Analysis - Computer Science at ...

268 Chap. 8 File Processing and External SortingThis requires the ability to guess accurately what information will be needed laterand store it in primary memory now. This is referred to as caching.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 locationof the data on disk. Likewise, when writing to a particular logical byte positionwith respect to the beginning of the file, this position must be converted by thefile manager into the corresponding physical location on the disk. To gain someappreciation for the the approximate time costs for these operations, you need tounderstand the physical structure and basic workings of a disk drive.Disk drives are often referred to as direct access storage devices. This meansthat it takes roughly equal time to access any record in the file. This is in contrastto sequential access storage devices such as tape drives, which require the tapereader to process data from the beginning of the tape until the desired position hasbeen reached. As you will see, the disk drive is only approximately direct access:At any given time, some records are more quickly accessible than others.8.2.1 Disk Drive ArchitectureA hard disk drive is composed of one or more round platters, stacked one on top ofanother and attached to a central spindle. Platters spin continuously at a constantrate. Each usable surface of each platter is assigned a read/write head or I/Ohead through which data are read or written, somewhat like the arrangement ofa phonograph player’s arm “reading” sound from a phonograph record. Unlike aphonograph needle, the disk read/write head does not actually touch the surface ofa hard disk. Instead, it remains slightly above the surface, and any contact duringnormal operation would damage the disk. This distance is very small, much smallerthan the height of a dust particle. It can be likened to a 5000-kilometer airplane tripacross the United States, with the plane flying at a height of one meter!A hard disk drive typically has several platters and several read/write heads, asshown in Figure 8.2(a). Each head is attached to an arm, which connects to theboom. 2 The boom moves all of the heads in or out together. When the heads arein some position over the platters, there are data on each platter directly accessible2 This arrangement, while typical, is not necessarily true for all disk drives. Nearly everythingsaid here about the physical arrangement of disk drives represents a typical engineering compromise,not a fundamental design principle. There are many ways to design disk drives, and the engineering