13.1 through 13.5, 13.10 and 13.11

.-.-dDChapter 13 Disk Storage, Basic File Structures, and Hashingciata structures, called indexes, which speed up the search tbr and retrieval oI records.Tl-rese techniques involve storage of auxiliary data, called incler files, in addition tothe file records thernselves.Chapters 13 and l4 n-ray be brorvsed through or even omitted by reaclers rvhohave already studied file organizations. The material covered here, in particularSections 13.1 through 13.8, is necessary for understanding Chapters l5 and 16,which deal with qr-rery processing ancl query optimization.13.1 IntroductionThe collection of data that makes up a computerized database must be stored physicallyon some computer storage medium. The DBMS software can then retrieve,tupdate, and process this data as needed. Cornputer storage media fbrm a -irr)ftiscItit'rnrclty lhat includes two nrain cateqories:'l Primary storage. This category includes storase media that can be operatedon directly by the computer cerrtral processirtg rllir (CPU), such as the computerr.nair.r memory and smaller but faster cache rremories. Prirnrrry storageusually provides fast access to data but is of limited storage capacity.,u: Secondary and tertiary storage. This category includes magnetic disks,optical disks, :rnd tapes. Hard clisk drives are classifled as secondary storage,whereas removable media are considered tertiary storage. These devices usuallyhave tr larger capacity, cclst less, and provide slower access to data than doprimary storage devices. Data in secondary or tertiary storage cannot beprocessedirectly by the CPU; first it rnust be copied into prirnary storage.We will give an overview of the variolrs storage devices used for primary and secondarrystorage in Section l3.l.l and then we will discuss horv databases ilrc t\'l-ricallyhandled in the storage hierarcl-r,v in Section 1-1.1.2.13.1.1 Memory Hierarchies and Storage DevicesIn a modern computer system, data resides and is transported throughout a hierarchyof storage media. The highest-speed rnemory is the nlost expensive and is thereforeavailable with the least capacity. The lowest-speecl r.nenrory is otlline tapestorage, which is essentially available in indefinite storige capacity.At the prinnry storage level, the memory hierarchy includes at the rnost expensiveend, cache memory, which is a static RAM (Random Access Memory). Cache memoryis typically used by the CPU to speed up executiorr of programs. The nert levelo[ primary storage is DRAM (Dynamic RAM), which provides the rnain rvork irreirfbr the CPU for keeping programs and data. It is popularly called main memory.The advanttrge of DRAM is its lorv cost, which colrtinues to decrease; the drawbackis its volatilityr and lower speed conrpirred with static RAM. hr rhe second(lr), dnd ter-1. Volat le memory typically loses ts contents in case of a power outage, whereas nonvo ati e memorydoes not.