Chapter 1 Introduction File Structures Readings: Folk, Chapter 1

Why File Structure Design? Data are organized into files. Files are stored on disks, instead of main memory, for the following reasons: –Disk is persistent -- data stay after power is shut down. –Disk is of enormous capacity. »The capacity of main memory on a typical PC ranges from 32 megabytes to 256 megabytes. »The capacity of a hard disk ranges from several hundred megabytes to several gigabytes. –Disk is much cheaper. However, disk is much much slower. –A read operation from a slow random access memory (RAM) is about 120 nanoseconds (1.2  seconds). –A read operation from a typical disk is about 30 milliseconds (3.0  seconds). –The ratio is about 1:2.5  10 5.

Major Issues Goal of file structure design: Get information sought in as few disk accesses as possible Solutions –Buffering –Grouping information in some way so the information can be retrieved in a small number of accesses to the disk. e.g. binary search is much faster then sequential search –Using auxiliary data structures, called indexes, to help locating information fast simple index, B-tree, hashing Difficulties in file structure design –Files may contain records of variable length –New data types, such as multimedia data, emerge –The contents of files may change insertion and deletion make indexing hard –The use of files may change remember the Y2K problem?