A database is mapped into a number of different files that are maintained by the
underlying operating system. These files reside permanently on disks. A file is
organized logically as a sequence of records. These records are mapped onto disk
blocks. Files are provided as a basic construct in operating systems, so we shall
assume the existence of an underlying file system. We need to consider ways of
representing logical data models in terms of files.
Each file is also logically partitioned into fixed-length storage units called
blocks, which are the units of both storage allocation and data transfer. Most
databases use block sizes of 4 to 8 kilobytes by default, but many databases allow
the block size to be specified when a database instance is created. Larger block
sizes can be useful in some database applications.
A block may contain several records; the exact set of records that a block
contains is determined by the form of physical data organization being used.We