Another part of the operating system is the file manager. While the memory manager is responsible for the maintenance of primary memory, the file manager is responsible for the maintenance of secondary storage (e.g., hard disks). Nutt [1997] describes the responsibility of the file manager and defines the file, the fundamental abstraction of secondary storage:
"Each file is a named collection of data stored in a device. The file manager implements this abstraction and provides directories for organizing files. It also provides a spectrum of commands to read and write the contents of a file, to set the file read/write position, to set and use the protection mechanism, to change the ownership, to list files in a directory, and to remove a file...The file manager provides a protection mechanism to allow machine users to administer how processes executing on behalf of different users can access the information in files. File protection is a fundamental property of files because it allows different people to store their information on a shared computer, with the confidence that the information can be kept confidential."
In addition to these functions, the file manager also provides a logical way for users to organize files in secondary storage. Brookshear [1997] elaborates:
"For the convenience of the machine's users, most file managers allow files to be grouped into a bundle called a directory or folder. This approach allows a user to organize his or her files according to their purpose by placing related files in the same directory.
Moreover, by allowing directories to contain other directories, called subdirectories, a hierarchical organization can be constructed. For example, a user may create a directory called Records that contains subdirectories called Financial Records, Medical Records, and Household Records. Within each of these subdirectories could be files that fall within that particular category. A sequence of directories within directories is called a directory path" [Brookshear 1997]
While users may need to store complex data structures in secondary storage, most storage devices including hard disks "are capable of storing only linearly addressed blocks of bytes." Thus, the file manager needs to provide some way of mapping user data to storage blocks in secondary storage and vice versa. These blocks can be managed in at least three different ways: "as a contiguous set of blocks on the secondary storage device, as a list of blocks interconnected with links, or as a collection of blocks interconnected by a file index" [Nutt 1997]. These three methods are commonly referred to as Contiguous Allocation, Linked Allocation, and Indexed Allocation. For each of these methods, read the text below and use the simulation applet to watch how the blocks are managed. These materials were provided courtesy of Haritha Moosani [Moosani 1998], and were developed as part of a research project under Professor Franz Kurfess at the New Jersey Institute of Technology.
