File System Interface

Let us quickly review the operating system design principles of a file system.
We want to be discuss the functions of the file systems, describe the interfaces to file systems,  discuss file system design tradeoffs and access methods and explore file-system protection.
Files are mapped by the operating system onto physical devices. It is usually the smallest allotment of logical secondary storage. A file can have attributes such as name, identifier, type, location, size and protection.
File operations include creating, writing, reading, repositioning, deleting and truncating a file. These six basic operations comprise the minimal set of required file operations
Most of the file operations mentioned involve searching the directory for the entry associated with the named file. To avoid this search, many systems require that an open() call is made prior to file use. And the file-system maintains an open-file table, containing information about all open files. The open system call typically returns a pointer to the entry in the open-file table. This pointer, not the actual file name is used in all file operations
Similarly the operating system maintains a file-open count so that it can reuse the open file table entries or it could run out of space in the table The file system also has to locate the file on the disk.
Each process opens a file in an access mode so that subsequent I/O requests can be allowed or denied. File Systems also provide facilities for locking an open file. File locks are usually the same kind as reader-writer locks - a shared lock is similar to reader lock so that several processes can acquire the lock concurrently and an exclusive lock is like a writer lock, allowing only one process to acquire it.
File types are also important to the file system. File extensions inform the files system on the programs associated with the file type and the actions to be taken.
File types can be used to indicate the internal structure of the file. For example, an executable might have a specific file structure so that it can determine where in memory to load the file and the starting point of the instructions. The Macintosh operating system for example requires that files have a resource fork and a data fork. The resource fork contains information of interest to the user. The data fork contains program code or data - the traditional file contents.
Access methods determine how the information stored in the file is accessed. Sequential access method uses rewind, read next and write next operations. Editors and compilers use the file in this manner. Read and write makes up the bulk of the operations on the file. Another method is direct access. The file is made up of fixed length logical records that allows programs to read and write in no particular order. This method supports operations such as read n and write n where n is the relative block number.
File Systems also have to keep track of the layout referred to as the storage structure. Directories are a useful way of organizing files and they support operations such as Search for a file, create a file, delete a file, list a directory and rename a file. Traversal of the file system to list all directories and files is another common operation. Layout of files and directories could have a backup copy.
File Systems is usually mounted before it can be used. The mount point defines the location within the file structure where the file system is to be attached. The file system is then checked if its valid.
A file system can be remote such as in the case of distributed file system where the remote file systems are visible from the local machine.