This article is about the way computers organise data stored on media such as disk. By separating the data into pieces and giving operating system concepts 6th edition silberschatz galvin pdf free download piece a name, the information is easily isolated and identified. The structure and logic rules used to manage the groups of information and their names is called a “file system”.
There are many different kinds of file systems. Each one has different structure and logic, properties of speed, flexibility, security, size and more. Some file systems have been designed to be used for specific applications. By 1961 the term was being applied to computerized filing alongside the original meaning. By 1964 it was in general use.
A file system consists of two or three layers. Sometimes the layers are explicitly separated, and sometimes the functions are combined. The logical file system “manage open file table entries and per-process file descriptors. This interface allows support for multiple concurrent instances of physical file systems, each of which is called a file system implementation. Note: this only applies to file systems used in storage devices. File systems allocate space in a granular manner, usually multiple physical units on the device. For a 512-byte allocation, the average unused space is 256 bytes.
For 64 KB clusters, the average unused space is 32 KB. The size of the allocation unit is chosen when the file system is created. Choosing the allocation size based on the average size of the files expected to be in the file system can minimize the amount of unusable space. Frequently the default allocation may provide reasonable usage. Choosing an allocation size that is too small results in excessive overhead if the file system will contain mostly very large files. As a file system is used, files are created, modified and deleted. When a file is created the file system allocates space for the data.
Some file systems permit or require specifying an initial space allocation and subsequent incremental allocations as the file grows. As files are deleted the space they were allocated eventually is considered available for use by other files. This creates alternating used and unused areas of various sizes. This is free space fragmentation. When a file is created and there is not an area of contiguous space available for its initial allocation the space must be assigned in fragments. When a file is modified such that it becomes larger it may exceed the space initially allocated to it, another allocation must be assigned elsewhere and the file becomes fragmented.
Most file systems have restrictions on the length of filenames. Other bookkeeping information is typically associated with each file within a file system. A file system stores all the metadata associated with the file—including the file name, the length of the contents of a file, and the location of the file in the folder hierarchy—separate from the contents of the file. Most file systems store the names of all the files in one directory in one place—the directory table for that directory—which is often stored like any other file. Most file systems also store metadata not associated with any one particular file.
Some file systems provide for user defined attributes such as the author of the document, the character encoding of a document or the size of an image. Some file systems allow for different data collections to be associated with one file name. Apple has long used a forked file system on the Macintosh, and Microsoft supports streams in NTFS. File systems include utilities to initialize, alter parameters of and remove an instance of the file system. Some include the ability to extend or truncate the space allocated to the file system.