File system (also “filesystem”, one word) is a term used to refer to the specific technique that allows files to be laid out and located on a hard disk or other random access storage device.
At its most basic, a file system is nothing more than the specific format of the overhead information used to keep track of what data is stored where on a hard disk, and the rules used to place and locate that data. Since most hard disks and similar devices store data as files, then this is the “system” by which those “files” are placed on the media.
In addition to defining exactly what gets stored where, file systems also define what capabilities are present, such as encryption, compression, the length of file names, the maximum size of files, and even whether or not file names can include both upper and lower case characters. File systems may also include information relating to security and permissions, allowing the operating system to control who may or may not perform operations on the files themselves.
There are dozens, if not hundreds, of different file systems, but the two most common in Windows-based computers are the FAT (File Allocation Table) file system and NTFS (New Technology File System). The FAT file system predates Windows, but is still commonly used on Windows-compatible memory cards used on mobile and small devices, due to its comparative simplicity. NTFS is the more common file system for hard disks, due to its generally better performance, increased capacity, and the ability to support important aspects of data security.
In computing, a file system or filesystem (often abbreviated to fs) controls how data is stored and retrieved. Without a file system, data placed in a storage medium would be one large body of data with no way to tell where one piece of data stops and the next begins. By separating the data into pieces and giving each piece a name, the data is easily isolated and identified. Taking its name from the way paper-based data management system is named, each group of data is called a "file." The structure and logic rules used to manage the groups of data 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. For example, the ISO 9660 file system is designed specifically for optical discs.
File systems can be used on numerous different types of storage devices that use different kinds of media. As of 2019, hard disk drives have been key storage devices and are projected to remain so for the foreseeable future. Other kinds of media that are used include SSDs, magnetic tapes, and optical discs. In some cases, such as with tmpfs, the computer's main memory (random-access memory, RAM) is used to create a temporary file system for short-term use.
Some file systems are used on local data storage devices; others provide file access via a network protocol (for example, NFS, SMB, or 9P clients). Some file systems are "virtual", meaning that the supplied "files" (called virtual files) are computed on request (such as procfs and sysfs) or are merely a mapping into a different file system used as a backing store. The file system manages access to both the content of files and the metadata about those files. It is responsible for arranging storage space; reliability, efficiency, and tuning with regard to the physical storage medium are important design considerations.