Editing File system (section)

=== Metadata ===

In addition to data, the file content, a file system also manages associated [[metadata]] which may include but is not limited to:

* name
* [[file size|size]] which may be stored as the number of blocks allocated or as a [[byte]] count
* [[system time|when]] created, last accessed, last backed-up
* owner [[user ID|user]] and [[group ID|group]]
* [[file system permissions|access permissions]]
* [[file attribute]]s such as whether the file is read-only, [[executable]], etc.
* [[device file|device type]] (e.g. [[Block devices|block]], [[Character devices|character]], [[Internet socket|socket]], [[subdirectory]], etc.)

A file system stores associated metadata separate from the content 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.
Many file systems put only some of the metadata for a file in the directory table, and the rest of the metadata for that file in a completely separate structure, such as the [[inode]].

Most file systems also store metadata not associated with any one particular file.
Such metadata includes information about unused regions—[[free space bitmap]], [[block availability map]]—and information about [[bad sector]]s.
Often such information about an [[allocation group]] is stored inside the allocation group itself.

Additional attributes can be associated on file systems, such as [[NTFS]], [[XFS]], [[ext2]], [[ext3]], some versions of [[Unix File System|UFS]], and [[HFS+]], using [[extended file attributes]]. 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. These separate collections may be referred to as ''streams'' or ''forks''. Apple has long used a forked file system on the Macintosh, and Microsoft supports streams in NTFS. Some file systems maintain multiple past revisions of a file under a single file name; the file name by itself retrieves the most recent version, while prior saved version can be accessed using a special naming convention such as "filename;4" or "filename(-4)" to access the version four saves ago.

See [[comparison of file systems#Metadata|comparison of file systems § Metadata]] for details on which file systems support which kinds of metadata.