NTFS

NTFS is the standard file system of Windows NT, including its later versions Windows 2000, Windows XP, Windows Server 2003, Windows Server 2008, and Windows Vista. NTFS supersedes the FAT file system as the preferred file system for Microsoft's "Windows"-branded operating systems. NTFS has several improvements over FAT and HPFS (High Performance File System) such as improved support for metadata and the use of advanced data structures to improve performance, reliability, and disk space utilization, plus additional extensions such as security access control lists (ACL) and file system journaling. The file system specification is a trade secret, although it can be licensed commercially from Microsoft through their Intellectual Property licensing program.

In the mid 1980s Microsoft and IBM formed a joint project to create the next generation graphical operating system. The result of the project was OS/2, but eventually Microsoft and IBM disagreed on many important issues and separated. OS/2 remained an IBM project. Microsoft started to work on Windows NT. The OS/2 filesystem HPFS contained several important new features. When Microsoft created their new operating system, they borrowed many of these concepts for NTFS. Probably as a result of this common ancestry, HPFS and NTFS share the same disk partition identification type code (07). Sharing an ID is unusual since there were dozens of available codes, and other major filesystems have their own code. FAT has more than nine (one each for FAT12, FAT16, FAT32, etc.). Algorithms which identify the filesystem in a partition type 07 must perform additional checks.

In NTFS, all file data-file name, creation date, access permissions, and contents-are stored as metadata in the Master File Table. This abstract approach allowed easy addition of file system features during Windows NT's development - an interesting example is the addition of fields for indexing used by the Active Directory software.

NTFS allows any sequence of 16-bit values for name encoding (file names, stream names, index names, etc.). This means UTF-16 codepoints are supported, but the file system does not check whether a sequence is valid UTF-16 (it allows any sequence of short values, not restricted to those in the Unicode standard).

Internally, NTFS uses B+ trees to index file system data. Although complex to implement, this allows faster file look up times in most cases. A file system journal is used to guarantee the integrity of the file system metadata but not individual files' content. Systems using NTFS are known to have improved reliability compared to FAT file systems.

The Master File Table (MFT) contains metadata about every file, directory, and metafile on an NTFS volume. It includes filenames, locations, size, and permissions. Its structure supports algorithms which minimize disk fragmentation. A directory entry consists of a filename and a "file ID" which is the record number representing the file in the Master File Table. The file ID also contains a reuse count to detect stale references. While this strongly resembles the W_FID of Files-11, other NTFS structures radically differ.