NTFS (New Technology File System) is the primary file system used by modern versions of Microsoft Windows. Introduced with Windows NT 3.1 in 1993, NTFS has undergone several revisions and improvements over the years to become the robust and reliable file system that underpins Windows today.
At its core, NTFS organizes a volume into files and folders stored on disk partitions. It uses advanced data structures and algorithms to track file information, support powerful features like permissions and encryption, and enable system recovery tools.
One of the main data structures in NTFS is the master file table (MFT). The MFT is an implementation of a relational database that contains entries for every file and folder on the volume. Each MFT entry, or record, contains a set of attributes that define the file’s name, creation/modification time stamps, security descriptors, and data content. Small files may be stored entirely within the MFT record itself, while larger files have attributes that point to their actual location on disk.
The structure of records in the MFT allows efficient lookup and traversal of files and directories. A hierarchy ID attribute establishes parent-child relationships between directories. A directory index tracks files by their names and allocation information. Entries can be looked up via a B+ tree structure for fast searches even on large volumes.
Another key component of NTFS is the Log File System, which maintains a transaction log file to record all changes to metadata. This transactional approach allows NTFS to recover damaged or interrupted disk writes that might leave file system structures in an inconsistent state. The log caches writes so they can be committed to disk asynchronously. Meanwhile it stores modifications in a careful order that will maintain integrity if applied to the volume again. The log file is cyclically overwritten but can optionally be duplicated on another disk for added redundancy.
Here are some other notable NTFS features:
- Permissions - NTFS enforces file access control through access control lists (ACLs) associated with each file and folder. ACLs regulate access for specific users and groups down to the level of allowing or denying reads, writes, or execution of a file. Complex rules can be implemented such as inheritance of permissions from parent directories.
- Encryption - With the Encrypting File System (EFS), NTFS can transparently encrypt files and folders using public key cryptography. Users are assigned public-private key pairs that are used to encrypt and decrypt files. Keys are derived from user account passwords via cryptographic hashing.
- Compression - To save disk space, NTFS can compress files and folders using the LZNT1 algorithm, a variant of Lempel-Ziv compression optimized for fast decompression.
- Sparse Files - Sparse files contain large regions of data not actually allocated on disk. Only nonzero data is stored, saving space for large data sets that are mostly empty.
- Junctions and Symbolic Links - These special files transparently reference other folders and volumes via absolute or relative paths. This provides namespace unification, hiding multi-volume complexity.
- Hard Links - Allow a file to have multiple directory entries that reference the same physical data. Used for consistency in data access.
- Alternate Data Streams - Allow storage of metadata and other data within a hidden alternate stream for each file. Used for compatibility with legacy applications.
Under the hood, Windows translates high-level file operations to NTFS transactions which manipulate file system structures. For example, creating a new file results in allocating an MFT entry, updating parent directory indices, logging the transaction in case it fails, and finally writing the file data out to the volume. The transactional nature of NTFS ensures that each operation transforms the state of the file system predictably.
NTFS code is implemented within the Windows I/O manager and runtime library. Areas like the cache manager handle caching of directory and file metadata to optimize performance. The memory manager maps NTFS structures into process virtual memory so applications can access them directly. Of course the actual NTFS driver handles lower level operations like managing physical disk blocks.
While NTFS excels in organizing structured data storage, it does have limitations. There is no built-in support for user tags, comments, deduplication of identical files, or snapshotting of incremental file versions. And as a monolithic file system, NTFS functionality is tied to Windows releases and cannot be extended by third parties.
Nevertheless, after continuous improvements over 30 years, NTFS provides an efficient, robust platform for Windows storage and data access. Its capabilities evolved from the specialized needs of server products to the demands of high-end workstations and now personal devices. It has incorporated new features like support for massive volume sizes, high reliability mechanisms like CHKDSK, and full 64-bit operational support. Undoubtedly it will continue adapting to new storage technologies like solid-state drives and cloud storage backends.