NTFS (NT file system; sometimes New Technology File System) is the file system that the Windows NT operating system uses for storing and retrieving files on a hard disk. NTFS is the Windows NT equivalent of the Windows 95 file allocation table (FAT) and the OS/2 High Performance File System (HPFS). However,

NTFS offers a number of improvements over FAT and HPFS in terms of performance, extendibility, and security.

Notable features of NTFS include:
• Use of a b-tree directory scheme to keep track of file clusters
• Information about a file’s clusters and other data is stored with each cluster, not just a governing table (as FAT is)
• Support for very large files (up to 2 to the 64th power or approximately 16 billion bytes in size)
• An access control list (ACL) that lets a server administrator control who can access specific files
• Integrated file compression
• Support for names based on Unicode
• Support for long file names as well as “8 by 3” names
• Data security on both removable and fixed disks

How NTFS Works
When a hard disk is formatted (initialized), it is divided into partitions or major divisions of the total physical hard disk space. Within each partition, the operating system keeps track of all the files that are stored by that operating system. Each file is actually stored on the hard disk in one or more clusters or disk spaces of a predefined uniform size. Using NTFS, the sizes of clusters range from 512 bytes to 64 kilobytes. Windows NT provides a recommended default cluster size for any given drive size. For example, for a 4 GB (gigabyte) drive, the default cluster size is 4 KB (kilobytes). Note that clusters are indivisible. Even the smallest file takes up one cluster and a 4.1 KB file takes up two clusters (or 8 KB) on a 4 KB cluster system.

The selection of the cluster size is a trade-off between efficient use of disk space and the number of disk accesses required to access a file. In general, using NTFS, the larger the hard disk the larger the default cluster size, since it’s assumed that a system user will prefer to increase performance (fewer disk accesses) at the expense of some amount of space inefficiency.

When a file is created using NTFS, a record about the file is created in a special file, the Master File Table (MFT). The record is used to locate a file’s possibly scattered clusters. NTFS tries to find contiguous storage space that will hold the entire file (all of its clusters).