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Q100108: Overview of FAT, HPFS, and NTFS File Systems

Article: Q100108
Product(s): Microsoft Windows NT
Version(s): ; winnt:
Operating System(s): 
Keyword(s): kbotherkbfaq
Last Modified: 19-FEB-2002

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The information in this article applies to:

- Microsoft Windows NT Server version 3.1 
- Microsoft Windows NT Workstation version 3.1 
- Microsoft Windows NT Advanced Server 
- Microsoft Windows NT Workstation 
- Microsoft Windows NT Server 
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SUMMARY
=======

This article explains the differences between FAT, HPFS, and NTFS under Windows
NT, and their advantages and disadvantages. It is divided into the following
sections:

- FAT Overview

- HPFS Overview

- NTFS Overview

NOTE: HPFS is only supported under Windows NT versions 3.1, 3.5, and 3.51.
Windows NT 4.0 does not support and cannot access HPFS partitions. Also, the
FAT32 file system is only supported in the Windows 98/95 and Windows 2000.

MORE INFORMATION
================

FAT OVERVIEW
------------

FAT is by far the most simplistic of the file systems supported by Windows NT.
The FAT file system is characterized by the file allocation table (FAT), which
is really a table that resides at the very "top" of the volume. To protect the
volume, two copies of the FAT are kept in case one becomes damaged. In addition,
the FAT tables and the root directory must be stored in a fixed location so that
the system's boot files can be correctly located.

A disk formatted with FAT is allocated in clusters, whose size are determined by
the size of the volume. When a file is created, an entry is created in the
directory and the first cluster number containing data is established. This
entry in the FAT table either indicates that this is the last cluster of the
file, or points to the next cluster.

Updating the FAT table is very important as well as time consuming. If the FAT
table is not regularly updated, it can lead to data loss. It is time consuming
because the disk read heads must be repositioned to the drive's logical track
zero each time the FAT table is updated.

There is no organization to the FAT directory structure, and files are given the
first open location on the drive. In addition, FAT supports only read-only,
hidden, system, and archive file attributes.

FAT Naming Convention
---------------------

FAT uses the traditional 8.3 file naming convention and all filenames must be
created with the ASCII character set. The name of a file or directory can be up
to eight characters long, then a period (.) separator, and up to a three
character extension. The name must start with either a letter or number and can
contain any characters except for the following:

  . " / \ [ ] : ; | = ,

If any of these characters are used, unexpected results may occur. The name
cannot contain any spaces.

The following names are reserved:

  CON, AUX, COM1, COM2, COM3, COM4, LPT1, LPT2, LPT3, PRN, NUL

All characters will be converted to uppercase.

Advantages of FAT
-----------------

It is not possible to perform an undelete under Windows NT on any of the
supported file systems. Undelete utilities try to directly access the hardware,
which cannot be done under Windows NT. However, if the file was located on a FAT
partition, and the system is restarted under MS-DOS, the file can be undeleted.
The FAT file system is best for drives and/or partitions under approximately 200
MB, because FAT starts out with very little overhead. For further discussion of
FAT advantages, see the following:

- Microsoft Windows NT Server "Concepts and Planning Guide," Chapter 5, section
  titled "Choosing a File System"

- Microsoft Windows NT Workstation 4.0 Resource Kit, Chapter 18, "Choosing a
  File System"

- Microsoft Windows NT Server 4.0 Resource Kit "Resource Guide," Chapter 3,
  section titled "Which File System to Use on Which Volumes"

Disadvantages of FAT
--------------------

Preferably, when using drives or partitions of over 200 MB the FAT file system
should not be used. This is because as the size of the volume increases,
performance with FAT will quickly decrease. It is not possible to set
permissions on files that are FAT partitions.

FAT partitions are limited in size to a maximum of 4 Gigabytes (GB) under Windows
NT and 2 GB in MS-DOS. For additional information on this limitation, please see
the following article in the Microsoft Knowledge Base:

  ARTICLE-ID: Q118335
  TITLE : Maximum Partition Size in MS-DOS

For further discussion of other disadvantages of FAT, see the following:

- Microsoft Windows NT Server "Concepts and Planning Guide," Chapter 5, section
  titled "Choosing a File System"

- Microsoft Windows NT Workstation 4.0 Resource Kit, Chapter 18, "Choosing a
  File System"

- Microsoft Windows NT Server 4.0 Resource Kit "Resource Guide," Chapter 3,
  section titled "Which File System to Use on Which Volumes"

HPFS OVERVIEW
-------------

The HPFS file system was first introduced with OS/2 1.2 to allow for greater
access to the larger hard drives that were then appearing on the market.
Additionally, it was necessary for a new file system to extend the naming
system, organization, and security for the growing demands of the network server
market. HPFS maintains the directory organization of FAT, but adds automatic
sorting of the directory based on filenames. Filenames are extended to up to 254
double byte characters. HPFS also allows a file to be composed of "data" and
special attributes to allow for increased flexibility in terms of supporting
other naming conventions and security. In addition, the unit of allocation is
changed from clusters to physical sectors (512 bytes), which reduces lost disk
space.

Under HPFS, directory entries hold more information than under FAT. As well as
the attribute file, this includes information about the modification, creation,
and access date and times. Instead of pointing to the first cluster of the file,
the directory entries under HPFS point to the FNODE. The FNODE can contain the
file's data, or pointers that may point to the file's data or to other
structures that will eventually point to the file's data.

HPFS attempts to allocate as much of a file in contiguous sectors as possible.
This is done in order to increase speed when doing sequential processing of a
file.

HPFS organizes a drive into a series of 8 MB bands, and whenever possible a file
is contained within one of these bands. Between each of these bands are 2K
allocation bitmaps, which keep track of which sectors within a band have and
have not been allocated. Banding increases performance because the drive head
does not have to return to the logical top (typically cylinder 0) of the disk,
but to the nearest band allocation bitmap to determine where a file is to be
stored.

Additionally, HPFS includes a couple of unique special data objects:

Super Block
-----------

The Super Block is located in logical sector 16 and contains a pointer to the
FNODE of the root directory. One of the biggest dangers of using HPFS is that if
the Super Block is lost or corrupted due to a bad sector, so are the contents of
the partition, even if the rest of the drive is fine. It would be possible to
recover the data on the drive by copying everything to another drive with a good
sector 16 and rebuilding the Super Block. However, this is a very complex task.

Spare Block
-----------

The Spare Block is located in logical sector 17 and contains a table of "hot
fixes" and the Spare Directory Block. Under HPFS, when a bad sector is detected,
the "hot fixes" entry is used to logically point to an existing good sector in
place of the bad sector. This technique for handling write errors is known as
hot fixing.

Hot fixing is a technique where if an error occurs because of a bad sector, the
file system moves the information to a different sector and marks the original
sector as bad. This is all done transparent to any applications that are
performing disk I/O (that is, the application never knows that there were any
problems with the hard drive). Using a file system that supports hot fixing will
eliminate error messages such as the FAT "Abort, Retry, or Fail?" error message
that occurs when a bad sector is encountered.

Note: The version of HPFS that is included with Windows NT does not support hot
fixing.

Advantages of HPFS
------------------

HPFS is best for drives in the 200-400 MB range. For more discussion of the
advantages of HPFS, see the following:

- Microsoft Windows NT Server "Concepts and Planning Guide," Chapter 5, section
  titled "Choosing a File System"

- Microsoft Windows NT Workstation 4.0 Resource Kit, Chapter 18, "Choosing a
  File System"

- Microsoft Windows NT Server 4.0 Resource Kit "Resource Guide," Chapter 3,
  section titled "Which File System to Use on Which Volumes"

Disadvantages of HPFS
---------------------

Because of the overhead involved in HPFS, it is not a very efficient choice for a
volume of under approximately 200 MB. In addition, with volumes larger than
about 400 MB, there will be some performance degradation. You cannot set
security on HPFS under Windows NT.

HPFS is only supported under Windows NT versions 3.1, 3.5, and 3.51. Windows NT
4.0 cannot access HPFS partitions.

For additional disadvantages of HPFS, see the following:

- Microsoft Windows NT Server "Concepts and Planning Guide," Chapter 5, section
  titled "Choosing a File System"

- Microsoft Windows NT Workstation 4.0 Resource Kit, Chapter 18, "Choosing a
  File System"

- Microsoft Windows NT Server 4.0 Resource Kit "Resource Guide," Chapter 3,
  section titled "Which File System to Use on Which Volumes"

NTFS OVERVIEW
-------------

From a user's point of view, NTFS continues to organize files into directories,
which, like HPFS, are sorted. However, unlike FAT or HPFS, there are no
"special" objects on the disk and there is no dependence on the underlying
hardware, such as 512 byte sectors. In addition, there are no special locations
on the disk, such as FAT tables or HPFS Super Blocks.

The goals of NTFS are to provide:

- Reliability, which is especially desirable for high end systems and file
  servers

- A platform for added functionality

- Support POSIX requirements

- Removal of the limitations of the FAT and HPFS file systems

Reliability
-----------

To ensure reliability of NTFS, three major areas were addressed: recoverability,
removal of fatal single sector failures, and hot fixing.

NTFS is a recoverable file system because it keeps track of transactions against
the file system. When a CHKDSK is performed on FAT or HPFS, the consistency of
pointers within the directory, allocation, and file tables is being checked.
Under NTFS, a log of transactions against these components is maintained so that
CHKDSK need only roll back transactions to the last commit point in order to
recover consistency within the file system.

Under FAT or HPFS, if a sector that is the location of one of the file system's
special objects fails, then a single sector failure will occur. NTFS avoids this
in two ways: first, by not using special objects on the disk and tracking and
protecting all objects that are on the disk. Secondly, under NTFS, multiple
copies (the number depends on the volume size) of the Master File Table are
kept.

Similar to OS/2 versions of HPFS, NTFS supports hot fixing.

Added Functionality
-------------------

One of the major design goals of Windows NT at every level is to provide a
platform that can be added to and built upon, and NTFS is no exception. NTFS
provides a rich and flexible platform for other file systems to be able to use.
In addition, NTFS fully supports the Windows NT security model and supports
multiple data streams. No longer is a data file a single stream of data.
Finally, under NTFS, a user can add his or her own user-defined attributes to a
file.

POSIX Support
-------------

NTFS is the most POSIX.1 compliant of the supported file systems because it
supports the following POSIX.1 requirements:

Case Sensitive Naming:

Under POSIX, README.TXT, Readme.txt, and readme.txt are all different files.

Additional Time Stamp:

The additional time stamp supplies the time at which the file was last accessed.

Hard Links:

A hard link is when two different filenames, which can be located in different
directories, point to the same data.

Removing Limitations
--------------------

First, NTFS has greatly increased the size of files and volumes, so that they can
now be up to 2^64 bytes (16 exabytes or 18,446,744,073,709,551,616 bytes). NTFS
has also returned to the FAT concept of clusters in order to avoid HPFS problem
of a fixed sector size. This was done because Windows NT is a portable operating
system and different disk technology is likely to be encountered at some point.
Therefore, 512 bytes per sector was viewed as having a large possibility of not
always being a good fit for the allocation. This was accomplished by allowing
the cluster to be defined as multiples of the hardware's natural allocation
size. Finally, in NTFS all filenames are Unicode based, and 8.3 filenames are
kept along with long filenames.

Advantages of NTFS
------------------

NTFS is best for use on volumes of about 400 MB or more. This is because
performance does not degrade under NTFS, as it does under FAT, with larger
volume sizes.

The recoverability designed into NTFS is such that a user should never have to
run any sort of disk repair utility on an NTFS partition. For additional
advantages of NTFS, see the following:

- Microsoft Windows NT Server "Concepts and Planning Guide," Chapter 5, section
  titled "Choosing a File System"

- Microsoft Windows NT Workstation 4.0 Resource Kit, Chapter 18, "Choosing a
  File System"

- Microsoft Windows NT Server 4.0 Resource Kit "Resource Guide," Chapter 3,
  section titled "Which File System to Use on Which Volumes"

Disadvantages of NTFS
---------------------

It is not recommended to use NTFS on a volume that is smaller than approximately
400 MB, because of the amount of space overhead involved in NTFS. This space
overhead is in the form of NTFS system files that typically use at least 4 MB of
drive space on a 100 MB partition.

Currently, there is no file encryption built into NTFS. Therefore, someone can
boot under MS-DOS, or another operating system, and use a low-level disk editing
utility to view data stored on an NTFS volume.

It is not possible to format a floppy disk with the NTFS file system; Windows NT
formats all floppy disks with the FAT file system because the overhead involved
in NTFS will not fit onto a floppy disk.

For further discussion of NTFS disadvantages, see the following:

- Microsoft Windows NT Server "Concepts and Planning Guide," Chapter 5, section
  titled "Choosing a File System"

- Microsoft Windows NT Workstation 4.0 Resource Kit, Chapter 18, "Choosing a
  File System"

- Microsoft Windows NT Server 4.0 Resource Kit "Resource Guide," Chapter 3,
  section titled "Which File System to Use on Which Volumes"

NTFS Naming Conventions
-----------------------

File and directory names can be up to 255 characters long, including any
extensions. Names preserve case, but are not case sensitive. NTFS makes no
distinction of filenames based on case. Names can contain any characters except
for the following:

  ?  "  /  \  <  >  *  |  :

Currently, from the command line, you can only create file names of up to 253
characters.

NOTE: Underlying hardware limitations may impose additional partition size
limitations in any file system. Particularly, a boot partition can be only 7.8
GB in size, and there is a 2-terabyte limitation in the partition table.

For more information about the supported file systems for Windows NT, please see
the Windows NT Resource Kit.

Additional query words: 3.10 3.50 3.51 4.00 prodnt gb eb

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Keywords          : kbother kbfaq
Technology        : kbWinNTsearch kbWinNTWsearch kbWinNTW310 kbWinNTSsearch kbWinNTS310 kbWinNTAdvSerSearch kbWinNTS310search kbWinNT310Search kbWinNTW310Search
Version           : :; winnt:

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