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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: 118335
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.