The precise manner in which data is organised on a hard disk drive is determined by the file system used. File systems are generally operating system dependent. However, since it is the most widely used PC operating system, most other operating systems’ file systems are at least read-compatible with Microsoft Windows.
The FAT file system was first introduced in the days of MS-DOS way back in 1981. The purpose of the File Allocation Table is to provide the mapping between clusters – the basic unit of logical storage on a disk at the operating system level – and the physical location of data in terms of cylinders, tracks and sectors – the form of addressing used by the drive’s hardware controller.
The FAT contains an entry for every file stored on the volume that contains the address of the file’s starting cluster. Each cluster contains a pointer to the next cluster in the file, or an end-of-file indicator at (0xFFFF), which indicates that this cluster is the end of the file. The diagram shows three files: File1.txt uses three clusters, File2.txt is a fragmented file that requires three clusters and File3.txt fits in one cluster. In each case, the file allocation table entry points to the first cluster of the file.
The first incarnation of FAT was known as FAT12, which supported a maximum partition size of 8MB. This was superseded in 1984 by FAT16, which increased the maximum partition size to 2GB. FAT16 has undergone a number of minor modifications over the years, for example, enabling it to handle file names longer than the original limitation of 8.3 characters. FAT16’s principal limitation is that it imposes a fixed maximum number of clusters per partition, meaning that the bigger the hard disk, the bigger the cluster size and the more unusable space on the drive. The biggest advantage of FAT16 is that it is compatible across a wide variety of operating systems, including Windows 95/98/Me, OS/2, Linux and some versions of UNIX.
Dating from the Windows 95 OEM Service Release 2 (OSR2), Windows has supported both FAT16 and FAT32. The latter is little more than an extension of the original FAT16 file system that provides for a much larger number of clusters per partition. As such, it offers greatly improved disk utilisation over FAT16. However, FAT32 shares all of the other limitations of FAT16 plus the additional one that many non-Windows operating systems that are FAT16-compatible will not work with FAT32. This makes FAT32 inappropriate for dual-boot environments, although while other operating systems such as Windows NT can’t directly read a FAT32 partition, they can read it across the network. It’s no problem, therefore, to share information stored on a FAT32 partition with other computers on a network that are running older versions of Windows.
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.
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