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Limitations in File Systems
Each file system supports a maximum volume size, file size, and number of files per volume. For example, generally FAT16 and FAT32 volumes are limited to 4 GB and 32 GB (Generally) respectively. There are some limitations related to FAT file systems you must know, given below:
FAT12: FAT volumes smaller than 16 MB are formatted as FAT12. It is the oldest FAT type and uses a 12-bit binary to hold cluster numbers. A volume formatted using FAT12 can hold a maximum of 4,086 clusters, which is equal to 2 12 minus a few reserved values to be used in FAT. (We shall discuss it in detail in the logical structure of the disk given next in this chapter). Therefore FAT12 is most suitable for smaller volumes. It is used on floppy disks and hard disk partitions smaller than about 16 MB.
FAT16: The FAT16 uses a 16-bit binary number to hold cluster numbers. A volume using FAT16 can hold a maximum of 65,526 clusters, which is equal to 2 16 minus a few reserved values to be used in FAT. (We shall discuss it in detail in the logical structure of the disk given next in this chapter). FAT16 is used for hard disk volumes ranging in size from 16 MB to 2,048 MB. FAT16 volumes larger than 2 GB are not accessible from computers running MS-DOS, Windows 95/98/ME and many other operating systems. This limitation occurs because these operating systems do not support cluster sizes larger than 32 KB, which results in the 2 GB limit. (See the Clusters limit given next in this chapter).
FAT32: In theory, the maximum FAT32 volumes may be up to 2048 GB (approximately 2 Terabytes). The FAT32 is supported by Windows 95's OEM SR2 release, as well as Windows 98/ME. FAT32 uses a 28-bit binary cluster number (Remember! not 32, because 4 of the 32 bits are "Reserved"). Thus theoretically FAT32 can handle volumes with over 268 million clusters (Actually 268,435,456 clusters), and will support drives up to 2 TB in size. However to do this the size of the FAT grows very large. (We are going to discuss it in the topics given next in this chapter).
The comparison of FAT Types has been given next, in the table.
 File System
 Attribute |
FAT12
|
FAT16
|
FAT32 |
Used For… |
Floppies and small hard disk volumes
|
Small to large hard disk volumes
|
Medium to very large hard disk volumes
|
Size of Each FAT Entry |
12 Bits
|
16 Bits (2 Bytes)
|
32 Bits ( 4 Bytes)
|
Maximum Number of Clusters |
4,086
|
65,526
|
~268,435,456
|
Cluster Size Used |
0.5 KiB to 4 KiB
|
2 KiB to 64 KiB
|
4 KiB to 32 KiB
|
Maximum Volume Size |
16,736,256 Bytes
|
2,147,483,648
Bytes
|
2,199,023,255,552 Bytes (about 2 Terabytes or 2 41 Bytes)
|
Maximum File Size |
Less then 16MB (Approximate) |
2,147,483,520 Bytes |
4,294,967,295 Bytes (2 32 – 1)
|
Maximum Files and Directories within the Root Directory |
128/512
(128 for 3½ 1.44MB FDD and 512 for Hard Disk Drives)
|
512
(Long file names can reduce the number of available files and Directories in the root Directory.)
|
65,534
(Long file names can reduce the number of available files and Directories in the root Directory.)
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• NTFS: NTFS stands for New Technology File System. It is used by Windows 2000/XP. In theory, the maximum NTFS partition size is (2 64 – 1) clusters.
The detailed description of NTFS File System is beyond the limit of this book however some limitations of it have been given in the following table:
Description
|
Limit |
Maximum file size
|
16 Exabytes – 1 KB (2 64 Bytes – 1 KB)
|
Maximum volume size
|
(2 64 – 1) clusters
|
Files (and Folders) per volume
|
4,294,967,295 (2 32 – 1 ) Flies and Folders
|
Cluster
The smallest unit of space on the hard disk for allocation that any software can access is the sector , which contains 512 bytes. It is possible to have an allocation system for the disk where each file is assigned as many individual sectors as it needs. For example, a 1 MB file would require approximately 2,048 individual sectors to store its data.
In case of FAT file system or rather we can say in most of the file systems, individual sectors are not used. There are several performance reasons for this. When DOS writes some information onto the hard disk, it does not allocate the space sector wise, instead it uses a new unit of storage called cluster.
FAT was designed many years ago and is a simple file system, and is not capable of managing individual sectors. What FAT does instead is to group sectors into larger blocks that are called clusters or allocation units.
A cluster is the smallest unit of disk space that can be allocated to a file. This is the reason that the clusters are often called allocation units. It may be very difficult to manage the disk when files are broken into 512-byte pieces.
A 20 GB disk volume using 512 byte sectors managed individually would contain over 41 million individual sectors, and keeping track of this many pieces of information is time and resource consuming. However some operating systems do allocate space to files by the sector but they require some advanced intelligence to do this properly.
Clusters are the minimum space allocated by the DOS when storing any information on the disk. Even to store only one byte long information on the disk requires minimum one cluster area on the disk surface.
If one cluster can store 512 bytes of information then to store 513 bytes you will require two clusters. Every file must be allocated an integer number of clusters. This means that if a volume uses clusters that contain 4,096 bytes then a 610 byte file will use one cluster thus 4,096 bytes on the disk but a 4,097 byte file uses two clusters thus 8,192 bytes on the disk.
This is the reason that cluster size is so important to make you sure to maximize the efficient use of the disk. Thus we can understand that the larger cluster sizes result the more wasted space.
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