3.4.2 File Allocation Table and nt File System

• Clusters and File Allocation Tables

• FAT16

• FAT32

• NT File System

Clusters and File Allocation Tables

Disks are divided into tracks and sectors. See the figure below.

Figure 1 Tracks and sectors on a disk

Sectors hold a fixed number of bytes, typically 512 bytes. One or more sectors are allocated to store a file. If the file contains only a line or two of text, it will fit into a fraction of one sector. In that case, the remainder of the sector is left unused. The unused portion is called slack space. If the file is large, perhaps tens of millions of bytes in length, it will not fit even on a single track. It will require thousands of sectors spread across multiple tracks (they need not be contiguous tracks). As there are likely to be hundreds or even thousands of files on the disk, and each needs one or more sectors, there is some bookkeeping to do. File systems differ in the details of how they solve the bookkeeping problem, but the basic principles are the same.

Because sectors are small, modern computer systems group them into clusters and read or write an entire cluster at a time. A cluster is the smallest amount of space any file can occupy on a disk. A cluster contains 4, 8, 16, 32, or 64 adjacent sectors (the number must be a power of 2). The choice of cluster size depends on the capacity of the drive—the bigger the drive, the bigger the cluster size. A small portion of the disk is reserved for the File Allocation Table (FAT). For each cluster that is part of a file, the FAT entry gives the number of the next cluster for that file. In this way, the clusters that make up a file are chained together, so if you know the address in the FAT of the first cluster of a file, you can find all the others by following the chain. The FAT entry for the last cluster in the chain contains a special marker to indicate that it is the end of the chain.

FAT16

In early versions of the Windows operating system and in MS-DOS, which preceded Windows, the FAT used 16 bits (two bytes) per entry, which allowed for a total of 2¹⁶ or 65,536 clusters. This scheme is now referred to as FAT16. As hard disk drives got larger, a problem developed— the FAT was too small to accommodate all the available clusters and only allowed for partitions up to 2GB. The following calculation shows how the partition limit of 2GB is derived: 512(2⁹) bytes per sector * 64 (2⁶) sectors per cluster * 2¹⁶ clusters in a FAT 16 partition = 2³¹ bytes = 2GB). In addition, small files on a large-cluster file system contain more slack space, wasting space on the disk that could be used to store additional files. For example, there are many files that are 1000 bytes or less, but a 2GB FAT16 partition will reserve 32KB of disk space for each one of those files: 512 (2⁹) bytes per sector * 64 (2⁶) sectors in a cluster = 32768 (2¹⁵) bytes = 32KB.

One solution to using FAT16 on hard disk drives that contain more than 2GB is to partition the drive into several logical drives—such as drive C, D, and E—each with its own FAT. This works, but it forces users to spread their files across several logical drives when they may not want to organize things that way. Also, if one logical drive completely fills up, no file on that logical drive can grow any larger, even if there is plenty of slack space elsewhere on the disk. Finally, some applications that require huge files, such as database systems, may find that even an entire FAT16 partition is not enough space for one file, although the disk as a whole has enough room.

FAT32

To address these problems, Windows 9x/2000/XP support a FAT32 file system. In this system, 32 bits (4 bytes) are used per entry, but the first 4 bits are reserved. Therefore, it has a total of 2^(32-4) = 2²⁸ = 268435456 clusters. In a FAT32 file system, smaller clusters can be used instead of larger FAT16 clusters. This leads to more efficient space allocation on the FAT32 drive. The FAT32 can support drives up to two terabytes in size.