The basic unit of storage is a disk. Regardless of the partition style or disk type, Windows Server 2003 reads data from disks and writes data to disks using the disk input/output (I/O) subsystem. The I/O subsystem understands the physical and logical structures of disks, which allows it to perform read and write operations. The basic physical structure of a disk includes:

Each disk has one or more platters. Platters are the physical media from which data is read and to which data is written. The disk head travels in a circular path over the platter. This circular path is called a track. Tracks are magnetically encoded when you format a disk. Tracks that reside in the same location on each platter form a cylinder. For example, if a disk has four platters, Cylinder 1 consists of Track 1 from all four platters.

Tracks are divided into sectors. Sectors represent a subsection within a track and are made up of individual bytes. The number of sectors in a track depends on the disk type and the location of the track on the platter. Tracks closer to the outside of the platter can have more sectors than tracks near the center of the platter.

When you format a disk with a file system, the file system structures the disk using clusters, which are logical groupings of sectors. Both FAT and NTFS use a fixed sector size of 512 bytes but allow the cluster size to be variable. For example, the cluster size might be 4096 bytes, and if there are 512 bytes per sector, each cluster is made up of eight sectors. Table 20-1 provides a summary of the default cluster sizes for FAT, FAT32, and NTFS. You have the option of specifying the cluster size when you create a file system on a disk, or you can accept the default cluster size setting. Either way, the cluster sizes available depend on the type of file system you are using.

The important thing to know about clusters is that they are the smallest unit in which disk space is allocated. Each cluster can hold one file at most. So, if you create a 1-kilobyte (KB) file and the cluster size is 4 KB, there will be 3 KB of empty space in the cluster that isn't available to other files. That's just the way it is. If a single cluster isn't big enough to hold an entire file, then the remaining file data will go into the next available cluster and the next until the file is completely stored. For FAT, for example, the first cluster used by the file has a pointer to the second cluster, and the second cluster has a pointer to the next, and so on until you get to the final cluster used by the file, which has an End Of File (EOF) marker.

While the disk I/O subsystem manages the physical structure of disks, Windows Server 2003 manages the logical disk structure at the file system level. The logical structure of a disk relates to the basic or dynamic volumes you create on a disk and the file systems with which those volumes are formatted. You can format both basic volumes and dynamic volumes using FAT or NTFS. As discussed in the next section, each file system type has a different structure, and there are advantages and disadvantages of each as well.