Magnetic disk storage is the packaging of magnetic media disks, packaged read/write head electronics on a spinning electro-mechanical device. Disks are made up of round platters coated with a magnetic material that can be written and read electronically through the encoding of the magnetic media with binary data.
Figure 6-8 shows the platters that make up the magnetic media and the read/write electronics that surround the disks, all of which resembles old analogue record players. The read/write electronics are recording heads that both read and write data. As discussed previously, the head assembly can house data buffers that compensate for delays in reading or writing data.
The platters are organized into tracks, concentric rings that form the writable surface of the platter. These are further divided into sectors which form the smallest readable/writable units on the disk. As the disk rotates, portions of the sectors become visible as the read/write heads are positioned to that track.
Drives are made up of more than one disk, with each storing data on both sides. Each surface must have its own read/write head and a set of tracks which can be accessed through a single position (for instance, when vertically aligned). This is called a cylinder. Given this organization, every sector on the drive can be uniquely addressed by its cylinder, head, and sector numbers .
Sectors are important because they are the fundamental metric that determines the size of data stored within the disk. The number of bytes in a sector corresponds to the disks formatted capacity. As indicated in Figure 6-8, the sector must contain information about addresses and synchronizing HDA error corrections in the header fields. Given that the header and associated synchronization gaps are not usable for data, the formatted capacity is always less. Formatting a drive writes the header information and arbitrary data pattern to verify correct error checking. Although entire drives are normally formatted at one time, a technique called soft sectoring allows a single track or hard sector to be formatted.
Technical specifications that surround disks include the capacity, transfer rate, and average access time. Each metric is important when considering the given workload supported. While capacity may be an issue, the number of transactions necessary to read data from the disk may compromise seek time. On the other hand, large transfers of data (like that used by datacentric applications in data warehousing) may push capacity and transfer rates to their optimum while sacrificing average seek time.
Capacity, as mentioned previously, comes in two forms: formatted and unformatted. As youve probably guessed, the larger the disk, the greater the amount of addressing and error correction code (ECC) overhead, thereby reducing space for actual user data.
Transfer rate and throughput refers to the speed at which the HDA processes read and write operations. Throughput is the amount of data the drive can deliver or accept at the interface on a sustained basis. This is important because the term interface can have many connotations and implementations .
Average access time is the time needed to position the HDA to a specific cylinder, as well as and the time it takes for the requested sector to rotate under the heads. Thus, disk rotation speeds are related to capacity.