A (hard) disk drive record data on the surfaces of metal plates called platters that are coated with a substance containing ground-up iron, or other substances that allow zeros and ones to be recorded as tiny spots of magnetization. Floppy disks (also called “diskettes'’ by those who think the term “floppy'’ is undignified) are similar, but use a sheet of plastic rather than metal, and permanently enclose it in a paper or plastic envelope. I won’t say anything more about floppy disks, but most of facts about hard disks are also true for floppies, but slower. It is customary to use the simple term “disk'’ to mean “hard disk drive'’ and say “platter'’ when you mean the disk itself.

When in use, the disk spins rapidly and a read/write head slides along the surface. Usually, both sides of a platter are used for recording, so there is a head for each surface. In some more expensive disk drives, there are several platters, all on a common axle spinning together. The heads are fixed to an arm that can move radially in towards the axle or out towards the edges of the platters. All of the heads are attached to the same arm, so they are all at the same distance from the centers of their platters at any given time.

To read or write a bit of data on the disk, a head has to be right over the spot where the data is stored. This may require three operations, giving rise to four kinds of delay.

· The correct head (i.e., the correct surface) must be selected. This is done electronicly, so it is very fast (at most a few microseconds).

· The head has to be moved to the correct distance from the center of the disk. This movement is called seeking and involves physically moving the arm in or out. Because the arm has mass (inertia), it must be accelerated and decelerated. When it finally gets where it’s going, the disk has to wait a bit for the vibrations caused by the jerky movement to die out. All in all, seeking can take several milliseconds, depending on how far the head has to move.

· The disk has to rotate until the correct spot is under the selected disk. Since the disk is constantly spinning, all the drive has to do is waiting for the correct spot to come around.

· Finally, the actual data has to be transferred. On a read operation, the data is usually transferred to a RAM buffer in the device and then copied, by DMA, to the computer’s main memory. Similarly, on write, the data is transferred by DMA to a buffer in the disk, and then copied onto the surface of a platter.

The total time spent getting to the right place on the disk is called latency and is divided into rotational latency and seek time (although sometimes people use the term “seek time'’ to cover both kinds of latency).

The data on a disk is divided up into fixed-sized disk blocks. The hardware only supports reading or writing a whole block at a time. If a program wants to change one bit (or one byte) on the disk, it has to read in an entire disk block, change the part of it want to change, and then write it back out. Each block has a location, sometimes called a disk address that consists of three numbers: surface, track, and sector. The part of the disk swept out by a head while it is not moving is a ring-shaped region on the surface called a track. The track number indicates how far the data is from the center of the disk (the axle). Each track is divided up into some number of sectors. On some disks, the outer tracks have more sectors than the inner ones because the outer tracks are longer, but all sectors are the same size. The set of tracks swept out by all the heads while the arm is not moving is a called a cylinder. Thus a seek operation moves to a new cylinder, positioning each on one track of the cylinder.

This basic picture of disk organization hasn’t changed much in forty years. What have changed are that disks keep getting smaller and cheaper and the data on the surfaces gets denser (the spots used to record bits are getting smaller and closer together). The first disks were several feet in diameter, cost tens of thousands of dollars, and held tens of thousands of bytes. Currently (2006) a typical disk is 3-1/2 inches or 1 inch in diameter, costs a few hundred dollars and holds several hundred gigabytes (billions of bytes) of data. What haven’t changed much are physical limitations. Early disks spun at 3600 revolutions per minute (RPM); currently they spin at about 7200 RPM, or 15,000 RPM for high-performance disks. At 7200 RPM, the rotational latency is at worst 1/7200 minute (8.33 milliseconds) and on the average it is half that (4.17 ms). The heads and the arm that moves them have gotten much smaller and lighter, allowing them to be moved more quickly, but the improvement has been modest. Current disks take anywhere from a millisecond to 10s of milliseconds to seek to particular cylinder.

Just for reference, here are the specs for a popular disk used in PC’s and currently selling at the ebay.com for about $220.

Disk manufacturers such as Seagate, Maxtor, and IBM have details of many more disks available online.