DISKS

DISKS

Let's begin studying some real I/O devices. We will begin with disks, which are conceptually simple, yet very important. After that we will look at clocks, keyboards, and displays.

Disk Hardware

Disks come in a variety of types. The most common ones are the magnetic disks (hard disks and floppy disks). They are characterized by the fact that reads and writes are equally fast, which makes them ideal as secondary memory (paging, file systems, etc.). Arrays of these disks are sometimes used to provide highly reliable storage. For distribution of programs, data, and movies, various kinds of optical disks (CD-ROMs, CD-Recordables, and DVDs) are also important. In the following sections we will first explain the hardware and then the software for these devices.

Magnetic Disks

Magnetic disks are organized into cylinders, each one containing as many tracks as there are heads stacked vertically. The tracks are divided into sectors, with the number of sectors around the circumference usually being 8 to 32 on floppy disks, and up to several hundred on hard disks. The number of heads varies from 1 to about 16.

Older disks have little electronics and just deliver a simple serial bit stream. On these disks, the controller does most of the work. On other disks, particularly, IDE (Integrated Drive Electronics) and SATA (Serial ATA) disks, the disk drive itself contains a microcontroller that does considerable work and allows the real controller to issue a set of higher-level commands. The controller often does track caching, bad block remapping, and much more.

A device feature that has important implications for the disk driver is the possibility of a controller doing seeks on two or more drives at the same time. These are known as overlapped seeks. While the controller and software are waiting for a seek to complete on one drive, the controller can initiate a seek on another drive. Many controllers can also read or write on one drive while seeking on one or more other drives, but a floppy disk controller cannot read or write on two drives at the same time. (Reading or writing requires the controller to move bits on a microsecond time scale, so one transfer uses up most of its computing power.) The situation is different for hard disks with integrated controllers, and in a system with more than one of these hard drives they can operate simultaneously, at least to the extent of transferring between the disk and the controller's buffer memory. Only one transfer between the controller and the main memory is possible at once, however. The ability to perform two or more operations at the same time can reduce the average access time significantly.

Figure 1 compares parameters of the standard storage medium for the original IBM PC with parameters of a disk made 20 years later to show how much disks changed in 20 years. It is interesting to note that not all parameters have improved as much.  Average seek time is seven times better than it was, transfer rate is 1300 times better, while capacity is up by a factor of 50,000. This pattern has to do with relatively gradual improvements in the moving parts, but much higher bit densities on the recording surfaces.

Disk parameters for the original IBM PC 360-KB floppy disk and a Western Digital WD 18300 hard disk

One thing to be aware of in looking at the specifications of modern hard disks is that the geometry specified, and used by the driver software, is almost always different from the physical format. On old disks, the number of sectors per track was the same for all cylinders. Modern disks are divided into zones with more sectors on the outer zones than the inner ones. Figure 2(a) shows a tiny disk with two zones. The outer zone has 32 sectors per track; the inner one has 16 sectors per track. A real disk, such as the WD 18300, typically has 16 or more zones, with the number of sectors increasing by about 4% per zone as one goes out from the innermost zone to the outermost zone.

To hide the details of how many sectors each track has, most modern disks have a virtual geometry that is presented to the operating system. The software is instructed to act as though there are x cylinders, y heads, and z sectors per track. The controller then remaps a request for (x, y, z ) onto the real cylinder, head, and sector. A possible virtual geometry for the physical disk of Figure 2(a) is illustrated in Figure 2(b). In both cases the disk has 192 sectors, only the published arrangement is different than the real one.

For PCs, the maximum values for these three parameters are often (65535, 16, and 63), due to the need to be backward compatible with the limitations of the original IBM PC. On this machine, 16-, 4-, and 6-bit fields were used to specify these numbers, with cylinders and sectors numbered starting at 1 and heads numbered starting at 0. With these parameters and 512 bytes per sector, the largest possible disk is 31.5 GB. To get around this limit, all modern disks now support a system called logical block addressing, in which disk sectors are just numbered consecutively starting at 0, without regard to the     .

Physical geometry of a disk with two zones



Tags

overlapped seeks, disk geometry, logical block addressing