Chapter 13. The Block I/O Layer
Block devices are hardware devices distinguished by the random (that is, not necessarily sequential) access of fixed-size chunks of data, called blocks. The most common block device is a hard disk, but many other block devices exist, such as floppy drives, CD-ROM drives, and flash memory. Notice how these are all devices on which you mount a filesystemfilesystems are the lingua franca of block devices.
The other basic type of device is a character device. Character devices, or char devices, are accessed as a stream of sequential data, one byte after another. Example character devices are serial ports and keyboards. If the hardware device is accessed as a stream of data, it is implemented as a character device. On the other hand, if the device is accessed randomly (nonsequentially), it is a block device.
Basically, the difference comes down to whether the device accesses data randomlyin other words, whether the device can seek to one position from another. As an example, consider the keyboard. As a driver, the keyboard provides a stream of data. You type "fox" and the keyboard driver returns a stream with those three letters in exactly that order. Reading the letters out of order, or reading any letter but the next one in the stream, makes little sense. The keyboard driver is thus a char device; the device provides a stream of characters that the user types onto the keyboard. Reading from the keyboard returns a stream first with f, then o, then x, and ultimately end of file (EOF). When no keystrokes are waiting, the stream is empty. A hard drive, conversely, is quite different. The hard drive's driver might ask to read the contents of one arbitrary block and then read the contents of a different block; the blocks need not be consecutive. Therefore, the hard disk's data is accessed randomly, and not as a stream, and thus the hard disk is a block device.
Managing block devices in the kernel requires more care, preparation, and work than managing character devices. Character devices have only one positionthe current onewhereas block devices must be able to navigate back and forth between any location on the media. Indeed, the kernel does not have to provide an entire subsystem dedicated to the management of character devices, but block devices receive exactly that. Such a subsystem is a necessity partly because of the complexity of block devices. A large reason, however, for such extensive support is that block devices are quite performance sensitive; getting every last drop out of your hard disk is much more important than squeezing an extra percent of speed out of your keyboard. Furthermore, as you will see, the complexity of block devices provides a lot of room for such optimizations. The topic of this chapter is how the kernel manages block devices and their requests. This part of the kernel is known as the block I/O layer. Interestingly, revamping the block I/O layer was the primary goal for the 2.5 development kernel. This chapter covers the all-new block I/O layer in the 2.6 kernel.