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Linux Kernel Development, Third Edition by Robert Love

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14. 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. The fixed-size chunks of data are called blocks. The most common block device is a hard disk, but many other block devices exist, such as floppy drives, Blu-ray readers, and flash memory. Notice how these are all devices on which you mount a filesystem—filesystems 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 ...

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