Devices that support a file system are known as block devices. Drivers written for these devices are known as block device drivers. Block device drivers take a file system request (in the form of a buf(9S) structure) and issue the I/O operations to the disk to transfer the specified block. The main interface to the file system is the strategy(9E) routine. See Chapter 10, Drivers for Block Devices, for more information.
Block device drivers can also provide a character driver interface that allows utility programs to bypass the file system and access the device directly. This device access is commonly referred to as the raw interface to a block device.