Character and Block Devices

A file in the device tree that is not a directory represents either a character device or a block device.

A block device can contain addressable, reusable data. An example of a block device is a file system. Any device can be a character device. Most block devices also have character interfaces. Disks have both block and character interfaces. In your /devices/pseudo directory, you might find devices such as the following:

brw-r-----   1 root     sys       85,  0 Nov  3 09:43 md@0:0,0,blk
crw-r-----   1 root     sys       85,  0 Nov  3 09:43 md@0:0,0,raw
brw-r-----   1 root     sys       85,  1 Nov  3 09:43 md@0:0,1,blk
crw-r-----   1 root     sys       85,  1 Nov  3 09:43 md@0:0,1,raw
brw-r-----   1 root     sys       85,  2 Nov  3 09:43 md@0:0,2,blk
crw-r-----   1 root     sys       85,  2 Nov  3 09:43 md@0:0,2,raw

Block devices have a b as the first character of their file mode. Character devices have a c as the first character of their file mode. In this example, the block devices have blk in their names and the character devices have raw in their names.

The md(7D) device is a metadevice that provides disk services. The block devices access the disk using the system's normal buffering mechanism. The character devices provide for direct transmission between the disk and the user's read or write buffer.