Block devices are also associated with a device node, which also has major and minor numbers.

With block devices, the major number is used to identify the device driver and the minor number is used to identify the partition. Let's look at the MMC driver as an example:

# ls -l /dev/mmcblk*

brw-------    1 root root  179,   0 Jan  1  1970 /dev/mmcblk0
brw-------    1 root root  179,   1 Jan  1  1970 /dev/mmcblk0p1
brw-------    1 root root  179,   2 Jan  1  1970 /dev/mmcblk0p2
brw-------    1 root root  179,   8 Jan  1  1970 /dev/mmcblk1
brw-------    1 root root  179,   9 Jan  1  1970 /dev/mmcblk1p1
brw-------    1 root root  179,  10 Jan  1  1970 /dev/mmcblk1p2

The major number is 179 (look it up in devices.txt!). The minor numbers are used in ranges to identify different mmc devices and the partitions of the storage medium on that device. In the case of the mmcblk driver, the ranges are eight minor numbers per device: the minor numbers from 0 to 7 are for the first device, the numbers from 8 to 15 are for the second, and so on. Within each range, the first minor number represents the entire device as raw sectors, and the others represent up to seven partitions.

You are probably aware of the SCSI disk driver, known as sd, which is used to control a range of disks that use the SCSI command set, which includes SCSI, SATA, USB mass storage, and UFS (universal flash storage). It has the major number eight and ranges of 16 minor numbers per interface (or disk). The minor numbers from 0 to 15 are for the first interface, with device nodes named sda up to sda15, the numbers from 16 to 31 are for the second disk with device nodes sdb up to sdb15, and so on. This continues up to the sixteenth disk from 240 to 255, with the node name sdp. There are other major numbers reserved for them because SCSI disks are so popular, but we needn't worry about that here.

The partitions are created using utilities such as fdisk, sfidsk, or parted. An exception is raw flash memory: the partition information for the MTD driver is part of the kernel command line or in the device tree, or one of the other methods described in Chapter 7, Creating a Storage Strategy.

A user-space program can open and interact directly with a block device through the device node. This is not a common thing to do, and is usually for performing administrative operations such as partitioning, formatting with a filesystem, and mounting. Once the filesystem is mounted, you interact with the block device indirectly through the files in that filesystem.