Chapter 4. Using Disks and Other Storage Media

Table of Contents
Two kinds of devices
Hard disks
Disks without filesystems
Allocating disk space

"On a clear disk you can seek forever. "

When you install or upgrade your system, you need to do a fair amount of work on your disks. You have to make filesystems on your disks so that files can be stored on them and reserve space for the different parts of your system.

This chapter explains all these initial activities. Usually, once you get your system set up, you won't have to go through the work again, except for using floppies. You'll need to come back to this chapter if you add a new disk or want to fine-tune your disk usage.

The basic tasks in administering disks are:

Chapter 5 contains information about virtual memory and disk caching, of which you also need to be aware when using disks.

Two kinds of devices

UNIX, and therefore Linux, recognizes two different kinds of device: random-access block devices (such as disks), and character devices (such as tapes and serial lines), some of which may be serial, and some random-access. Each supported device is represented in the filesystem as a device file. When you read or write a device file, the data comes from or goes to the device it represents. This way no special programs (and no special application programming methodology, such as catching interrupts or polling a serial port) are necessary to access devices; for example, to send a file to the printer, one could just say

$ cat filename > /dev/lp1
and the contents of the file are printed (the file must, of course, be in a form that the printer understands). However, since it is not a good idea to have several people cat their files to the printer at the same time, one usually uses a special program to send the files to be printed (usually lpr). This program makes sure that only one file is being printed at a time, and will automatically send files to the printer as soon as it finishes with the previous file. Something similar is needed for most devices. In fact, one seldom needs to worry about device files at all.

Since devices show up as files in the filesystem (in the /dev directory), it is easy to see just what device files exist, using ls or another suitable command. In the output of ls -l, the first column contains the type of the file and its permissions. For example, inspecting a serial device gives on my system

$ ls -l /dev/cua0
crw-rw-rw-   1 root     uucp       5,  64 Nov 30  1993 /dev/cua0
The first character in the first column, i.e., `c' in crw-rw-rw- above, tells an informed user the type of the file, in this case a character device. For ordinary files, the first character is `-', for directories it is `d', and for block devices `b'; see the ls man page for further information.

Note that usually all device files exist even though the device itself might be not be installed. So just because you have a file /dev/sda, it doesn't mean that you really do have an SCSI hard disk. Having all the device files makes the installation programs simpler, and makes it easier to add new hardware (there is no need to find out the correct parameters for and create the device files for the new device).