Operating Systems Technology



System Administration

Course Notes #7

Most computers today already come with an OS installed. Why then do you have to learn about installing an OS? There are several reasons:

1. Some computers will not come with an OS installed, this is particularly true if you buy a workstation or PC with the specific intent of loading your own OS onto it

2. You may wish to remove the factory-installed OS in favor of one you prefer

3. Some of the installation steps will be similar to steps involved in installing other software (for instance, a server) so the more experience you get, the better off you are

4. Sometimes a hard disk will become corrupted or will die on you, or the portion of the disk storing the OS may become corrupted, therefore you will have to install an OS on a new or refurbished hard disk, or reinstall the OS on your current hard disk

5. You may wish to install a second OS and use dual booting, or as is the case in this course, you may want to install an OS inside of VMware to create a virtual machine

OS installation has become somewhat simplified thanks to installation programs. In many cases, the installation will only require minimal input from you. Linux allows you to specify the extent of your own involvement although even if you perform a custom installation, there are only a few steps for you to participate in. The most important single step is configuring the hard disk. This is discussed below. You will also specify the root password during installation. It is important that you remember the root password because forgetting it can lead to you being unable to perform any system administration duties, or having to reinstall the OS, which might result in loss of data files!

Disk Partitioning

In Linux, the hard disk is partitioned into logical sections that store different types of information. Commonly, there are at least three partitions: root, home, swap. The root partition contains the Linux kernel. This partition must be big enough to store the Linux kernel with some space left over for various system data files. The home partition will store the home directory(ies) including all user files. Often, this will be the largest of the partitions because it is expected to grow in size as more and more users are added and users add to their home directories. The swap partition is used for swap space. Swap space is virtual memory – that is, it is an extension to the computer’s main memory. Whenever a user wishes to run a program, the executable program and storage space for data are placed on the swap space partition. As the program runs, the OS moves the parts of the program and data that are needed right then from disk into main memory. Since main memory is limited it size, at some point the user(s) of the system will need more memory than exists, so the remember of the program and data that are not currently in use for the active programs reside in swap space. Therefore, swap space must be large enough to store all programs and data that might ever be run at one time. If you do not specify enough space for one of these partitions, it might lead to

1. poor system performance

2. lack of storage space for users

3. inability to execute some programs

Note that while partitioning is critical when installing, you may not necessarily be stuck with the partitions that you create. For instance, imagine that you provided too little space for the swap partition, and later, you want to move some space from the home partition to the swap partition. One program to manipulate the partition table is called fdisk. This program however is not designed for large partitions. Other programs that you might examine if you ever have to manually change partition sizes are cfdisk, parted, and sfdisk.

Installation Options

With the CentOS DVD, you have a number of options. You can use it as a boot disk so that you can “run off the disk” rather than running off of the OS installed onto your hard disk. This may be desirable if you are only experimenting with CentOS and want to see what it is like, or if you have to do a Linux operation but it is a one-time event. Unfortunately, by booting off the hard disk, you have no storage space available, so whatever process(es) you work on, you will not be able to save the work since you cannot write back to the DVD. You may be able to save your work through some external storage medium like a floppy disk or flash drive.

Otherwise, you will use the CentOS DVD to install the OS anew, or update/repair your current installation. Once inserted, the CentOS DVD instructs your computer what to do. If your computer already finds a CentOS installation, it will ask if you want to re-install the OS, or update or repair the current installation. If you are to update or repair, the process takes less time although may still be time consuming. Most commonly though, the user wishes to install the OS or re-install the OS, thus wiping out whatever may have been on the disk to begin with. The steps to perform the OS installation are those that you will work through in lab 4. Typically, you will do a standard install, which is takes some of the decision making away from you. However, it is far more convenient. A custom installation instead allows you to tailor the installation to your own desires such as where Linux software is placed. For the most part though, your choices boil down to:

1. What partitions to create (discussed above)

2. What application software to install (the CentOS DVD comes with a number of software packages, you merely select what you want) – as we will see later in the semester, you can always return to this step after installation is over so that if you want to install additional software that you did not install initially, that is no problem

3. Creating a root password and creating an initial user account

The reason for the creation of an initial user account is that it is common for users to boot up CentOS in the GUI mode. From the login window of the GUI, you cannot log in as root. So, unless you log in initially in the text-only mode, you will have to have at least one non-root account created. You would log into the GUI, then open either a terminal window and su (switch user) to root or open proper administration tools to create additional accounts and start managing your Linux environment.

When you do your own installation, you will spend the most effort in creating your partition, as described above. Prior to creating your partition, your disk may be formatted in a way that differs from what Linux expects. So the partitioning process allows you to first delete any physical or logical partitions. You will take these steps. The primary reason for this is that most disks come formatted for a Windows OS and Linux will want to have a different formatting. In essence, you want to delete any partitions so that you have one large partition. Now, you can start partitioning up the disk as you desire.

Bootloaders

A bootloader is a small program that executes when the machine first is turned on (or rebooted) that performs the boot process. The boot process includes loading and starting the operating system. We will cover the boot process in more detail later in the semester. The bootloader that you will install is called Grub, it is installed automatically with CentOS. There are several other bootloaders available for Linux though. Grub is part of the GNUs Linux. Lilo however is the oldest and most popular bootloader. The textbook discusses how you can modify Lilo’s processes by altering the lilo-initrd.conf script. Syslinux is a bootloader specifically for booting off of floppy disk and so is outmoded. Loadlin is a bootloader that can boot Linux from DOS. Thus, from Windows, you can open a DOS prompt and then use Loadlin to boot Linux.

In most computer systems, there is only a single OS. So when you turn on the computer, the boot loader runs to and loads and initializes that OS. However, many Linux users may want to have both Linux and Windows (or two versions of Linux) running on their machine. This requires that you have a multiboot bootloader. In this way, a menu is presented at boot time to allow you to choose which OS to boot. It should be noted that Grub works well as a multiboot bootloader if you are selecting between Windows and Linux, but it does not work well between two or more versions of Linux.

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