Network Plus 2005 In Depth

424 Chapter 9 NETWORKING WITH UNIX-TYPE OF OPERATING SYSTEMS

NET+ UNIX Multiprocessing

3.1As you have learned, a process represents an instance of a program running in memory (RAM). In addition to processes, UNIX-type of systems also support threads, which are self-contained subsets of a process. Any modern NOS must handle multiple processes and threads in an efficient manner. UNIX-type of systems allocate separate resources (such as memory space) to each process as it is created. They also manage all programs’ access to these resources. This approach enables partitioning of processes in memory, thereby preventing one program from disrupting the operation of the entire system. When one program ends unexpectedly on a UNIX-type of system, it doesn’t cause the whole computer to crash.

Like Windows Server 2003, modern UNIX-type of systems support SMP (symmetric multiprocessing). Different flavors of UNIX support different numbers of processors:

Solaris supports up to 128 processors per server (although Sun does not make any hardware containing that many processors).

Linux supports SMP using a maximum of 32 processors per server.

Mac OS X Server supports up to two processors per server.

You must know how your servers will be used and plan for multiprocessing servers according to your estimated application-processing loads.

The UNIX Memory Model

From early on, UNIX-type systems were created to use both physical and virtual memory efficiently. Like Windows Server 2003, UNIX-type of systems allocate a memory area for each application. They attempt to decrease the inefficiency of this practice, however, by sharing memory between programs wherever they can. For example, if five people are using FTP on your UNIX server, five instances of the FTP program will run. In reality, only a small part of each FTP program (called the private data region—the part that stores the user name, for example) will receive its own memory space; most of the program will remain in a region of memory shared by all five instances of the program. In this case, rather than using five times the memory required by one instance of the program, a UNIX-type of system sets aside only a little more memory for five FTP users than it does for one FTP user.

Most current UNIX-type of systems use a 32-bit addressing scheme that enables programs to access 4 GB of memory. Most of these systems also run on CPUs that employ 64-bit addresses, enabling programs to access more than 18 exabytes (264 bytes) of memory. That’s more than 18 billion billion bytes of data—by one estimate, three times the total number of words ever spoken by human beings! Virtual memory in a UNIX server can take the form of a disk partition, or it can be in a file (much like the virtual memory file pagefile.sys in Windows Server 2003).

The UNIX Kernel

The core of all UNIX-type of systems is called the kernel. The kernel is loaded into memory and runs when you turn on your computer. Its primary function is to coordinate access to all

3.1remove functionality on a running UNIX-type of system by loading and unloading kernel modules. A UNIX kernel module is a file that contains instructions for performing a specific task such as reading data from and writing data to a hard drive.

The Solaris kernel is derived from the original AT&T UNIX software from Bell Labs. The Linux kernel is the software Linus Torvalds wrote and released to the public in 1991. The Mac OS X Server kernel (called XNU) is derived from an operating system called Mach, which was developed at Carnegie Mellon University in the 1990s.

UNIX System File and Directory Structure

The UNIX system was one of the first operating systems to implement a hierarchical file system (a method of organizing files and directories on a disk in which directories may contain files and other directories). The notion of a file system organized in this way was considered revolutionary at the time of UNIX’s inception. Today, most operating systems, including all Microsoft operating systems and Novell NetWare, use hierarchical file systems. Figure 9-1 shows a typical UNIX file system hierarchy.

FIGURE 9-1 UNIX file system hierarchy

On a UNIX-type of system, the /boot directory contains the kernel and other system initialization files. Applications and services are stored in the /bin and /sbin directories. (The applications and services in the /sbin directory support the system initialization process; you’ll rarely use these programs.) The /var directory holds variable data (such as log files, users’ unread e-mail, and print jobs waiting to be printed). The file /var/log/messages, for example, stores system log messages, such as a notification of a disk drive that is running out of space. Users’ login directories typically appear in /home. Mac OS X Server creates users’ login directories in /Users. When you create a new user account, the system assigns a directory in /home to that user (/Users in Mac OS X Server). The login (or home) directory matches the account’s user name. Thus, /home/jones is the login (or home) directory for the user name jones on a UNIX system. /Users/jones is the login (or home) directory for the user jones on a Mac OS X Server system.

426 Chapter 9 NETWORKING WITH UNIX-TYPE OF OPERATING SYSTEMS

NET+ UNIX System File Services

2.13UNIX file services fall into two broad categories: disk file systems and network file systems.

3.1Disk file systems are used to organize the information on a hard drive. Network file systems enable users to access files on other servers via the network.

Disk File Systems

The UNIX disk file system is the operating system’s facility for organizing, managing, and accessing files through logical structures and software routines. Just as Windows Server 2003 supports FAT, NTFS, and other file systems, UNIX-type systems also support multiple file system types. The native file system type on Linux, called ext3, is the “third extended” file system for Linux. Solaris employs the file system called UFS (for “UNIX File System”) for its native file system type. Apple’s HFS+ (for hierarchical file system) is the native file system for Mac OS X Server. On UNIX-type of systems, you can access disk partitions formatted with the DOS FAT file system as well as Windows Server 2003 NTFS. This applies to partitions on disks that are physically attached to computers that are running a UNIX or UNIX-type of system. UNIX systems have access to nonnative file system types, such as NTFS, over the network with network file systems.

Network File Systems

UNIX-type of systems also support network file systems, which are analogous to Windows shares or NetWare network volumes. From a UNIX-type of host, the network file system allows you to attach shared file systems (or drives) from Windows, NetWare, or other UNIX servers and share files with users on other computers. Sun Microsystems’ NFS (Network File System) is a popular remote file system type supported by UNIX. Sun Microsystems published the specification for NFS, and most vendors of UNIX and UNIX-type systems include NFS applications for sharing and accessing files over a network. Another network file system, called Samba, is an open source application that implements the Windows SMB and CIFS file system protocols. Samba is included with Solaris, most Linux distributions, and Mac OS X Server systems by default.

Apple’s Macintosh computers have long had a built-in network file system called AFP (AppleTalk Filing Protocol or Apple File Protocol). Though Mac OS X Server includes complete support for NFS and Samba network file systems, AFP remains its primary network file system. You must use AFP on a Mac OS X Server system to share files with computers running older versions of the Macintosh operating system.

A UNIX Command Sampler

3.1when you’re running a GUI, the GUI actually executes commands in response to your mouse clicks. This section discusses some of the basics of the UNIX user interface, interaction with the UNIX command line, and some fundamental UNIX commands.

3.1for you is called a command interpreter. Also known as a shell, a command interpreter translates your typed commands into machine instructions that a UNIX-type of system can understand. In other words, the command interpreter is a program that runs other programs. UNIX command interpreters also perform file globbing (described later) and keep track of what commands you’ve entered previously. The primary UNIX command interpreter is the file /bin/sh. To use the shell effectively, you should be familiar with at least some basic commands.

Every UNIX-type of system contains full documentation of UNIX commands in the manual pages (or man pages). The manual pages describe each command’s function and proper execution. Although their organization differs slightly in various flavors of UNIX, manual pages are typically arranged in nine sections:

Section 1 covers the commands that you most typically enter while typing in a command window.

Sections 2 through 5 document the programmer’s interface to the UNIX system.

Section 6 documents some of the amusements and games that are included in the UNIX system.

Section 7 describes the device drivers for the system.

Section 8 covers the commands used by administrators to manage the system.

Section 9 documents the UNIX kernel functions programmers use when writing device drivers.

You can access manual pages by entering the man command in a UNIX command window. For example, to read the manual page entry for the telnet command, you would type man telnet in a command window, and then press Enter.

Although the UNIX manual pages are accurate and complete, UNIX newcomers often complain that they can’t find the appropriate manual page if they don’t know the name of the command they want to use. That’s why the apropos command exists. It enables you to find possible manual page entries for the command you want to use. For example, you might type apropos list to search for a command that lists files. The apropos command would then display all commands and programming functions that include the keyword list in their manual page entries. Type man <command> (where <command> is a command name displayed by apropos), and press Enter when you find a command name that looks like it might do what you want.

Commands function in much the same way as sentences in ordinary language. Some of these sentences are one-word directives to the system requesting that it perform a simple task on your behalf (such as date for “tell me the current date and time”). Other sentences are detailed instructions to the system containing the equivalent of nouns, adjectives, and adverbs and creating a precise description of the task you want the system to perform. For example, to instruct the system to “display the names of all files in the current directory that have been accessed in the past five days,” you would type: find . –type f –atime -5 -print.