Operating Systems and File Systems Unix and Linux

Operating Systems and their File Systems

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UNIX is one of the oldest operating systems in the world of computing, and widely used even today. Many people consider UNIX arcane both in its operation and interface, however it is ideally suited to the needs of large and small computing systems. It is also the most common server based operating system on the Internet. Indeed you may never use UNIX on your local PC, there’s no doubt that you are using it indirectly, when you log on to the Internet.

There are many flavors of UNIX available, and millions of users have chosen to install a UNIX operating system on their own personal computers and work stations. There are dozens of variants of the basic UNIX interface, and today, one of the most popular is that of the Linux operating system. It too is available in several flavors. While both UNIX and Linux operating systems can be difficult to set up, with each requiring specific system level knowledge to operate, both are extremely stable and robust, and very system resource efficient.

UNIX operating systems rely on the UNIX file system only, however there is no single UNIX file system, any more than there is a single UNIX operating system. The file systems used by the majority of UNIX operating systems is fairly similar, and unusually distinct from the file systems used by other operating systems, such as DOS or Windows. As you are probably aware, Linux is geared specifically for use on the desktop and small server environments, while UNIX gets the most heavy stress server environments. At this writing, programmers are continually working to update and improve Linux, adding support for most of the other operating systems. Linux will read and write to FAT partitions, and newer versions include FAT 32 support. Although we have not examined it ourselves, we are advised that Linux can also read from and write to HPFS and read NTFS volumes.

How the UNIX file system works

Every item in a UNIX file system can de defined as belonging to one of four possible types: Ordinary files: Ordinary files can contain text, data, or program information. An ordinary file cannot contain another file, or directory. An ordinary file can be thought of as a one-dimensional array of bytes. Directories: In a previous section, we described directories as containers that can hold files, and other directories. A directory is actually implemented as a file that has one line for each item contained within the directory. Each line in a directory file contains only the name of the item, and a numerical reference to the location of the item. The reference is called an i-number, and is an index to a table known as the i-list. The i-list is a complete list of all the storage space available to the file system. Special files: Special files represent input/output (i/o) devices, like a tty (terminal), a disk drive, or a printer. Because UNIX treats such devices as files, a degree of compatibility can be achieved between device i/o, and ordinary file i/o, allowing for the more efficient use of software. Special files can be either character special files, that deal with streams of characters, or block special files, that operate on larger blocks of data. Typical block sizes are 512 bytes, 1024 bytes, and 2048 bytes. Links: A link is a pointer to another file. Remember that a directory is nothing more than a list of the names and i-numbers of files. A directory entry can be a hard link, in which the i-number points directly to another file. A hard link to a file is indistinguishable from the file itself. When a hard link is made, then the i-numbers of two different directory file entries point to the same inode. For that reason, hard links cannot span across file systems. A soft link (or symbolic link) provides an indirect pointer to a file. A soft link is implemented as a directory file entry containing a pathname. Soft links are distinguishable from files, and can span across file systems. Not all versions of UNIX support soft links.

The I-List

When we speak of a UNIX file system, we are actually referring to an area of physical memory represented by a single i-list. A UNIX machine may be connected to several file systems, each with its own i-list. One of those i-lists points to a special storage area, known as the root file system. The root file system contains the files for the operating system itself, and must be available at all times. Other file systems are removable. Removable file systems can be attached, or mounted, to the root file system. Typically, an empty directory is created on the root file system as a mount point, and a removable file system is attached there. When you issue a cd command to access the files and directories of a mounted removable file system, your file operations will be controlled through the i-list of the removable file system.

The purpose of the i-list is to provide the operating system with a map into the memory of some physical storage device. The map is continually being revised, as the files are created and removed, and as they shrink and grow in size. Thus, the mechanism of mapping must be very flexible to accomodate drastic changes in the number and size of files. The i-list is stored in a known location, on the same memory storage device that it maps.

Each entry in an i-list is called an i-node. An i-node is a complex structure that provides the necessary flexibility to track the changing file system. The i-nodes contain the information necessary to get information from the storage device, which typically communicates in fixed-size disk blocks. An i-node contains 10 direct pointers, which point to disk blocks on the storage device. In addition, each i-node also contains one indirect pointer, one double indirect pointer, and one triple indirect pointer. The indirect pointer points to a block of direct pointers. The double indirect pointer points to a block of indirect pointers, and the triple indirect pointer points to a block of double indirect pointers. By structuring the pointers in a geometric fashion, a single i-node can represent a very large file.

It now makes a little more sense to view a UNIX directory as a list of i-numbers, each i-number referencing a specific i-node on a specific i-list. The operating system traces its way through a file path by following the i-nodes until it reaches the direct pointers that contain the actual location of the file on the storage device.

The file system table

Each file system that is mounted on a UNIX machine is accessed through its own block special file. The information on each of the block special files is kept in a system database called the file system table, and is usually located in /etc/fstab. It includes information about the name of the device, the directory name under which it will be mounted, and the read and write privileges for the device. It is possible to mount a file system as “read-only,” to prevent users from changing anything.

File system quotas

Although not originally part of the UNIX filesystem, quotas quickly became a widely-used tool. Quotas allow the system administrator to place limits on the amount of space the users can allocate. Quotas usually place restrictions on the amount of space, and the number of files, that a user can take. The limit can be a soft limit, where only a warning is generated, or a hard limit, where no further operations that create files will be allowed.

The command

quota will let you know if you’re over your soft limit. Adding the -v option will provide statistics about your disk usage.

File system related commands

Here are some commands related to file system usage, and other topics discussed in this section: bdf – On HP-UX systems, reports file system usage statistics df – On HP-UX systems, reports on free disk blocks, and i-nodes du – Summarizes disk usage in a specified directory hierarchy ln – Creates a hard link (default), or a soft link (with -s option) mount, umount – Attaches, or detaches, a file system (super user only) mkfs – Constructs a new file system (super user only) fsck – Evaluates the integrity of a file system (super user only)

A brief tour of the UNIX filesystem

The actual locations and names of certain system configuration files will differ under different inplementations of UNIX. Here are some examples of important files and directories under version 9 of the HP-UX operating system: /hp-ux – The kernel program /dev/ – Where special files are kept /bin/ – Executable system utilities, like sh, cp, rm /etc/ – System configuration files and databases /lib/ – Operating system and programming libraries /tmp/ – System scratch files (all users can write here) /lost+found/ – Where the file system checker puts detached files /usr/bin – Additional user commands /usr/include/ – Standard system header files /usr/lib/ – More programming and system call libraries /usr/local/ – Typically a place where local utilities go /usr/man – The manual pages are kept here
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