From the Publisher

• Covers all versions of UNIX, as well as Linux, operating systems that are used by the majority of Fortune 1000 companies for their mission-critical data
• Offers more detail than other books on the file input/output aspects of UNIX programming
• Describes implementation of UNIX filesystems over a thirty year period
• Demonstrates VERITAS and other filesystem examples

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Table of Contents

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UNIX Filesystems

John Wiley & Sons

Chapter One

This chapter introduces UNIX from a historical perspective, showing how the various UNIX versions have evolved over the years since the very first implementation in 1969 to the present day. The chapter also traces the history of the different attempts at standardization that have produced widely adopted standards such as POSIX and the Single UNIX Specification.

The material presented here is not intended to document all of the UNIX variants, but rather describes the early UNIX implementations along with those companies and bodies that have had a major impact on the direction and evolution of UNIX.

A Brief Walk through Time

There are numerous events in the computer industry that have occurred since UNIX started life as a small project in Bell Labs in 1969. UNIX history has been largely influenced by Bell Labs' Research Editions of UNIX, AT&T's System V UNIX, Berkeley's Software Distribution (BSD), and Sun Microsystems' SunOS and Solaris operating systems.

The following list shows the major events that have happened throughout the history of UNIX. Later sections describe some of these events in more detail.

1969. Development on UNIX starts in AT&T's Bell Labs.

1971. 1st Edition UNIX is released.

1973. 4th Edition UNIX is released. Thisis the first version of UNIX that had the kernel written in C.

1974. Ken Thompson and Dennis Ritchie publish their classic paper, "The UNIX Timesharing System" [RITC74].

1975. 6th Edition, also called V6 UNIX, becomes the first version of UNIX to be used outside Bell Labs. The University of California at Berkeley starts development on the Berkeley Software Distribution or more commonly called BSD.

1977. At this stage there were 500 sites running UNIX. Universities accounted for about 20 percent of those sites.

1979. 7th Edition UNIX was rewritten to make it more portable. Microsoft licenses 7th Edition and starts development of Xenix.

1980. Microsoft releases Xenix, a PC-based version of UNIX. 1982. AT&T's UNIX Systems Group releases System III UNIX. The Santa Cruz Operation (SCO) licenses Xenix from Microsoft.

1983. AT&T's UNIX System Development Labs release System V Release 1 UNIX.

1984. 4.2BSD is released including TCP/IP. System V Release 2 is released and the number of installations of UNIX worldwide exceeds 100,000. Digital Equipment Corporation's (DEC's) 4.2BSD-based Ultrix is released.

1986. 4.3BSD is released. 4.2BSD-based HP-UX first appears. IBM releases AIX 2 for the RT server.

1987. AT&T releases System V Release 3, which includes STREAMS, the Network File System (NFS), and the Transport Level Interface (TLI).

1989. As a joint venture between AT&T's Unix System Laboratories (USL) and Sun Microsystems, System V Release 4.0 is released.

1990. Based on SVR2 with enhancements from 4.2BSD and 4.3BSD, IBM releases AIX 3.1.

1991. Linus Torvalds announces Linux 0.0.1. 1992. USL releases System V Release 4.2 that includes the VERITAS filesystem VxFS and Volume Manager VxVM.

1993. 4.4BSD, the last release from Berkeley, is released. SVR4.2MP is released by Novell following their purchase of USL from AT&T.

1994. 4.4BSD Lite, which was free of copyrighted UNIX source code, is released.

1995. SCO buys Novell's UNIX business.

1996. The Linux 2.0 kernel is released.

1997. UnixWare 7, a merge of SVR4.2MP and SCO OpenServer, is released.

2001. SCO's UNIX business is sold to Linux distributor Caldera. The Linux 2.4 kernel emerges after many delays.

How Many Versions of UNIX Are There?

Most versions of UNIX have stemmed from System V or BSD, with many taking enhancements from both. The 1980s saw a proliferation of versions of UNIX. Although it is difficult to give an exact figure on how many versions of UNIX have actually been produced, by the late 1980s it is safe to say that there were close to 100 different UNIX variants employed throughout the world. It is no wonder why UNIX has had its critics and not surprising that attempts at standardization have proceeded for much of UNIX's history.

Developing operating systems is a costly business, however, which has resulted in consolidation throughout the mid to late 1990s. On the low end, Linux and SCO variants tend to dominate while in the mid to high-end, Sun's Solaris, Hewlett Packard's HP-UX, and IBM's AIX account for most of the market share.

As time goes by there is likely to be further consolidation at the low to mid-range of PCs, workstations, and servers with Linux clearly leading the way. UNIX is still a clear leader at the high end of the market with Sun, HP, and IBM all having enterprise level capabilities in their UNIX offerings. While it is difficult to see further consolidation at this end of the market, only time will tell.

Why Is UNIX So Successful?

Although the large number of different versions of UNIX can be considered a weakness since it raised the possibility of incompatibilities, it also demonstrates one of the great strengths of UNIX: the ease by which UNIX has been ported to a wide number of different hardware architectures and platforms, a task that was addressed very early on during development at Bell Labs.

Even though the number of versions of UNIX increased dramatically over the years, porting applications between one version and the next was still considerably easier than porting between one proprietary OS and the next. This task has been made easier by the introduction of relevant standards that nearly all of the major UNIX vendors have adopted. No other operating system can claim this level of dominance across such a wide range of platforms. The proliferation of UNIX resulted in a huge amount of development pushing UNIX way ahead of its nearest proprietary competitors.

The Early Days of UNIX

The research arm of the Bell Telephone Company, Bell Labs, had seen the need for a new computer operating system in the late 1950s. This resulted in the BESYS operating system which, although used internally, had limited distribution outside of Bell Labs. By the mid 1960s, third-generation computer equipment was emerging and the people at Bell Labs had to decide whether to create a new operating system or to adopt a third party OS. The decision was eventually made to join General Electric and MIT to create a new timesharing system called the MULTIplexed Information and Computing Service (MULTICS). This collaborative venture aimed to show that general purpose, multiuser operating systems were a viable solution. Based on a research operating system from MIT called the Compatible Time Sharing System (CTSS), the MULTICS project resulted in a wide range of new approaches. Amongst those working on MULTICS were Bell Lab researchers Ken Thomson and Dennis Ritchie, who would take many of the ideas from MULTICS and evolve them into what would become UNIX. As an example, many of the UNIX commands were based on commands of the same name in MULTICS, namely ls, cd, and pwd. Due to the high cost of development and the amount of time that it was believed MULTICS would take to complete, AT&T withdrew from the MULTICS project in 1969.

On an internal GE-645 computer at Bell Labs, the GECOS operating system was installed, which proved inadequate to many of the researchers. For many this was seen as being back at square one. This resulted in a proposal by Thompson and Ritchie to get Bell Labs to buy a new machine so they could start work on their own interactive time-sharing system. The proposal was rejected and Thompson started work on an old PDP-7. Developing initially on a GE-635, Thompson produced a primitive kernel, assembler, shell, and a few utilities (rm, cat, cp) that were then transported to the PDP-7 by paper tape. Even at this stage the new primitive OS was sufficient for all further development to proceed on the PDP-7 itself. As a pun on MULTICS, the new environment was named UNIX.

In 1970 the UNIX pioneers acquired a DEC PDP-11 that was initially diskless and had 24KB of memory. They used 12KB for the operating system, allowed a small amount of space for user programs, and the rest was used as a RAM disk. It was around this time that the first 1st Edition UNIX appeared.

The Early History of the C Language

Following the early assembler versions of UNIX, Thompson worked on a Fortran compiler that evolved to support the language B, a cut-down version of BCPL. The B compiler was provided as part of 1st Edition UNIX in 1971, and some of the first utilities were even written in B. It was Ritchie's work over the next two years that resulted in the first C compiler, which became part of 3rd Edition UNIX in 1973. Note that it would still be another 5 years before the appearance of Kernighan and Ritchie's book, The C Programming Language [KERN78]. Following an abortive attempt by Thompson to write part of the kernel in an early version of C which did not support structures, by 4th Edition UNIX that appeared in 1973, Thompson and Ritchie had rewritten the kernel in C.

Research Editions of UNIX

There were a total of ten research editions of UNIX from Bell Labs. Perhaps the most famous was 6th Edition UNIX which was described in John Lions' book Lions' Commentary on UNIX 6th Edition [LION96], which until it was published in 1996 remained an underground work. One thing that distinguished each research edition was the introduction of a new Programmer's Reference Manual. Following is a brief summary of the different research editions and which UNIX features they introduced:

1st Edition-1971. As well as the B compiler, 1st Edition UNIX introduced a number of well-known UNIX commands including cat, chdir, chmod, chown, cp, ed, find, mkdir, mkfs, mount, mv, rm, rmdir, wc, and who.

2nd Edition-1972. One amusing note on 2nd Edition was a comment in the Programmer's Reference Manual that the number of UNIX installations had now grown to 10!

3th Edition-1973. The UNIX C compiler (cc) first appeared. The kernel was still written in assembler and the number of installations had grown to 16. Pipes were also introduced.

4th Edition-1973. The kernel was rewritten in C.

5th Edition-1974. This edition appeared around the time of Thompson and Ritchie's paper "The UNIX Time Sharing System" [RITC74]. The source code was made freely available to universities for educational purposes.

6th Edition-1975. This edition, also known as V6, was the first edition widely available outside of Bell Labs. Most of the operating system by this time had been written in C.

7th Edition-1979. The first K&R (Kernighan and Ritchie) compliant C compiler made its appearance with 7th edition together with Steve Bourne's shell (sh). The kernel was rewritten to make it more portable to other architectures. At this time the UNIX Systems Group was created and started working on enhancing 7th Edition (on which System V UNIX would be based). Microsoft also licensed 7th Edition, which it used to develop the Xenix operating system. Note that the size of the 7th Edition kernel was only 40KB, a tiny program by today's standards.

8th Edition-1985. 8th Edition UNIX picked up some enhancements developed from 4.1BSD. This edition was used as the basis for System V Release 3.

9th Edition-1988. This edition picked up enhancements made for 4.3BSD.

10th Edition-1989. This was the last edition.

AT&T's Commercial Side of UNIX

In the late 1960s, while Bell Labs was looking for a new timesharing system, the Bell Telephone company was looking for a way to automate their telephone operations using minicomputers to switch over from their existing system of people and paper.

It was Berkley Tague, the head of the computer planning department, who, having seen the capabilities of UNIX, realized its potential and saw how it could ease their job. By 1971 Tague gained approval for the adoption of UNIX to support Bell Telephone operations. By 1973 he formed the UNIX Support Group (USG) which worked closely with the UNIX team from Bell Labs. During the same year, the first UNIX applications started to appear, initially involved in updating customer directory information and intercepting calls to phone numbers that had been changed. 1973 also saw the first C version of UNIX released internally together with the first Programmer's Work Bench, which included sccs and other tools.

Around the time of 7th Edition UNIX, USG took responsibility for UNIX and after a number of internal-only releases, System III UNIX became the first version of UNIX that was available for use outside Bell Labs.

USG later became the UNIX System Development Laboratory (USDL). In 1984, this group released System V Release 2 (SVR2) which was the first version of UNIX to support paging, copy-on-write semantics, shared memory, and file locking. SVR2 UNIX is described in Bach's classic book The Design of the UNIX Operating System [BACH86]. At this time there were about 100,000 installations of UNIX worldwide running on a whole host of different platforms and architectures. The fact that no other operating system had achieved this goal was perhaps the single greatest reason why UNIX became so popular.

Following yet another name change to AT&T Information Systems (ATTIS), the group released System V Release 3 (SVR3) in 1987. This included a number of enhancements:

* The File System Switch (FSS) provided an architecture under which multiple filesystems could coexist in the kernel at the same time. The FSS provided a layer by which the rest of the kernel could perform file and filesystem related operations in a filesystem independent manner through a well defined interface.

* The RFS (Remote File Sharing) filesystem provided a fully distributed, cache-coherent file system.

* The STREAMS subsystem for building networking stacks. The initial implementation of STREAMS was first introduced in 8th Edition UNIX.

* The Transport Layer Interface (TLI) for network programming.

* Shared libraries which can reduce the amount of memory used.

System V Release 3.2 was released in 1987 which involved a merge of SVR3 and Xenix, produced by Microsoft and the Santa Cruz Operation (SCO).

Continues...

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Excerpted from UNIX Filesystems by Steve D. Pate Excerpted by permission.
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