The practice of law is a knowledge, information, and document-intensive profession. In many respects, lawyers ply their trade in the same way independent programmers do: we sell our expertise, experience and technical skill in using what is, essentially, the aboriginal “open” source code — the code of laws and courtroom procedure.

In 2004, after practicing law for 23 years in a mid-sized downtown Seattle law firm, I opened my own boutique law practice and I decided to make GNU/Linux the centerpiece of a completely free/open software law firm environment.

Almost everything about the practice of law is open and public. You do not “lease” the law from any corporation and you do not acquire a license to use any federal or state statute, municipal code or administrative regulation. What the client pays for is the lawyer’s knowledge about how to use a highly complicated set of frequently asynchronous laws and procedures, much like what a client pays for when hiring a programmer.

In the Anglo-American legal system, the attorney deals with “legacy” code (otherwise known as “Common Law”) built up through the accretion of judicial case decisions over time (as in centuries, on occasion) and utility “programs” for special applications like federal taxation, bankruptcy, land use, criminal prosecutions, and divorce proceedings.

The expertise in law is knowing what sources to look in, how to read the code (law), what it means, and, most importantly, how to creatively and productively apply it in the context of a particular problem. In short, lawyers are not “law processors,” but “code programmers” working like free and open source computer consultants. Lawyers are naturals for GNU/Linux.

Cultural Inertia

Before making the cold-penguin conversion, I first had to overcome professional cultural inertia in creating a purely free and open law firm. I had spent 23 years working in a Windows world. All of my practicing colleagues used Microsoft products. All of my clients used Microsoft products. All of the courts used Microsoft products.

The first three questions I had to answer were:

Could I actually run my practice using free/open source software?
How would my GNU/Linux system interact with everyone else’s systems.
And finally, why bother?
Addressing the last question first, the answer was simple: why not? The 20th Century gee-whiz world of office technology has long since morphed into the world of office technology break-downs, power failures and mean-spirited invasions of viruses, worms, phishers, and spammers.

Simply put, I wanted, no, I urgently needed, a computer software and information management system that was both robust and fixable quickly, efficiently, and inexpensively. Although speed, efficiency and cost were virtues mostly forgotten in the big law firm environment, I knew that they would be critical in a small boutique practice. So it was a natural choice for me to choose a core technology — GNU/Linux — that fit all basic criteria.

Let me point out that I am, strictly speaking, a mere user of computers and computer software. I am not a techie in any sense except as a marginally sophisticated consumer of technology. Most lawyers do not know the difference between source code, machine code and the DaVinci Code.

Fortunately, I am married to a woman who speaks geek and who can program as readily as Gandalf blows smoke rings. She agreed to be the resident IT manager, my in-house information technology expert who would assemble the hardware and software systems, integrate them, tweak and refine them and, when necessary, rip them apart to fix whatever had gone wrong.

Purists might argue that a true user of GNU/Linux software should be able to do it all by oneself. I say that is a load of penguin poop. Although Renaissance Man might have been able to do it all, not since the 13th Century and Roger Bacon has it truly been possible for one person to know everything about everything. In fact, if there is any advantage to civilization, it is that I do not have to know everything about everything.

For ordinary humanoids, it simply is not possible for one person in one day to learn to play Rachmaninoff’s Piano Concerto No. 3, try a lawsuit, write a Java script program, fix the chain on your bicycle, weed your vegetable garden, run a marathon, earn a black belt in karate, frame your own house, write the great American novel, eat pizza, drink beer and occasionally socialize with a member of the opposite sex. In short, let me practice law, you program code, and we’ll both make a symbiotic living doing our own thing. That’s civilization.

The Building Blocks

My system is built around Debian, primarily because it is clean, reliable and because its proponents are dedicated to the principles of FOSS (Free and Open Source Software). Although my wife runs her machines on testing, or the bleeding edge versions of the operating system, I strictly run stable configurations. In part, I am chicken and in part I am prudent. I cannot afford to have the whole system melt down on me while responding to a 48 hour “motion to shorten time.”

In 2004, I purchased an IBM Thinkpad that came loaded with Windows and, after partitioning the hard drive, we loaded Debian Sarge. Now, in 2007, we are running Etch, the current stable release of Debian. My office runs a basic Pogolinux server.

Email has become an essential component of the practice of law. In fact, email now has supplanted the telephone, mail, and the facsimile machine as the workhorse of the law office. Literally everything is conducted by email, including correspondence with the judges’ clerks, communications with clients, and flamemail launched at opposing counsel. My email utility is Evolution and, although it periodically crashes or hangs, it is no less reliable than what I used to use.

My Web browser is Mozilla Firefox: although, on occasion, I will use one of the several other browsers that come loaded with my version of Debian. I use NoScript 1.3.1 (a FOSS program written by Giorgio Maone of InformAction, Palermo, Sicily) to preemptively block a lot of advertising and to protect myself from various Web security vulnerabilities.

Between my FOSS browser, Evolution, and GNU/Linux operating system, I like the fact that I am almost virus and worm free. My lawyer friends go through periodic emergency upgrades of their more prosaic software while I experience no such emergencies. Although my former partners are forced periodically to abandon their operating systems for something newer, shiner, and just as flaky, my operating system is always backwards and forwards compatible.

When Debian releases its next version of its stable system, I will simply download it without charge. Ubuntu wasn’t available when I set up my “free and open” law firm in 2003. Now it is and I might try it, too.

Although Free Software does not cost anything, there truly is no such thing as a free lunch. If you like and support something, you should help pay for it. My in house IT manager contributes her time and programs to the common cause. I contribute money to the Debian Foundation and donate some of my professional time proselytizing in articles like these or sharing law knowledge with local FOSS communities.

It is proper and sporting to give back and make those contributions. Businesses who think that the main attraction of FOSS is that it is, literally, free miss the point altogether. It is not about freeloading; it is about being part of, and contributing to what is literally an international software cooperative. In a way, that is how the legal codes, the common law develops, too: only much, much, much more slowly and without the intentionality of writing computer code.


Read More...

This article will discuss the differences between the Linux and Windows operating software’s; we discuss some of the pro’s and con’s of each system.

Let us first start out with a general overview of the Linux operating system. Linux at its most basic form is a computer kernel. The Kernel is the underlying computer code, used to communicate with hardware, and other system software, it also runs all of the basic functions of the computer.

The Linux Kernel is an operating system, which runs on a wide variety of hardware and for a variety of purposes. Linux is capable of running on devices as simple as a wrist watch, or a cell phone, but it can also run on a home computer using, for example Intel, or AMD processors, and its even capable of running on high end servers using Sun Sparc CPU’s or IBM power PC processors. Some Linux distro’s can only run one processor, while others can run many at once.

Common uses for Linux include that of a home desktop computing system, or more commonly for a server application, such as use as a web server, or mail server. You can even use Linux as a dedicated firewall to help protect other machines that are on the same network.

A programmer student named Linus Torvalds first made Linux as a variant of the Unix operating system in 1991. Linus Torvalds made Linux open source with the GNU (GPL) (General Public License), so other programmers could download the source code free of charge and alter it any way they see fit. Thousands of coders throughout the world began downloading and altering the source code of Linux, applying patches, and bug fixes, and other improvements, to make the OS better and better. Over the years Linux has gone from a simple text based clone of Unix, to a powerful operating software, with full-featured desktop environments, and unprecedented portability, and a variety of uses. Most of the original Unix code has also been gradually written out of Linux over the years.

As a result of Linux being open source software, there is no one version of Linux; instead there are many different versions or distributions of Linux, that are suited for a variety of different users and task. Some Distributions of Linux include Gentoo, and Slackware, which due to the lack of a complete graphical environment is best, suited for Linux experts, programmers, and other users that know their way around a command prompt. Distributions that lack a graphical environment are best suited for older computers lacking the processing power necessary to process graphics, or for computers performing processor intensive task, where it is desirable to have all of the system resources focused on the task at hand, rather than wasting resources by processing graphics. Other Linux distributions aim at making the computing experience as easy as possible. Distributions such as Ubuntu, or Linspire make Linux far easier to use, by offering full-featured graphical environments that help eliminate the need for a command prompt. Of course the downside of ease of use is less configurability, and wasted system resources on graphics processing. Other distributions such as Suse try to find a common ground between ease of use and configurability.

“Linux has two parts, they include the Kernel mentioned previously, and in most circumstances it will also include a graphical user interface, which runs atop the Kernel” reference #3. In most cases the user will communicate with the computer via the graphical user interface.

(ref #6) Some of the more common graphical environments that can run on Linux include the following. The KDE GUI (Graphical user interface). Matthias Ettrich developed KDE in 1996. He wanted a GUI for the Unix desktop that would make all of the applications look and feel alike. He also wanted a desktop environment for Unix that would be easier to use than the ones available at the time. KDE is a free open source project, with millions of coders working on it throughout the world, but it also has some commercial support from companies such as Novell, Troltech, and Mandriva. KDE aims to make an easy to use desktop environment without sacrificing configurability. Windows users might note that KDE has a similar look to Windows. Another popular GUI is (ref #7) GNOME. GNOME puts a heavy emphasis on simplicity, and user ability. Much like KDE GNOME is open source and is free to download. One notable feature of GNOME is the fact that it supports many different languages; GNOME supports over 100 different languages. Gnome is license under the LGPL license (lesser general public license). The license allows applications written for GNOME to use a much wider set of licenses, including some commercial applications. The name GNOME stands for GNU Network object model environment. GNOME’s look and feel is similar to that of other desktop environments. Fluxbox is another example of a Linux GUI. With less of an emphasis on ease of use and eye candy, Fluxbox aims to be a very lightweight, and a more efficient user of system resources. The interface has only a taskbar and a menu bar, which is accessed by right clicking over the desktop. Fluxbox is most popular for use with older computers that have a limited abundance of system resources.

Although most Linux distributions offer a graphical environment, to simplify the user experience, they all also offer a way for more technically involved users to directly communicate with the Kernel via a shell or command line. The command line allows you to run the computer without a GUI, by executing commands from a text-based interface. An advantage of using the command prompt is it uses less system resources and enables your computer to focus more of its energy on the task at hand. Examples of commands include the cd command for changing your directory, or the halt command for shutting down your system, or the reboot command for restarting the computer ect.

Now that we are more familiar with the Linux operating system, we can note the many ways in which Linux differs from the worlds most popular OS, Microsoft Windows. From this point forward we will discuss some of the more prominent ways in which Linux deferrers from Windows.

For starters there is only one company that releases a Windows operating system, and that company is Microsoft. All versions of Windows, weather Windows XP Home, Business, or Vista, all updates, security patches, and service patches for Windows comes from Microsoft. With Linux on the other hand there is not one company that releases it. Linux has millions of coders and companies throughout the world, volunteering their time to work on patches, updates, newer versions, and software applications. Although some companies, charge for TECH support, and others charge for their distribution of Linux, by packaging it with non-free software, you will always be able to get the Linux Kernel for free, and you can get full-featured Linux desktops with all the necessary applications for general use, for free as well. The vendors that charge money for their distribution of Linux are also required to release a free version in order to comply with the GPL License agreement. With Microsoft Windows on the other hand you have to pay Microsoft for the software, and you will also have to pay for most of the applications that you will use.

Windows and Linux also differ on TECH support issues. Windows is backed by the Microsoft Corporation, which means that if you have an issue with any of their products the company should resolve it. For example if Microsoft Windows is not working right, then you should be able to call Microsoft and make use of their TECH support to fix the issue. TECH support is usually included with the purchase of the product for a certain amount of time, maybe a two year period, and from there on you may be charged for the service. Although IBM backs their Linux products, for the most part if you use Linux you are on your own. If you have a problem with Ubuntu Linux you cannot call Ubuntu and expect any help. Despite the lack of professional help, you can however receive good TECH advice, from the thousands or millions of Linux forums that are on the web. You ca also get great help from social networking sites such as Myspace, by posting questions in the many Linux groups. You can usually receive responses for your questions in a matter of hours form many qualified people.

Configurability is another key difference between the two operating software’s. Although Windows offers its control panel to help users configure the computer to their liking, it does not match the configuring options that Linux provides especially if you are a real TECH savvy user. In Linux the Kernel is open source, so if you have the know how, you can modify it in virtually any way that you see fit. Also Linux offers a variety of Graphical environments to further suit your needs. As mentioned earlier Linux is capable of running full-featured graphical environments like KDE, or more lightweight and resource friendly GUI’s like Fluxbox, or Blackbox, to suit users with older computers. There are also versions of Linux that are designed to emulate the Windows look and feel as closely as possible. Distributions such as Linspire are best suited for users that are migrating over from the Windows world. There are also distributions that include no graphical environment at all to better suit users that need to squeeze out all of the computing power that they can get for various computing activities, and for users that are more advanced than others. All of this configurability can be problematic sometimes, as you will have to make a decision on which desktop is right for you, and to make things easier on yourself you will need to only install applications that are native to your distribution and graphical environment.

(ref #1) The cost effectiveness of Linux is another way it separates itself from Windows. For home use Linux is cheap and in most cases completely free, while Windows varies in cost depending on which version you buy. With Linux most of the applications will also be free, however for Windows in the majority of cases you are suppose to pay for the applications. For most cases, with Linux there is no need to enter a product activation key when performing an installation, you are free to install it on as many computers as you’d like. With Windows you are only allowed to install it on one computer and Microsoft uses product activation software to enforce this rule. When installing Window’s you must enter a product activation key, which will expire after so many uses. If you wish too, you can purchase Linux from a variety of vendors, which will include a boxed set of CDs, Manuals, and TECH support for around 40-130$. Of course If you purchase a high-end version of Linux used for servers it may cost any where from 400$- 2000$. “In 2002 computer world magazine quoted the chief technology architect at Merrill Lynch in New York, as saying “the cost of running Linux is typically a tenth of the cost of running Unix or Windows alternatively.” (ref#1)

(ref #1) Installation of Windows is generally easier, than installing Linux. “With Windows XP there are three main ways to install. There is a clean install, in which you install Windows on a blank hard drive. There is also an upgrade install, in which you start with an older version of Windows and “upgrade” to a newer one. An advantage of upgrading is that all of the files on the older system should remain intact throughout the process. You can also perform a repair install, in which case you are installing the same version of Windows on top of itself in order to fix a damaged version of Windows. There is also a recovery, which Technically is not an install; it is used to restore a copy of Windows back to its factory settings. The disadvantage of recovering Windows is the fact that you will loose all of your data, which resides on the damaged copy of Windows.” (ref#1) Also with Windows you can rest assured that your hardware will most likely be supported by the operating software, although this is not much of a problem with Linux you cant be sure if Linux will support all of your hardware. With Linux installation varies greatly from Distro to Distro. You may be presented with a graphical installer or it may be a text-based installer, these variations make Linux a bit more difficult and unpredictable to install than is Windows, (although the difficulty is disappearing). You may perform a clean install of Linux or dual boot it, to co-exist with another operation software. With Linux rather than having to buy an upgrade Cd, you can install updates by downloading and then installing them while your desktop is running. With Linux it is also not necessary to reboot your computer after most upgrades, It is only necessary to reboot after an upgrade to the kernel. It is also possible to run Linux without ever needing to install it on a hard drive; there are many distributions of Linux that will allow you to run it straight off of a live cd. The advantage of this is that you do not need to alter your system in order to

try Linux. You can run Linux off of the CD so you do not have to damage your Windows partition. Other advantages include the ability to rescue a broken Linux system. If your Linux computer will not boot, then you may insert a live cd and boot off it, so you can repair the damaged version of Linux. Also you may use a Linux live cd to recover files from a damaged Windows computer that will no longer boot up. Since Linux is capable of reading NTFS files you may copy files form a Windows computer to a USB flash drive or floppy drive ect.

Another major difference between Linux and Windows is the applications that you will use with either OS. Windows includes a much wider abundance of commercially backed applications than does Linux. It is much easier to find the software that you are looking for with Windows than it is with Linux, because so many software vendors make their products compatible with Windows only. With Linux you will for the most part be forced to let go of the familiar applications that you have grown accustomed to with Windows, in favor of lesser-known open source apps that are made for Linux. Applications such as Microsoft office, Outlook, Internet Explorer, Adobe Creative suite, and chat clients such as MSN messenger, do not work natively with Linux. Although with Linux you can get Microsoft office and Adobe creative suite to work using software from codeWeavers called cross Over Office. Instead of using these applications you will need to use Linux apps such as open office, The Gimp Image Editor, The ThunderBird email client, Instead of the MSN messenger you can use the GAIM messenger, and you can use Firefox as your web browser. Also with Linux it can be difficult to install software even if it is made for Linux. This is due to the fact that Linux has so many different versions. Software that is made to install on one version probably will require some configuration in order to install on another version. An example would be if you were trying to install software that was made for the KDE graphical environment, on the GNOME GUI, This app would not easily install on the GNOME GUI, and would require some configuring on your part to successfully install it.

The type of hard ware that Linux and windows runs on also causes them to differ. Linux will run on many different hardware platforms, from Intel and AMD chips, to computers running IBM power Pc processors. Linux will run on the slowest 386 machines to the biggest mainframes on the planet, newer versions of Windows will not run on the same amount of hardware as Linux. Linux can even be configured to run on apples, Ipod’s, or smart phones. A disadvantage of Linux is when it comes to using hardware devices such as Printers, Scanners, or Digital camera’s. Where as the driver software for these devices will often be easily available for Windows, with Linux you are for the most part left on your own to find drivers for these devices. Most Linux users will find comfort in the fact that drivers for the latest hardware are constantly being written by coders throughout the world and are usually very quickly made available.

(ref #1) One of the most notable differences between the two operating software’s is Windows legendary problems with malicious code, known as Viruses and Spy ware. Viruses, Spy-ware and a general lack of security are the biggest problems facing the Windows community. Under Windows Viruses and Spy-ware have the ability to execute themselves with little or no input from the user. This makes guarding against them a constant concern for any Windows user. Windows users are forced to employ third party anti virus software to help limit the possibility of the computer being rendered useless by malicious code. Anti virus software often has the negative side effect of hogging system resources, thus slowing down your entire computer, also most anti virus software requires that you pay a subscription service, and that you constantly download updates in order to stay ahead of the intruders. With Linux on the other hand problems with viruses are practically non-existent, and in reality you do not even need virus protection for your Linux machine. One reason why Viruses and Spy-ware are not a problem for Linux is simply due to the fact that there are far fewer being made for Linux. A more important reason is that running a virus on a Linux machine is more difficult and requires a lot more input from the user. With Windows you may accidentally run and execute a virus, by opening an email attachment, or by double clicking on a file that contains malicious code. However with Linux a virus would need to run in the terminal, which requires the user to give the file execute permissions, and then open it in the terminal. And in order to cause any real damage to the system the user would have to log in as root, by typing a user name and password before running the virus. Foe example to run a virus that is embedded in an email attachment the user would have to, open the attachment, then save it, then right click the file and chose properties form the menu, in properties they can give it execute permissions, they would then be able to

open the file in the terminal to run the virus. And even then the user would only be able to damage his or her home folder, all other users data will be left untouched, and all root system files would also remain untouched, because Linux would require a root password to make changes to these files. The only way the user can damage the whole computer would be if he or she logged in as root user by providing the root user name and password to the terminal before running the virus. Unlike Windows in Linux an executable file cannot run automatically, It needs to be given execute permissions manually this significantly improves security. In Linux the only realistic reason you would need virus protection is if you share files with Windows users, and that is to protect them not you, so you are not to accidentally pass a virus to the Windows computer that you are sharing files with.

The above was a general over view of some differences between the Windows operating system, and Linux. To recap we started with the fact that Windows has only one vendor that releases the software, while Linux comes from millions of different coders throughout the world. We also commented on the fact that the Linux Kernel and much of the applications used with it are completely free of charge, where as with windows you are forced to pay for most of the software. Unlike Widows Linux is often lacking in professional Tech support, and Linux users are often left on their own to solve Technical issues. Linux users can either pay for Tech support or rely on the many Linux Forums and groups available on the Internet. Due to the fact that the kernel is open source, Linux has a huge advantage over Windows in configurability. You can configure Linux to run almost any way you see fit by manipulating the Kernel. Installing the Windows Operating software and applications is easier due to the fact that it has a universal installer. Also finding applications for Windows is easier because of its popularity most apps are available for Windows only, and are made easily available. Linux will run on a greater variety of hard ware than does Windows, from mainframe super computers running multiple IBM Power PC Chips, to a small laptop running an AMD processor. And of course the biggest difference in this writer’s opinion is the fact that Linux does not suffer from an onslaught of Viruses and other malicious code, unlike Windows which is plagued by countless number of malicious code that can easily destroy your system if not properly guarded against.

In conclusion we will conclude that the Linux OS really is the superior software. Other than a few minor nuisances, linux out performs Windows in most categories. The fact that Linux is more secure is the tipping point, that tilts the scales in the favor of Linux. Windows simply suffers from far to many security vulnerabilities for it to be considered the better over all desktop environment.

References

http://www.michaelhorowitz.com/Linux.vs.Windows.html Reference #1

http://www.theinquirer.net/en/inquirer/news/2004/10/27/linux-more-secure-than-windows-says-study Reference #2

http://www.linux.com/whatislinux/ reference number 3

http://www.linux.org/info/

Reference #4

http://en.wikipedia.org/wiki/Linux%5Fkernel Reference #5

http://en.wikipedia.org/wiki/KDE Reference #6

http://en.wikipedia.org/wiki/GNOME Reference #7




Read More...

Ensuring a computer is not only displaying the correct time but that it is being maintained accurately is not as straight-forward as it first sounds.

Most Linux systems have two clocks. The hardware clock, also known as the CMOS of Bios clock, is usually a simple crystal oscillator with battery back-up that maintains time when your system is off or boots up. This clock is usually located on the motherboard and will run all the time, however these clock chips tend to lose time as the computer ages and the battery weakens.

The other clock, the system clock, is a software clock and it starts when you boot up your system often getting an initial time from the hardware clock. System clocks keep time by adding seconds on to a prime epoch, a base time that for Linux and Unix, began at midnight on January 1, 1970.

However, the hardware clock is a cheap electronic oscillator and cannot maintain time to any useful degree of accuracy. They often drift several seconds a day which for day-to-day process is probably adequate but with time sensitive applications it can cause serious problems.

A better way is to set the system clock using the time from a NTP (Network Time Protocol) time server. These dedicated time servers get a UTC (Coordinated Universal Time) time from an atomic clock which are the most accurate time keepers in the world, not losing a second in time in several millions of years.

Dedicated NTP servers use either a radio receiver to pick-up a radio time and frequency broadcast which are transmitted by several national laboratories or by using the timing signal broadcast from the GPS network.

Linux uses a NTP service called NTP Daemon (ntpd). This uses NTP to adjust the system clock for any drift in time as it frequently check the UTC time source.

To configure the NTP daemon the ntp.conf file in the /etc directory can be used. From here more than one time server can be used as a reference and also the frequency it is checked can also be altered.

Richard N Williams is a technical author and specialist in atomic clocks, telecommunications, NTP and network time synchronisation helping to develop dedicated NTP clocks. Please visit us for more information about a network time server or other ntp server solutions.

Article Source: http://EzineArticles.com/?expert=Richard_N_Williams Read More...

Conventions used in this document                                     

=================================

Each section in this document will have the string "
" at the
right-hand side of the section title. Each subsection will have
"" at the right-hand side. These strings are meant to make
it easier to search through the document.

NOTE that the master copy of this document is available online at:
http://www.atnf.csiro.au/~rgooch/linux/docs/vfs.txt


What is it?                                                           

===========

The Virtual File System (otherwise known as the Virtual Filesystem
Switch) is the software layer in the kernel that provides the
filesystem interface to userspace programmes. It also provides an
abstraction within the kernel which allows different filesystem
implementations to co-exist.


A Quick Look At How It Works                                          

============================

In this section I'll briefly describe how things work, before
launching into the details. I'll start with describing what happens
when user programmes open and manipulate files, and then look from the
other view which is how a filesystem is supported and subsequently
mounted.

Opening a File                                                     
--------------

The VFS implements the open(2), stat(2), chmod(2) and similar system
calls. The pathname argument is used by the VFS to search through the
directory entry cache (dentry cache or "dcache"). This provides a very
fast lookup mechanism to translate a pathname (filename) into a
specific dentry.

An individual dentry usually has a pointer to an inode. Inodes are the
things that live on disc drives, and can be regular files (you know:
those things that you write data into), directories, FIFOs and other
beasts. Dentries live in RAM and are never saved to disc: they exist
only for performance. Inodes live on disc and are copied into memory
when required. Later any changes are written back to disc. The inode
that lives in RAM is a VFS inode, and it is this which the dentry
points to. A single inode can be pointed to by multiple dentries
(think about hardlinks).

The dcache is meant to be a view into your entire filespace. Unlike
Linus, most of us losers can't fit enough dentries into RAM to cover
all of our filespace, so the dcache has bits missing. In order to
resolve your pathname into a dentry, the VFS may have to resort to
creating dentries along the way, and then loading the inode. This is
done by looking up the inode.

To lookup an inode (usually read from disc) requires that the VFS
calls the lookup() method of the parent directory inode. This method
is installed by the specific filesystem implementation that the inode
lives in. There will be more on this later.

Once the VFS has the required dentry (and hence the inode), we can do
all those boring things like open(2) the file, or stat(2) it to peek
at the inode data. The stat(2) operation is fairly simple: once the
VFS has the dentry, it peeks at the inode data and passes some of it
back to userspace.

Opening a file requires another operation: allocation of a file
structure (this is the kernel-side implementation of file
descriptors). The freshly allocated file structure is initialised with
a pointer to the dentry and a set of file operation member functions.
These are taken from the inode data. The open() file method is then
called so the specific filesystem implementation can do it's work. You
can see that this is another switch performed by the VFS.

The file structure is placed into the file descriptor table for the
process.

Reading, writing and closing files (and other assorted VFS operations)
is done by using the userspace file descriptor to grab the appropriate
file structure, and then calling the required file structure method
function to do whatever is required.

For as long as the file is open, it keeps the dentry "open" (in use),
which in turn means that the VFS inode is still in use.

All VFS system calls (i.e. open(2), stat(2), read(2), write(2),
chmod(2) and so on) are called from a process context. You should
assume that these calls are made without any kernel locks being
held. This means that the processes may be executing the same piece of
filesystem or driver code at the same time, on different
processors. You should ensure that access to shared resources is
protected by appropriate locks.

Registering and Mounting a Filesystem                              
-------------------------------------

If you want to support a new kind of filesystem in the kernel, all you
need to do is call register_filesystem(). You pass a structure
describing the filesystem implementation (struct file_system_type)
which is then added to an internal table of supported filesystems. You
can do:

% cat /proc/filesystems

to see what filesystems are currently available on your system.

When a request is made to mount a block device onto a directory in
your filespace the VFS will call the appropriate method for the
specific filesystem. The dentry for the mount point will then be
updated to point to the root inode for the new filesystem.

It's now time to look at things in more detail.


struct file_system_type                                               

=======================

This describes the filesystem. As of kernel 2.1.99, the following
members are defined:

struct file_system_type {
 const char *name;
 int fs_flags;
 struct super_block *(*read_super) (struct super_block *, void *, int);
 struct file_system_type * next;
};

 name: the name of the filesystem type, such as "ext2", "iso9660",
 "msdos" and so on

 fs_flags: various flags (i.e. FS_REQUIRES_DEV, FS_NO_DCACHE, etc.)

 read_super: the method to call when a new instance of this
 filesystem should be mounted

 next: for internal VFS use: you should initialise this to NULL

The read_super() method has the following arguments:

 struct super_block *sb: the superblock structure. This is partially
 initialised by the VFS and the rest must be initialised by the
 read_super() method

 void *data: arbitrary mount options, usually comes as an ASCII
 string

 int silent: whether or not to be silent on error

The read_super() method must determine if the block device specified
in the superblock contains a filesystem of the type the method
supports. On success the method returns the superblock pointer, on
failure it returns NULL.

The most interesting member of the superblock structure that the
read_super() method fills in is the "s_op" field. This is a pointer to
a "struct super_operations" which describes the next level of the
filesystem implementation.


struct super_operations                                               

=======================

This describes how the VFS can manipulate the superblock of your
filesystem. As of kernel 2.1.99, the following members are defined:

struct super_operations {
 void (*read_inode) (struct inode *);
 void (*write_inode) (struct inode *);
 void (*put_inode) (struct inode *);
 void (*delete_inode) (struct inode *);
 int (*notify_change) (struct dentry *, struct iattr *);
 void (*put_super) (struct super_block *);
 void (*write_super) (struct super_block *);
 int (*statfs) (struct super_block *, struct statfs *, int);
 int (*remount_fs) (struct super_block *, int *, char *);
 void (*clear_inode) (struct inode *);
};

All methods are called without any locks being held, unless otherwise
noted. This means that most methods can block safely. All methods are
only called from a process context (i.e. not from an interrupt handler
or bottom half).

 read_inode: this method is called to read a specific inode from the
 mounted filesystem. The "i_ino" member in the "struct inode"
 will be initialised by the VFS to indicate which inode to
 read. Other members are filled in by this method

 write_inode: this method is called when the VFS needs to write an
 inode to disc

 put_inode: called when the VFS inode is removed from the inode
 cache. This method is optional

 delete_inode: called when the VFS wants to delete an inode

 notify_change: called when VFS inode attributes are changed. If this
 is NULL the VFS falls back to the write_inode() method. This
 is called with the kernel lock held

 put_super: called when the VFS wishes to free the superblock
 (i.e. unmount). This is called with the superblock lock held

 write_super: called when the VFS superblock needs to be written to
 disc. This method is optional

 statfs: called when the VFS needs to get filesystem statistics. This
 is called with the kernel lock held

 remount_fs: called when the filesystem is remounted. This is called
 with the kernel lock held

 clear_inode: called then the VFS clears the inode. Optional

The read_inode() method is responsible for filling in the "i_op"
field. This is a pointer to a "struct inode_operations" which
describes the methods that can be performed on individual inodes.


struct inode_operations                                               

=======================

This describes how the VFS can manipulate an inode in your
filesystem. As of kernel 2.1.99, the following members are defined:

struct inode_operations {
 struct file_operations * default_file_ops;
 int (*create) (struct inode *,struct dentry *,int);
 int (*lookup) (struct inode *,struct dentry *);
 int (*link) (struct dentry *,struct inode *,struct dentry *);
 int (*unlink) (struct inode *,struct dentry *);
 int (*symlink) (struct inode *,struct dentry *,const char *);
 int (*mkdir) (struct inode *,struct dentry *,int);
 int (*rmdir) (struct inode *,struct dentry *);
 int (*mknod) (struct inode *,struct dentry *,int,int);
 int (*rename) (struct inode *, struct dentry *,
   struct inode *, struct dentry *);
 int (*readlink) (struct dentry *, char *,int);
 struct dentry * (*follow_link) (struct dentry *, struct dentry *);
 int (*readpage) (struct file *, struct page *);
 int (*writepage) (struct file *, struct page *);
 int (*bmap) (struct inode *,int);
 void (*truncate) (struct inode *);
 int (*permission) (struct inode *, int);
 int (*smap) (struct inode *,int);
 int (*updatepage) (struct file *, struct page *, const char *,
    unsigned long, unsigned int, int);
 int (*revalidate) (struct dentry *);
};

Again, all methods are called without any locks being held, unless
otherwise noted.

 default_file_ops: this is a pointer to a "struct file_operations"
 which describes how to open and then manipulate open files

 create: called by the open(2) and creat(2) system calls. Only
 required if you want to support regular files. The dentry you
 get should not have an inode (i.e. it should be a negative
 dentry). Here you will probably call d_instantiate() with the
 dentry and the newly created inode

 lookup: called when the VFS needs to lookup an inode in a parent
 directory. The name to look for is found in the dentry. This
 method must call d_add() to insert the found inode into the
 dentry. The "i_count" field in the inode structure should be
 incremented. If the named inode does not exist a NULL inode
 should be inserted into the dentry (this is called a negative
 dentry). Returning an error code from this routine must only
 be done on a real error, otherwise creating inodes with system
 calls like create(2), mknod(2), mkdir(2) and so on will fail.
 If you wish to overload the dentry methods then you should
 initialise the "d_dop" field in the dentry; this is a pointer
 to a struct "dentry_operations".
 This method is called with the directory inode semaphore held

 link: called by the link(2) system call. Only required if you want
 to support hard links. You will probably need to call
 d_instantiate() just as you would in the create() method

 unlink: called by the unlink(2) system call. Only required if you
 want to support deleting inodes

 symlink: called by the symlink(2) system call. Only required if you
 want to support symlinks. You will probably need to call
 d_instantiate() just as you would in the create() method

 mkdir: called by the mkdir(2) system call. Only required if you want
 to support creating subdirectories. You will probably need to
 call d_instantiate() just as you would in the create() method

 rmdir: called by the rmdir(2) system call. Only required if you want
 to support deleting subdirectories

 mknod: called by the mknod(2) system call to create a device (char,
 block) inode or a named pipe (FIFO) or socket. Only required
 if you want to support creating these types of inodes. You
 will probably need to call d_instantiate() just as you would
 in the create() method

 readlink: called by the readlink(2) system call. Only required if
 you want to support reading symbolic links

 follow_link: called by the VFS to follow a symbolic link to the
 inode it points to. Only required if you want to support
 symbolic links


struct file_operations                                                

======================

This describes how the VFS can manipulate an open file. As of kernel
2.1.99, the following members are defined:

struct file_operations {
 loff_t (*llseek) (struct file *, loff_t, int);
 ssize_t (*read) (struct file *, char *, size_t, loff_t *);
 ssize_t (*write) (struct file *, const char *, size_t, loff_t *);
 int (*readdir) (struct file *, void *, filldir_t);
 unsigned int (*poll) (struct file *, struct poll_table_struct *);
 int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long);
 int (*mmap) (struct file *, struct vm_area_struct *);
 int (*open) (struct inode *, struct file *);
 int (*release) (struct inode *, struct file *);
 int (*fsync) (struct file *, struct dentry *);
 int (*fasync) (struct file *, int);
 int (*check_media_change) (kdev_t dev);
 int (*revalidate) (kdev_t dev);
 int (*lock) (struct file *, int, struct file_lock *);
};

Again, all methods are called without any locks being held, unless
otherwise noted.

 llseek: called when the VFS needs to move the file position index

 read: called by read(2) and related system calls

 write: called by write(2) and related system calls

 readdir: called when the VFS needs to read the directory contents

 poll: called by the VFS when a process wants to check if there is
 activity on this file and (optionally) go to sleep until there
 is activity. Called by the select(2) and poll(2) system calls

 ioctl: called by the ioctl(2) system call

 mmap: called by the mmap(2) system call

 open: called by the VFS when an inode should be opened. When the VFS
 opens a file, it creates a new "struct file" and initialises
 the "f_op" file operations member with the "default_file_ops"
 field in the inode structure. It then calls the open method
 for the newly allocated file structure. You might think that
 the open method really belongs in "struct inode_operations",
 and you may be right. I think it's done the way it is because
 it makes filesystems simpler to implement. The open() method
 is a good place to initialise the "private_data" member in the
 file structure if you want to point to a device structure

 release: called when the last reference to an open file is closed

 fsync: called by the fsync(2) system call

 fasync: called by the fcntl(2) system call when asynchronous
 (non-blocking) mode is enabled for a file

Note that the file operations are implemented by the specific
filesystem in which the inode resides. When opening a device node
(character or block special) most filesystems will call special
support routines in the VFS which will locate the required device
driver information. These support routines replace the filesystem file
operations with those for the device driver, and then proceed to call
the new open() method for the file. This is how opening a device file
in the filesystem eventually ends up calling the device driver open()
method. Note the devfs (the Device FileSystem) has a more direct path
from device node to device driver (this is an unofficial kernel
patch).


struct dentry_operations                                              

========================

This describes how a filesystem can overload the standard dentry
operations. Dentries and the dcache are the domain of the VFS and the
individual filesystem implementations. Device drivers have no business
here. These methods may be set to NULL, as they are either optional or
the VFS uses a default. As of kernel 2.1.99, the following members are
defined:

struct dentry_operations {
 int (*d_revalidate)(struct dentry *);
 int (*d_hash) (struct dentry *, struct qstr *);
 int (*d_compare) (struct dentry *, struct qstr *, struct qstr *);
 void (*d_delete)(struct dentry *);
 void (*d_release)(struct dentry *);
 void (*d_iput)(struct dentry *, struct inode *);
};

 d_revalidate: called when the VFS needs to revalidate a dentry. This
 is called whenever a name lookup finds a dentry in the
 dcache. Most filesystems leave this as NULL, because all their
 dentries in the dcache are valid

 d_hash: called when the VFS adds a dentry to the hash table

 d_compare: called when a dentry should be compared with another

 d_delete: called when the last reference to a dentry is
 deleted. This means no-one is using the dentry, however it is
 still valid and in the dcache

 d_release: called when a dentry is really deallocated

 d_iput: called when a dentry looses its inode (just prior to its
 being deallocated). The default when this is NULL is that the
 VFS calls iput(). If you define this method, you must call
 iput() yourself

Each dentry has a pointer to its parent dentry, as well as a hash list
of child dentries. Child dentries are basically like files in a
directory.

There are a number of functions defined which permit a filesystem to
manipulate dentries:

 dget: open a new handle for an existing dentry (this just increments
 the usage count)

 dput: close a handle for a dentry (decrements the usage count). If
 the usage count drops to 0, the "d_delete" method is called
 and the dentry is placed on the unused list if the dentry is
 still in its parents hash list. Putting the dentry on the
 unused list just means that if the system needs some RAM, it
 goes through the unused list of dentries and deallocates them.
 If the dentry has already been unhashed and the usage count
 drops to 0, in this case the dentry is deallocated after the
 "d_delete" method is called

 d_drop: this unhashes a dentry from its parents hash list. A
 subsequent call to dput() will dellocate the dentry if its
 usage count drops to 0

 d_delete: delete a dentry. If there are no other open references to
 the dentry then the dentry is turned into a negative dentry
 (the d_iput() method is called). If there are other
 references, then d_drop() is called instead

 d_add: add a dentry to its parents hash list and then calls
 d_instantiate()

 d_instantiate: add a dentry to the alias hash list for the inode and
 updates the "d_inode" member. The "i_count" member in the
 inode structure should be set/incremented. If the inode
 pointer is NULL, the dentry is called a "negative
 dentry". This function is commonly called when an inode is
 created for an existing negative dentry

Read More...