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diff --git a/hurd/documentation.mdwn b/hurd/documentation.mdwn index a32bc84f..be0874e7 100644 --- a/hurd/documentation.mdwn +++ b/hurd/documentation.mdwn @@ -24,6 +24,8 @@ is included in the section entitled * [[*The_Hurd*|hurd-talk]], a presentation by Marcus Brinkmann. + * A document about *[[translators]]* by Marcus Brinkmann. + * [[*A_Critique_of_the_GNU_Hurd_Multi-server_Operating_System*|critique]], an analysis of the GNU Hurd on GNU Mach system, written by Neal Walfield and Marcus Brinkmann. diff --git a/hurd/documentation/translators.html b/hurd/documentation/translators.html new file mode 100644 index 00000000..4e47a9c0 --- /dev/null +++ b/hurd/documentation/translators.html @@ -0,0 +1,236 @@ +[[meta copyright="Copyright © 1998, 1999, 2007, 2008 Free Software Foundation, +Inc."]] + +[[meta license="Verbatim copying and distribution of this entire article is +permitted in any medium, provided this notice is preserved."]] + +[[meta title="Translators"]] + +By Marcus Brinkmann. + +<ul> +<li><a href="#concept" name="TOC_concept">Concept</a></li> +<li><a href="#examples" name="TOC_examples">Examples</a></li> +<li><a href="#actpas" name="TOC_actpas">Passive Translators, Active Translators</a></li> +<li><a href="#manage" name="TOC_manage">Managing Translators</a></li> +</ul> +<h3><a href="#TOC_concept" name="concept">Concept</a></h3> +<p> +Before we take a closer look at translators, let us consider regular +filesystems. A filesystem is store for a hierarchical tree of directories +and files. You access directories and files by a special character string, +the path. Furthermore, there are symbolic links to refer to one file at +several places in the tree, there are hard links to give one and the same +file several names. There are also special device files for communication +with the hardware device drivers of the kernel, and there are mount points +to include other stores in the directory tree. Then there are obscure +objects like fifos and hard links.</p> +<p> +Although these objects are very different, they share some common +properties, for example, they have all an owner and a group associated with +them as well as access rights (permissions). This information is written in +inodes. This is a actually a further commonality: Every object has exactly +one inode associated with it (hard links are somewhat special as they share +one and the same inode). Sometimes, the inode has further information +stored in it. For example, the inode can contain the target of a symbolic +link.</p> +<p> +However, these commonalities are usually not exploited in the +implementations, despite the common programming interface to them. All +inodes can be accessed through the standard POSIX calls, for example +<code>read()</code> and <code>write()</code>. For example, to add a new +object type (for example a new link type) to a common monolithic unix +kernel, you would need to modify the code for each filesystem +seperately.</p> +<p> +In the Hurd, things work differently. Although in the Hurd a special +filesystem server can exploit special properties of standard object types +like links (in the ext2 filesystem with fast links, for example), it has a +general interface to add such features without modifying existing code.</p> +<p> +The trick is to allow a program to be inserted between the actual content of +a file and the user accessing this file. Such a program is called a +translator, because it is able to process the incoming requests in many +different ways. In other words, a translator is a Hurd server which provides +the basic filesystem interface.</p> +<p> +Translators have very interesting properties. From the kernels point of +view, they are just another user process. This means, translators can be run +by any user. You don't need root priviligies to install or modify a +translator, you only need the access rights for the underlying inode the +translator is attached to. Many translators don't require an actual file to +operate, they can provide information by their own means. This is why +the information about translators is stored in the inode.</p> +<p> +Translators are responsible to serve all file system operations that involve +the inode they are attached to. Because they are not restricted to the usual +set of objects (device file, link etc), they are free to return anything +that makes sense to the programmer. One could imagine a translator that +behaves like a directory when accessed by <code>cd</code> or +<code>ls</code> and at the same time behaves like a file when accessed by +<code>cat</code>.</p> +<h3><a href="#TOC_examples" name="examples">Examples</a></h3> +<h4>Mount Points</h4> +<p> +A mount point can be seen as an inode that has a special translator attached +to it. Its purpose would be to translate filesystem operations on the mount +point in filesystem operations on another store, let's say, another +partition.</p> +<p> +Indeed, this is how filesystems are implemented under the Hurd. A +filesystem is a translator. This translator takes a store as its argument, +and is able to serve all filesystem operations transparently.</p> +<h4>Device Files</h4> +<p> +There are many different device files, and in systems with a monolithical +kernel, they are all provided by the kernel itself. In the Hurd, all device +files are provided by translators. One translator can provide support for +many similar device files, for example all hard disk partitions. This way, +the number of actual translators needed is quite small. However, note that +for each device file accessed, a seperate translator task is started. +Because the Hurd is heavily multi threaded, this is very cheap.</p> +<p> +When hardware is involved, a translator usually starts to communicate with +the kernel to get the data from the hardware. However, if no hardware access +is necessary, the kernel does not need to be involved. For example, +<code>/dev/zero</code> does not require hardware access, and can therefore +be implemented completely in user space.</p> +<h4>Symbolic Links</h4> +<p> +A symbolic link can be seen as a translator. Accesing the symbolic link +would start up the translator, which would forward the request to the +filesystem that contains the file the link points to.</p> +<p> +However, for better performance, filesystems that have native support +for symbolic links can take advantage of this feature and implement +symbolic links differently. Internally, accessing a symbolic link would not +start a new translator process. However, to the user, it would still look +as if a passive translator is involved (see below for an explanation what a +passsive translator is).</p> +<p> +Because the Hurd ships with a symlink translator, any filesystem server that +provides support for translators automatically has support for symlinks (and +firmlinks, and device files etc)! This means, you can get a working +filesystem very fast, and add native support for symlinks and other features +later.</p> +<h3><a href="#TOC_actpas" name="actpas">Passive Translators, Active Translators</a></h3> +<p> +There are two types of translators, passive and active. They are really +completely different things, so don't mix them up, but they have a close +relation to each other.</p> +<h4>Active Translators</h4> +<p> +An active translator is a running translator process, as introduced above. +You can set and remove active translators using the +<code>settrans -a</code></a> +command. The <code>-a</code> option is necessary to tell +<code>settrans</code> that you want to modify the active translator.</p> +<p> +The <code>settrans</code> command takes three kind of arguments. First, you +can set options for the <code>settrans</code> command itself, like +<code>-a</code> to modify the active translator. Then you set the inode you +want to modify. Remember that a translator is always associated with an +inode in the directory hierarchy. You can only modify one inode at a time. +If you do not specify any more arguments, <code>settrans</code> will try to +remove an existing translator. How hard it tries depends on the force +options you specify (if the translator is in use by any process, you will +get "device or resource busy" error message unless you force it to go away).</p> +<p> +But if you specify further arguments, it will be interpreted as a command +line to run the translator. This means, the next argument is the filename of +the translator executable. Further arguments are options to the translator, +and not to the <code>settrans</code> command.</p> +<p> +For example, to mount an ext2fs partition, you can run +<code>settrans -a -c /mnt /hurd/ext2fs /dev/hd2s5</code>. The +<code>-c</code> option will create the mount point for you if it doesn't +exist already. This does not need to be a directory, by the way. To unmount, +you would try <code>settrans -a /mnt</code>.</p> +<h4>Passive Translators</h4> +<p> +A passive translator is set and modified with the same syntax as the active +translator (just leave away the <code>-a</code>, so everything said above is +true for passive translators, too. However, there is a difference: passive +translators are not yet started.</p> +<p> +This makes sense, because this is what you usually want. You don't want the +partition mounted unless you really access files on this partition. You +don't want to bring up the network unless there is some traffic and so +on.</p> +<p> +Instead, the first time the passive translator is accessed, it is +automatically read out of the inode and an active translator is started on +top of it using the command line that was stored in the inode. This is +similar to the Linux automounter functionality. However, it does not come as +an additional bonus that you have to set up manually, but an integral part of +the system. So, setting passive translators defers starting the translator +task until you really need it. By the way, if the active translator dies for +some reason, the next time the inode is accessed the translator is +restarted.</p> +<p> +There is a further difference: active translators can die or get lost. As +soon as the active translator process is killed (for example, because you +reboot the machine) it is lost forever. Passive translators are not transient +and stay in the inode during reboots until you modify them with the +<code>settrans</code> program or delete the inodes they are attached to. +This means, you don't need to maintain a configuration file with your mount +points.</p> +<p> +One last point: Even if you have set a passive translator, you can still +set a different active translator. Only if the translator is automatically +started because there was no active translator the time the inode was +accessed the passive translator is considered.</p> +<h3><a href="#TOC_manage" name="manage">Managing Translators</a></h3> +<p> +As mentioned above, you can use +<code>settrans</code></a> +to set and alter passive and active translators. There are a lot of options +to change the behaviour of <code>settrans</code> in case something goes +wrong, and to conditionalize its action. Here are some common usages:</p> +<ul><li><code>settrans -c /mnt /hurd/ext2fs /dev/hd2s5</code> mounts a +partition, the translator will stay across reboots.</li> +<li><code>settrans -a /mnt /hurd/ext2fs ~/dummy.fs</code> mounts a +filesystem inside a data file, the translator will go away if it dies.</li> +<li><code>settrans -fg /nfs-data</code> forces a translator to go away.</li> +</ul> +<p> +You can use the <code>showtrans</code></a> +command to see if a translator is attached to an inode. This will only show +you the passive translator though.</p> +<p> +You can change the options of an active (filesystem) translator with +<code>fsysopts</code> without actually restarting it. This is very +convenient. For example, you can do what is called "remounting a +partition read-only" under Linux simply by running <code>fsysopts +/mntpoint --readonly</code>. The running active translator +will change its behaviour according to your request if possible. +<code>fsysopts /mntpoint</code> without a parameter shows you the current +settings.</p> +<h4>Examples</h4> +<p> +I recommend that you start by reading the <code>/bin/mount</code> command, +it is only a small script. Because setting filesystem translators is +similar to mounting partitions, you can easily grasp the concept this way. +Make a file system image with <code>dd if=/dev/zero of=dummy.fs bs=1024k +count=8; mke2fs dummy.fs</code> and "mount" it with <code>settrans -c dummy +/hurd/ext2fs `pwd`/dummy.fs</code>. Note that the translator is not started +yet, no new <code>ext2fs</code> process is running (verify with <code>ps +Aux</code>). Check that everything is correct using <code>showtrans</code></p> +<p> +Now type <code>ls dummy</code> and you will notice the short delay that +occurs while the translator is started. After that, there will be no more +delays accessing dummy. Under Linux, one would say that you automounted a +loop file system. Check with <code>ps Aux</code> that there is an <code>ext2fs +dummy</code> process up and running now. Now put some files into the new +directory. Try to make the filesystem read-only with <code>fsysopts</code>. +Note how further write attempts fail now. Try to kill the active translator +with <code>settrans -g</code>.</p> +<p> +You should have some understanding of what is going on now. Now remember +that this was only <em>one</em> special server, the Hurd ext2fs server. +There are many more server in the <code>hurd</code> directory. Some of them +are for filesystems. Some are needed for file system features like links. +Some are needed for device files. Some are useful for networking. Imagine +"mounting" an FTP Server with <code>settrans</code> and downloading files +simply with the standard <code>cp</code> command. Or editing your web sites +with <code>emacs /ftp/homepage.my.server.org/index.html</code>!</p> diff --git a/hurd/translator.mdwn b/hurd/translator.mdwn index 889f02a6..1a987b09 100644 --- a/hurd/translator.mdwn +++ b/hurd/translator.mdwn @@ -40,6 +40,8 @@ Also there is an [[writing/example]] about how to write a simple translator. See some [[examples]] about how to use translators. +Marcus Brinkmann has written a document about [[documentation/translators]]. + # Existing Translators |