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| author | Laurent Bercot <ska-skaware@skarnet.org> | 2014-12-05 22:26:11 +0000 |
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| committer | Laurent Bercot <ska-skaware@skarnet.org> | 2014-12-05 22:26:11 +0000 |
| commit | 90b12bd71bb9fc79a4640b9112c13ef529d0196a (patch) | |
| tree | 523b3f4ee2969e7a729bab2ba749c4b924ae62af /doc/scandir.html | |
| download | s6-90b12bd71bb9fc79a4640b9112c13ef529d0196a.tar.xz | |
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diff --git a/doc/scandir.html b/doc/scandir.html new file mode 100644 index 0000000..366bd08 --- /dev/null +++ b/doc/scandir.html @@ -0,0 +1,145 @@ +<html> + <head> + <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> + <meta http-equiv="Content-Language" content="en" /> + <title>s6: scan directories</title> + <meta name="Description" content="s6: scan directory" /> + <meta name="Keywords" content="s6 scandir supervision svscan s6-svscan scan directory servicedir" /> + <!-- <link rel="stylesheet" type="text/css" href="http://skarnet.org/default.css" /> --> + </head> +<body> + +<p> +<a href="index.html">s6</a><br /> +<a href="http://skarnet.org/software/">Software</a><br /> +<a href="http://skarnet.org/">skarnet.org</a> +</p> + +<h1> Scan directories </h1> + +<p> + A <em>scan directory</em> is a directory containing a list of +<a href="servicedir.html">service directories</a>, or symbolic links +pointing to service directories. +</p> + +<p> + A scan directory represents a list of services that are supposed to +be supervised. Running <a href="s6-svscan.html">s6-svscan</a> on this +scan directory launches a <em>supervision tree</em>: every service +listed in the scan directory will be supervised. +</p> + +<p> + There is normally only one scan directory per system, although nothing +prevents a system administrator from having more. +<a href="http://cr.yp.to/daemontools.html">daemontools</a> traditionally +uses <tt>/service</tt>, and <a href="http://smarden.org/runit/">runit</a> +traditionally uses <tt>/etc/service</tt>. s6 does not care where your +scan directory is, but I would advise <tt>/service</tt> for compatibility +with daemontools. Depending on your installation, <tt>/service</tt> could +be a symbolic link and point to a place either in a RAM filesystem or in +<tt>/var</tt>. +</p> + +<a name="where"> +<h2> Where and how to build a scan directory </h2> +</a> + +<p> + Opinions and practices differ. +</p> + +<p> + It is generally accepted that the place where you store all your +service directories (your "service repository") should <em>not</em> be +used as a scan directory - for a simple reason: you might want to have +service directories for more services than what you want to start at +any given time. In other words, your scan directory will be a <em>subset</em> +of your service repository, so you cannot just run s6-svscan on every +service you have a service directory for. So, the first thing is to +separate your <em>service repository</em>, which is just a storage place +for all the services you might want to manage someday, and your <em>scan +directory</em>, which is a directory representing all the services that +you are <em>currently</em> managing. +</p> + +<h3> Service repository </h3> + +<p> + Where to store your service repository is purely a matter of personal +preference. You just have to be aware that <a href="s6-supervise.html"> +s6-supervise</a> needs writable <tt>supervise</tt> and <tt>event</tt> +subdirectories in a service directory it monitors. +</p> + +<h3> Scan directory </h3> + +<p> + Where and how to build your scan directory depends heavily on your boot +system - and on your personal preference too. +</p> + +<p> + Standard <a href="http://cr.yp.to/daemontools.html">daemontools</a> and +<a href="http://smarden.org/runit/">runit</a> installations like to have +a fixed scan directory containing symlinks to service directories located +in the service repository. In other words, the service repository contains +the real <em>working copies</em> of the service directories. This works, +as long as: +</p> + +<ul> + <li> It is possible to create writable <tt>supervise</tt> and <tt>event</tt> +subdirectories in every managed service directory. This can be achieved for +instance via symlinks, or by having the service repository stored on a writable +filesystem. </li> + <li> The scan program (<a href="s6-svscan.html">s6-svscan</a>, +<a href="http://cr.yp.to/daemontools/svscan.html">svscan</a>, +<a href="http://smarden.org/runit/runsvdir.8.html">runsvdir</a>...) is +started late enough for all the necessary filesystems to be mounted. </li> +</ul> + +<p> + My own recommendation would be to have working copies of the service +directories <em>entirely separate</em> from the service repository. The +service repository can be safely stored on the root filesystem, and the +needed directories copied to a RAM filesystem at boot time. The scan +directory can be either the place where the working copies are written, +or another directory containing symlinks to those working copies. (The +latter is useful if you are not using <tt>s6-svscan -t0</tt>: copying a +directory is not atomic, but making a symlink is, so there is no risk +of your scanner finding an incomplete directory.) +</p> + +<p> + An example: +</p> + +<ul> + <li> Have your service repository in <tt>/img/services</tt>, i.e. have +service directories in <tt>/img/services/ftpd</tt>, <tt>/img/services/httpd</tt>, +<tt>/img/services/sshd</tt>, etc. </li> + <li> When booting, make <tt>/tmp</tt> a RAM filesystem, and create the +directories <tt>/tmp/services</tt> and <tt>/tmp/service</tt>. </li> + <li> Have s6-svscan run on <tt>/tmp/service</tt>, as early as possible in your +boot sequence. This is possible whether you want to run s6-svscan +<a href="s6-svscan-1.html">as process 1</a> or <a href="s6-svscan-not-1.html">not</a>. </li> + <li> During the boot sequence, populate <tt>/tmp/services</tt> with copies of the +service directories you need: for instance, + <ul> + <li> <tt>cp -a /img/services/sshd /tmp/services/sshd</tt> </li> + <li> <tt>cp -a /img/services/ftpd /tmp/services/ftpd</tt> </li> + <li> etc. </li> + </ul> </li> + <li> When you are ready to start a service, make a symlink in the +<tt>/tmp/service</tt> <em>scan directory</em> pointing to the working copy of +the service directory you need in <tt>/tmp/services</tt>, then notify s6-svscan. +For instance, to start ftpd and httpd together: +<pre> ln -s ../services/ftpd /tmp/service + ln -s ../services/httpd /tmp/service + s6-svscanctl -a /tmp/service</pre> </li> +</ul> + +</body> +</html> |