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diff --git a/doc/faq.html b/doc/faq.html index b54b79a..8df36cb 100644 --- a/doc/faq.html +++ b/doc/faq.html @@ -18,7 +18,10 @@ <h1> s6-rc: Frequently Asked Questions </h1> + +<a name="quickrecipes"> <h2> Quick recipes </h2> +</a> <h3> How do I... </h3> @@ -67,7 +70,10 @@ will bring the new services up. The services that will start are listed after <tt>-- change</tt>. </p> + +<a name="sourceformat"> <h2> The s6-rc-compile source format </h2> +</a> <h3> The source format for <a href="s6-rc-compile.html">s6-rc-compile</a> is not very convenient. @@ -161,7 +167,82 @@ Bundles have multiple uses, and virtual services are definitely one of them. </p> + +<a name="compiledmanagement"> +<h2> Managing compiled databases </h2> +</a> + +<h3> How do I safely update my compiled database? It is a directory, so +I cannot atomically replace my old database with the new one. </h3> + +<p> + Use symbolic links. You can atomically replace a symbolic link, and the +s6-rc programs have been designed with that mechanism in mind. Here is +the simplest way of managing your compiled databases: (change the names +according to your preferences, but keep the same idea) +</p> + +<ul> + <li> Make sure your <a href="s6-rc-init.html">s6-rc-init</a> invocation, +in your init scripts, always uses the <tt>/etc/s6-rc/compiled</tt> name +to refer to the database to use at boot time. </li> + <li> Make sure <tt>/etc/s6-rc/compiled</tt> is a <em>symbolic link</em> +that points to your current compiled database. For instance, the first +time you compile a service database, you could run: +<pre> + s6-rc-compile /etc/s6-rc/compiled-initial /etc/s6-rc/source-initial + ln -sf compiled-initial /etc/s6-rc/compiled +</pre> </li> + <li> When you compile a new service database, always compile it to a +unique name, preferrably in the same directory as your current compiled +database. You can for instance use a TAI64N timestamp, obtained by +<a href="http://skarnet.org/software/s6-portable-utils/s6-clock.html">s6-clock</a>, +to create such a name: +<pre> + stamp=`s6-clock` + s6-rc-compile /etc/s6-rc/compiled-$stamp /etc/s6-rc/source-current +</pre> </li> + <li> To replace your current compiled database with the one you just +created, run <a href="s6-rc-update.html">s6-rc-update</a> on the new +name: +<pre> + s6-rc-update /etc/s6-rc/compiled-$stamp +</pre> </li> + <li> At this point, your current database is the new one, but if +your machine reboots, it will still use the old one (because +the <tt>/etc/s6-rc/compiled</tt> link is still pointing to the old one). +To ensure that the +new database will be used on the next boot, atomically update the link: +<pre> + olddb=`s6-linkname -f /etc/s6-rc/compiled` + s6-ln -sf compiled-$stamp /etc/s6-rc/compiled +</pre> + The use of the +<a href="http://skarnet.org/software/s6-portable-utils/s6-ln.html">s6-ln</a> +utility is recommended, because not all implementations of the +<a href="http://pubs.opengroup.org/onlinepubs/9699919799/utilities/ln.html">ln</a> +standard actually perform an atomic replacement. (For instance, <tt>ln -sf</tt> +from GNU coreutils first removes the old link, then creates the new one: +this is not atomic and may break your system if you're unlucky and get a +power failure at the wrong time.) s6-ln is guaranteed to do the right thing. +Alternatively, you could run something like <tt>ln -sf compiled-$stamp +/etc/s6-rc/compiled.new ; mv -f /etc/s6-rc/compiled.new +/etc/s6-rc/compiled</tt>, but that's more complex and may also fail with +bad <tt>mv</tt> implementations. </li> + <li> Your current compiled database, stored in +<tt>/etc/s6-rc/compiled-$stamp</tt>, is now accessible via the +<tt>/etc/s6-rc/compiled</tt> symlink and will be used at boot time. You do +not need to modify your init scripts. If you wish, you can dispose of the +old database you just replaced: +<pre> + rm -rf $olddb +</pre> </li> +</ul> + + +<a name="switching"> <h2> Switching from another service manager </h2> +</a> <h3> I have a collection of init scripts in another format, but don't want to wait until the whole collection is converted @@ -315,7 +396,10 @@ state as "down" allows users to apply their chosen policies - see below. Keep it simple, stupid. </p> + +<a name="nopolicy"> <h2> Mechanism vs. policy </h2> +</a> <h3> s6-rc feels bare: there are tools, but no wrappers, no pre-packaged scripts to boot my machines, no default runlevels. By comparison, OpenRC |