mirror of
https://gitlab.com/libvirt/libvirt.git
synced 2024-07-06 18:05:45 +00:00
f2f9742d4d
The rule generating the HTML docs passing the --html flag to xsltproc. This makes it use the legacy HTML parser, which either ignores or tries to fix all sorts of broken XML tags. There's no reason why we should be writing broken XML in the first place, so removing --html and adding the XHTML doctype to all files forces us to create good XML. This adds the XHTML doc type and fixes many, many XML tag problems it exposes. Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
254 lines
13 KiB
XML
254 lines
13 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
|
|
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
|
|
<html xmlns="http://www.w3.org/1999/xhtml">
|
|
<body>
|
|
<h1>Hooks for specific system management</h1>
|
|
|
|
<ul id="toc"></ul>
|
|
|
|
<h2><a name="intro">Custom event scripts</a></h2>
|
|
<p>Beginning with libvirt 0.8.0, specific events on a host system will
|
|
trigger custom scripts.</p>
|
|
<p>These custom <b>hook</b> scripts are executed when any of the following
|
|
actions occur:</p>
|
|
<ul>
|
|
<li>The libvirt daemon starts, stops, or reloads its
|
|
configuration<br/><br/></li>
|
|
<li>A QEMU guest is started or stopped<br/><br/></li>
|
|
<li>An LXC guest is started or stopped<br/><br/></li>
|
|
</ul>
|
|
|
|
<h2><a name="location">Script location</a></h2>
|
|
<p>The libvirt hook scripts are located in the directory
|
|
<code>$SYSCONFDIR/libvirt/hooks/</code>.</p>
|
|
<ul>
|
|
<li>In Linux distributions such as Fedora and RHEL, this is
|
|
<code>/etc/libvirt/hooks/</code>. Other Linux distributions may do
|
|
this differently.</li>
|
|
<li>If your installation of libvirt has instead been compiled from
|
|
source, it is likely to be
|
|
<code>/usr/local/etc/libvirt/hooks/</code>.</li>
|
|
</ul>
|
|
<p>To use hook scripts, you will need to create this <code>hooks</code>
|
|
directory manually, place the desired hook scripts inside, then make
|
|
them executable.</p>
|
|
<br/>
|
|
|
|
<h2><a name="names">Script names</a></h2>
|
|
<p>At present, there are three hook scripts that can be called:</p>
|
|
<ul>
|
|
<li><code>/etc/libvirt/hooks/daemon</code><br/><br/>
|
|
Executed when the libvirt daemon is started, stopped, or reloads
|
|
its configuration<br/><br/></li>
|
|
<li><code>/etc/libvirt/hooks/qemu</code><br/><br/>
|
|
Executed when a QEMU guest is started, stopped, or migrated<br/><br/></li>
|
|
<li><code>/etc/libvirt/hooks/lxc</code><br /><br/>
|
|
Executed when an LXC guest is started or stopped</li>
|
|
</ul>
|
|
<br/>
|
|
|
|
<h2><a name="structure">Script structure</a></h2>
|
|
<p>The hook scripts are executed using standard Linux process creation
|
|
functions. Therefore, they must begin with the declaration of the
|
|
command interpreter to use.</p>
|
|
<p>For example:</p>
|
|
<pre>#!/bin/bash</pre>
|
|
<p>or:</p>
|
|
<pre>#!/usr/bin/python</pre>
|
|
<p>Other command interpreters are equally valid, as is any executable
|
|
binary, so you are welcome to use your favourite languages.</p>
|
|
<br/>
|
|
|
|
<h2><a name="arguments">Script arguments</a></h2>
|
|
<p>The hook scripts are called with specific command line arguments,
|
|
depending upon the script, and the operation being performed.</p>
|
|
<p>The guest hook scripts, qemu and lxc, are also given the <b>full</b>
|
|
XML description for the domain on their stdin. This includes items
|
|
such the UUID of the domain and its storage information, and is
|
|
intended to provide all the libvirt information the script needs.</p>
|
|
|
|
<p>The command line arguments take this approach:</p>
|
|
<ol>
|
|
<li>The first argument is the name of the <b>object</b> involved in the
|
|
operation, or '-' if there is none.<br/><br/>
|
|
For example, the name of a guest being started.<br/><br/></li>
|
|
<li>The second argument is the name of the <b>operation</b> being
|
|
performed.<br/><br/>
|
|
For example, "start" if a guest is being started.<br/><br/></li>
|
|
<li>The third argument is a <b>sub-operation</b> indication, or '-' if there
|
|
is none.<br/><br/></li>
|
|
<li>The last argument is an <b>extra argument</b> string, or '-' if there is
|
|
none.</li>
|
|
</ol>
|
|
|
|
<h4><a name="arguments_specifics">Specifics</a></h4>
|
|
<p>This translates to the following specifics for each hook script:</p>
|
|
|
|
<h5><a name="daemon">/etc/libvirt/hooks/daemon</a></h5>
|
|
<ul>
|
|
<li>When the libvirt daemon is started, this script is called as:<br/>
|
|
<pre>/etc/libvirt/hooks/daemon - start - start</pre></li>
|
|
<li>When the libvirt daemon is shut down, this script is called as:<br/>
|
|
<pre>/etc/libvirt/hooks/daemon - shutdown - shutdown</pre></li>
|
|
<li>When the libvirt daemon receives the SIGHUP signal, it reloads its
|
|
configuration and triggers the hook script as:<br/>
|
|
<pre>/etc/libvirt/hooks/daemon - reload begin SIGHUP</pre></li>
|
|
</ul>
|
|
<p>Please note that when the libvirt daemon is restarted, the <i>daemon</i>
|
|
hook script is called once with the "shutdown" operation, and then once
|
|
with the "start" operation. There is no specific operation to indicate
|
|
a "restart" is occurring.</p>
|
|
|
|
<h5><a name="qemu">/etc/libvirt/hooks/qemu</a></h5>
|
|
<ul>
|
|
<li>Before a QEMU guest is started, the qemu hook script is
|
|
called in three locations; if any location fails, the guest
|
|
is not started. The first location, <span class="since">since
|
|
0.9.0</span>, is before libvirt performs any resource
|
|
labeling, and the hook can allocate resources not managed by
|
|
libvirt such as DRBD or missing bridges. This is called as:<br/>
|
|
<pre>/etc/libvirt/hooks/qemu guest_name prepare begin -</pre>
|
|
The second location, available <span class="since">Since
|
|
0.8.0</span>, occurs after libvirt has finished labeling
|
|
all resources, but has not yet started the guest, called as:<br/>
|
|
<pre>/etc/libvirt/hooks/qemu guest_name start begin -</pre>
|
|
The third location, <span class="since">0.9.13</span>,
|
|
occurs after the QEMU process has successfully started up:<br/>
|
|
<pre>/etc/libvirt/hooks/qemu guest_name started begin -</pre>
|
|
</li>
|
|
<li>When a QEMU guest is stopped, the qemu hook script is called
|
|
in two locations, to match the startup.
|
|
First, <span class="since">since 0.8.0</span>, the hook is
|
|
called before libvirt restores any labels:<br/>
|
|
<pre>/etc/libvirt/hooks/qemu guest_name stopped end -</pre>
|
|
Then, after libvirt has released all resources, the hook is
|
|
called again, <span class="since">since 0.9.0</span>, to allow
|
|
any additional resource cleanup:<br/>
|
|
<pre>/etc/libvirt/hooks/qemu guest_name release end -</pre></li>
|
|
<li><span class="since">Since 0.9.11</span>, the qemu hook script
|
|
is also called at the beginning of incoming migration. It is called
|
|
as: <pre>/etc/libvirt/hooks/qemu guest_name migrate begin -</pre>
|
|
with domain XML sent to standard input of the script. In this case,
|
|
the script acts as a filter and is supposed to modify the domain
|
|
XML and print it out on its standard output. Empty output is
|
|
identical to copying the input XML without changing it. In case the
|
|
script returns failure or the output XML is not valid, incoming
|
|
migration will be canceled. This hook may be used, e.g., to change
|
|
location of disk images for incoming domains.</li>
|
|
<li><span class="since">Since 0.9.13</span>, the qemu hook script
|
|
is also called when the libvirtd daemon restarts and reconnects
|
|
to previously running QEMU processes. If the script fails, the
|
|
existing QEMU process will be killed off. It is called as:
|
|
<pre>/etc/libvirt/hooks/qemu guest_name reconnect begin -</pre>
|
|
</li>
|
|
<li><span class="since">Since 0.9.13</span>, the qemu hook script
|
|
is also called when the QEMU driver is told to attach to an
|
|
externally launched QEMU process. It is called as:
|
|
<pre>/etc/libvirt/hooks/qemu guest_name attach begin -</pre>
|
|
</li>
|
|
</ul>
|
|
|
|
<h5><a name="lxc">/etc/libvirt/hooks/lxc</a></h5>
|
|
<ul>
|
|
<li>Before a LXC guest is started, the lxc hook script is
|
|
called in three locations; if any location fails, the guest
|
|
is not started. The first location, <span class="since">since
|
|
0.9.13</span>, is before libvirt performs any resource
|
|
labeling, and the hook can allocate resources not managed by
|
|
libvirt such as DRBD or missing bridges. This is called as:<br/>
|
|
<pre>/etc/libvirt/hooks/lxc guest_name prepare begin -</pre>
|
|
The second location, available <span class="since">Since
|
|
0.8.0</span>, occurs after libvirt has finished labeling
|
|
all resources, but has not yet started the guest, called as:<br/>
|
|
<pre>/etc/libvirt/hooks/lxc guest_name start begin -</pre>
|
|
The third location, <span class="since">0.9.13</span>,
|
|
occurs after the LXC process has successfully started up:<br/>
|
|
<pre>/etc/libvirt/hooks/lxc guest_name started begin -</pre>
|
|
</li>
|
|
<li>When a LXC guest is stopped, the lxc hook script is called
|
|
in two locations, to match the startup.
|
|
First, <span class="since">since 0.8.0</span>, the hook is
|
|
called before libvirt restores any labels:<br/>
|
|
<pre>/etc/libvirt/hooks/lxc guest_name stopped end -</pre>
|
|
Then, after libvirt has released all resources, the hook is
|
|
called again, <span class="since">since 0.9.0</span>, to allow
|
|
any additional resource cleanup:<br/>
|
|
<pre>/etc/libvirt/hooks/lxc guest_name release end -</pre></li>
|
|
<li><span class="since">Since 0.9.13</span>, the lxc hook script
|
|
is also called when the libvirtd daemon restarts and reconnects
|
|
to previously running LXC processes. If the script fails, the
|
|
existing LXC process will be killed off. It is called as:
|
|
<pre>/etc/libvirt/hooks/lxc guest_name reconnect begin -</pre>
|
|
</li>
|
|
</ul>
|
|
<br/>
|
|
|
|
<h2><a name="execution">Script execution</a></h2>
|
|
<ul>
|
|
<li>The "start" operation for the guest hook scripts, qemu and lxc,
|
|
executes <b>prior</b> to the guest being created. This allows the
|
|
guest start operation to be aborted if the script returns indicating
|
|
failure.<br/><br/></li>
|
|
<li>The "shutdown" operation for the guest hook scripts, qemu and lxc,
|
|
executes <b>after</b> the guest has stopped. If the hook script
|
|
indicates failure in its return, the shut down of the guest cannot
|
|
be aborted because it has already been performed.<br/><br/></li>
|
|
<li>Hook scripts execute in a synchronous fashion. Libvirt waits
|
|
for them to return before continuing the given operation.<br/><br/>
|
|
This is most noticeable with the guest start operation, as a lengthy
|
|
operation in the hook script can mean an extended wait for the guest
|
|
to be available to end users.<br/><br/></li>
|
|
<li>For a hook script to be utilised, it must have its execute bit set
|
|
(ie. chmod o+rx <i>qemu</i>), and must be present when the libvirt
|
|
daemon is started.<br/><br/></li>
|
|
<li>If a hook script is added to a host after the libvirt daemon is
|
|
already running, it won't be used until the libvirt daemon
|
|
next starts.</li>
|
|
</ul>
|
|
<br/>
|
|
|
|
<h2><a name="qemu_migration">QEMU guest migration</a></h2>
|
|
<p>Migration of a QEMU guest involves running hook scripts on both the
|
|
source and destination hosts:</p>
|
|
<ol>
|
|
<li>At the beginning of the migration, the <i>qemu</i> hook script on
|
|
the <b>destination</b> host is executed with the "migrate"
|
|
operation.</li>
|
|
<li>Before QEMU process is spawned, the two operations ("prepare" and
|
|
"start") called for domain start are executed on
|
|
<b>destination</b> host.</li>
|
|
<li>If both of these hook script executions exit successfully (exit
|
|
status 0), the migration continues. Any other exit code indicates
|
|
failure, and the migration is aborted.</li>
|
|
<li>The QEMU guest is then migrated to the destination host.</li>
|
|
<li>Unless an error occurs during the migration process, the <i>qemu</i>
|
|
hook script on the <b>source</b> host is then executed with the
|
|
"stopped" and "release" operations to indicate it is no longer
|
|
running on this host. Regardless of the return codes, the
|
|
migration is not aborted as it has already been performed.</li>
|
|
</ol>
|
|
<br/>
|
|
|
|
<h2><a name="recursive">Calling libvirt functions from within a hook script</a></h2>
|
|
<p><b>DO NOT DO THIS!</b></p>
|
|
<p>A hook script must not call back into libvirt, as the libvirt daemon
|
|
is already waiting for the script to exit.</p>
|
|
<p>A deadlock is likely to occur.</p>
|
|
<br/>
|
|
|
|
<h2><a name="return_codes">Return codes and logging</a></h2>
|
|
<p>If a hook script returns with an exit code of 0, the libvirt daemon
|
|
regards this as successful and performs no logging of it.</p>
|
|
<p>However, if a hook script returns with a non zero exit code, the libvirt
|
|
daemon regards this as a failure, logs its return code, and
|
|
additionally logs anything on stderr the hook script returns.</p>
|
|
<p>For example, a hook script might use this code to indicate failure,
|
|
and send a text string to stderr:</p>
|
|
<pre>echo "Could not find required XYZZY" >&2
|
|
exit 1</pre>
|
|
<p>The resulting entry in the libvirt log will appear as:</p>
|
|
<pre>20:02:40.297: error : virHookCall:285 : Hook script execution failed: internal error Child process (LC_ALL=C PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
|
|
HOME=/root USER=root LOGNAME=root /etc/libvirt/hooks/qemu qemu prepare begin -) unexpected exit status 1: Could not find required XYZZY</pre>
|
|
</body>
|
|
</html>
|