[libvirt] [PATCH] docs: significant expansion of custom hook script information
Justin Clift
jclift at redhat.com
Thu Aug 5 16:41:59 UTC 2010
---
Also available for easy review online here for a while:
http://justinclift.fedorapeople.org/libvirt_docs/hooks.html
docs/hooks.html.in | 226 +++++++++++++++++++++++++++++++++++++--------------
1 files changed, 164 insertions(+), 62 deletions(-)
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<h1>Hooks for specific system management</h1>
- <p>Libvirt includes synchronous hooks, starting from version 0.8.0, as a
- way to tie in specific tailored system actions at a specific time.
- If these scripts are present on the host where the hypervisor
- is running, then they are called when the libvirt daemon is doingi
- some significant action.</p>
- <p>The scripts are expected to execute quickly, return a zero exit
- status if all conditions are set for the daemon to continue the
- action (non zero will be considered a failure which may
- be ignored but in general will stop the ongoing operation).
- The script also should not call back into libvirt as the daemon
- is waiting for the script exit and deadlock is likely to occur.</p>
- <p>The scripts are stored in the directory <code>/etc/libvirt/hooks/</code>
- when using a standard installation path
- (<code>$SYSCONF_DIR/libvirt/hooks/</code> in general).</p>
- <p>Each script is given the following command line arguments:</p>
- <ul>
- <li> the first argument is the name of the object involved in the
- operation, or '-' if there is none.
- <li> the second argument is the name of the operation.
- <li> the third argument is a suboperation indication like 'start' or
- 'end', or '-' if there is none.
- <li> the last argument is an extra argument string or '-' if there
- is none.
- </ul>
- <p>There are currently scripts for 3 domains of operation:
+
+ <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><p><code>/etc/libvirt/hooks/daemon</code> script if
- present is called at 3 points in time:</p>
- <p>at daemon startup, typically started with the following
- arguments:</p>
- <pre>/etc/libvirt/hooks/daemon - start - start</pre>
- <p>at daemon shutdown when it is about to exit, with the following
- arguments:</p>
- <pre>/etc/libvirt/hooks/daemon - shutdown - shutdown</pre>
- <p>When the daemon is asked to reload its driver state when
- receiving the SIGHUP signal, arguments are:</p>
- <pre>/etc/libvirt/hooks/daemon - reload begin SIGHUP</pre>
- </li>
- <li><p><code>/etc/libvirt/hooks/qemu</code> script and <br/>
- <code>/etc/libvirt/hooks/lxc</code> associate hooks for domain
- operation on the respective QEmu/KVM and LXC drivers.</p>
- <p> The domain related hooks also receive the full XML description
- for the concerned domain on their stdin, which allows them to get
- all the information from the domain, including UUID or storage
- if that is needed for the script operation.</p>
- <p> Currently only domain startup and domain end operations
- involve the hook, the first one just before the domain gets
- created.
- For example if starting a QEmu domain named <code>test</code>
- the following script will get called:</p>
- <pre>/etc/libvirt/hooks/qemu test start begin -</pre>
- <p> note that a non-zero return value from the script will abort the
- domain startup operation, and if an error string is passed on
- stderr by the hook script, it will be provided back to the user
- at the libvirt API level.</p>
- <p> For domain shutdown, the script will be called just after the
- domain has finished execution, and the script will get:</p>
- <pre>/etc/libvirt/hooks/qemu test stopped end -</pre>
- <p> It is expected that other operations will be associated to hooks
- but at the time of 0.8.0 only those 2 are associated to the
- domains life cycle</p>
- </li>
+ <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>
- <p></p>
+
+ <h2><a name="location">Script Location</a></h2>
+ <p>The libvirt hook scripts are located in the directory
+ <code>$SYSCONF_DIR/etc/libvirt/hooks/</code>.
+ <p>With Fedora and RHEL, this is <code>/etc/libvirt/hooks/</code>.
+ On other Linux distributions this may be different.</p>
+ <br/>
+
+ <h2><a name="names">Script names</a></h2>
+ <p>At present, there are three hook scripts:</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>
+
+ <h5><a name="qemu">/etc/libvirt/hooks/qemu</a></h5>
+ <ul>
+ <li>When a QEMU guest is started, the qemu hook script is called as:<br/>
+ <pre>/etc/libvirt/hooks/qemu guest_name start begin -</pre></li>
+ <li>When a QEMU guest is stopped, the qemu hook script is called
+ as:<br/>
+ <pre>/etc/libvirt/hooks/qemu guest_name stopped end -</pre></li>
+ </ul>
+
+ <h5><a name="lxc">/etc/libvirt/hooks/lxc</a></h5>
+ <ul>
+ <li>When an LXC guest is started, the lxc hook script is called as:<br/>
+ <pre>/etc/libvirt/hooks/lxc guest_name start begin -</pre></li>
+ <li>When a LXC guest is stopped, the lxc hook script is called
+ as:<br/>
+ <pre>/etc/libvirt/hooks/lxc guest_name stopped end -</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 have it's execute bit set
+ (ie. chmod +x <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 the 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 "start"
+ operation.<br/><br/></li>
+ <li>If this hook script returns indicating success (error code 0), the
+ migration continues. Any other return code indicates failure, and
+ the migration is aborted.<br/><br/></li>
+ <li>The QEMU guest is then migrated to the destination host.<br/>
+ <br/></li>
+ <li>Unless an error occurs during the migration process, the <i>qemu</i>
+ hook script on the source host is then executed with the "stopped"
+ operation, to indicate it is no longer running on this
+ host.<br/><br/>
+ Regardless of the return code from this hook script, 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 it with return code 256, 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:416 : Hook script execution failed: Hook script /etc/libvirt/hooks/qemu qemu failed with error code 256:Could not find required XYZZY</pre>
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