mirror of
https://gitlab.com/libvirt/libvirt.git
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8ec0e9a800
Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: Erik Skultety <eskultet@redhat.com>
519 lines
18 KiB
ReStructuredText
519 lines
18 KiB
ReStructuredText
.. role:: since
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====================================
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Hooks for specific system management
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====================================
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.. contents::
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Custom event scripts
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--------------------
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Beginning with libvirt 0.8.0, specific events on a host system will trigger
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custom scripts.
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These custom **hook** scripts are executed when any of the following actions
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occur:
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- The libvirt daemon starts, stops, or reloads its configuration (
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:since:`since 0.8.0` )
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- A QEMU guest is started or stopped ( :since:`since 0.8.0` )
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- An LXC guest is started or stopped ( :since:`since 0.8.0` )
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- A libxl-handled Xen guest is started or stopped ( :since:`since 2.1.0` )
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- A network is started or stopped or an interface is plugged/unplugged to/from
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the network ( :since:`since 1.2.2` )
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Script location
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---------------
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The libvirt hook scripts are located in the directory
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``$SYSCONFDIR/libvirt/hooks/``.
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- In Linux distributions such as Fedora and RHEL, this is
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``/etc/libvirt/hooks/``. Other Linux distributions may do this differently.
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- If your installation of libvirt has instead been compiled from source, it is
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likely to be ``/usr/local/etc/libvirt/hooks/``.
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- :since:`Since 6.5.0` , you can also place several hook scripts in the
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directories ``/etc/libvirt/hooks/<driver>.d/``.
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To use hook scripts, you will need to create this ``hooks`` directory manually,
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place the desired hook scripts inside, then make them executable.
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Script names
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------------
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At present, there are five hook scripts that can be called:
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- ``/etc/libvirt/hooks/daemon``
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Executed when the libvirt daemon is started, stopped, or reloads its
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configuration
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- ``/etc/libvirt/hooks/qemu``
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Executed when a QEMU guest is started, stopped, or migrated
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- ``/etc/libvirt/hooks/lxc``
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Executed when an LXC guest is started or stopped
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- ``/etc/libvirt/hooks/libxl``
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Executed when a libxl-handled Xen guest is started, stopped, or migrated
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- ``/etc/libvirt/hooks/network``
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Executed when a network is started or stopped or an interface is
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plugged/unplugged to/from the network
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:since:`Since 6.5.0` , you can also have several scripts with any name in the
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directories ``/etc/libvirt/hooks/<driver>.d/``. They are executed in
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alphabetical order after main script.
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Script structure
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----------------
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The hook scripts are executed using standard Linux process creation functions.
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Therefore, they must begin with the declaration of the command interpreter to
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use.
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For example:
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::
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#!/bin/bash
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or:
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::
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#!/usr/bin/python
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Other command interpreters are equally valid, as is any executable binary, so
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you are welcome to use your favourite languages.
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Script arguments
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----------------
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The hook scripts are called with specific command line arguments, depending upon
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the script, and the operation being performed.
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The guest hook scripts, qemu and lxc, are also given the **full** XML
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description for the domain on their stdin. This includes items such the UUID of
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the domain and its storage information, and is intended to provide all the
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libvirt information the script needs.
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For all cases, stdin of the network hook script is provided with the full XML
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description of the network status in the following form:
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::
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<hookData>
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<network>
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<name>$network_name</name>
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<uuid>afca425a-2c3a-420c-b2fb-dd7b4950d722</uuid>
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...
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</network>
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</hookData>
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In the case of an network port being created / deleted, the network XML will be
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followed with the full XML description of the port:
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::
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<hookData>
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<network>
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<name>$network_name</name>
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<uuid>afca425a-2c3a-420c-b2fb-dd7b4950d722</uuid>
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...
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</network>
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<networkport>
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<uuid>5d744f21-ba4a-4d6e-bdb2-30a35ff3207d</uuid>
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...
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<plug type='direct' dev='ens3' mode='vepa'/>
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</networkport>
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</hookData>
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Please note that this approach is different from other cases such as ``daemon``,
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``qemu`` or ``lxc`` hook scripts, because two XMLs may be passed here, while in
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the other cases only a single XML is passed.
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The command line arguments take this approach:
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#. The first argument is the name of the **object** involved in the operation,
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or '-' if there is none.
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For example, the name of a guest being started.
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#. The second argument is the name of the **operation** being performed.
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For example, "start" if a guest is being started.
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#. The third argument is a **sub-operation** indication, or '-' if there is
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none.
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#. The last argument is an **extra argument** string, or '-' if there is none.
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Specifics
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~~~~~~~~~
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This translates to the following specifics for each hook script:
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/etc/libvirt/hooks/daemon
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^^^^^^^^^^^^^^^^^^^^^^^^^
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- | When the libvirt daemon is started, this script is called as:
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::
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/etc/libvirt/hooks/daemon - start - start
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- | When the libvirt daemon is shut down, this script is called as:
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::
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/etc/libvirt/hooks/daemon - shutdown - shutdown
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- | When the libvirt daemon receives the SIGHUP signal, it reloads its
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configuration and triggers the hook script as:
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::
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/etc/libvirt/hooks/daemon - reload begin SIGHUP
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Please note that when the libvirt daemon is restarted, the *daemon* hook script
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is called once with the "shutdown" operation, and then once with the "start"
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operation. There is no specific operation to indicate a "restart" is occurring.
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/etc/libvirt/hooks/qemu
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^^^^^^^^^^^^^^^^^^^^^^^
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- | Before a QEMU guest is started, the qemu hook script is called in three
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locations; if any location fails, the guest is not started. The first
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location, :since:`since 0.9.0` , is before libvirt performs any resource
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labeling, and the hook can allocate resources not managed by libvirt such
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as DRBD or missing bridges. This is called as:
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::
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/etc/libvirt/hooks/qemu guest_name prepare begin -
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has finished labeling all resources, but has not yet started the guest,
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called as:
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::
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/etc/libvirt/hooks/qemu guest_name start begin -
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successfully started up:
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::
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/etc/libvirt/hooks/qemu guest_name started begin -
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- | When a QEMU guest is stopped, the qemu hook script is called in two
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locations, to match the startup. First, :since:`since 0.8.0` , the hook is
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called before libvirt restores any labels:
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::
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/etc/libvirt/hooks/qemu guest_name stopped end -
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:since:`since 0.9.0` , to allow any additional resource cleanup:
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::
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/etc/libvirt/hooks/qemu guest_name release end -
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- :since:`Since 0.9.11` , the qemu hook script is also called at the beginning
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of incoming migration. It is called as:
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::
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/etc/libvirt/hooks/qemu guest_name migrate begin -
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with domain XML sent to standard input of the script. In this case, the
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script acts as a filter and is supposed to modify the domain XML and print it
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out on its standard output. Empty output is identical to copying the input
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XML without changing it. In case the script returns failure or the output XML
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is not valid, incoming migration will be canceled. This hook may be used,
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e.g., to change location of disk images for incoming domains.
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- :since:`Since 1.2.9` , the qemu hook script is also called when restoring a
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saved image either via the API or automatically when restoring a managed save
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machine. It is called as:
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::
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/etc/libvirt/hooks/qemu guest_name restore begin -
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with domain XML sent to standard input of the script. In this case, the
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script acts as a filter and is supposed to modify the domain XML and print it
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out on its standard output. Empty output is identical to copying the input
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XML without changing it. In case the script returns failure or the output XML
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is not valid, restore of the image will be aborted. This hook may be used,
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e.g., to change location of disk images for restored domains.
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- :since:`Since 6.5.0` , you can also place several hook scripts in the
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directory ``/etc/libvirt/hooks/qemu.d/``. They are executed in alphabetical
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order after main script. In this case each script also acts as filter and can
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modify the domain XML and print it out on its standard output. This script
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output is passed to standard input next script in order. Empty output from
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any script is also identical to copying the input XML without changing it. In
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case any script returns failure common process will be aborted, but all
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scripts from the directory will are executed.
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- :since:`Since 0.9.13` , the qemu hook script is also called when the libvirtd
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daemon restarts and reconnects to previously running QEMU processes. If the
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script fails, the existing QEMU process will be killed off. It is called as:
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::
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/etc/libvirt/hooks/qemu guest_name reconnect begin -
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- :since:`Since 0.9.13` , the qemu hook script is also called when the QEMU
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driver is told to attach to an externally launched QEMU process. It is called
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as:
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::
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/etc/libvirt/hooks/qemu guest_name attach begin -
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/etc/libvirt/hooks/lxc
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^^^^^^^^^^^^^^^^^^^^^^
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- | Before a LXC guest is started, the lxc hook script is called in three
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locations; if any location fails, the guest is not started. The first
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location, :since:`since 0.9.13` , is before libvirt performs any resource
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labeling, and the hook can allocate resources not managed by libvirt such
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as DRBD or missing bridges. This is called as:
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::
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/etc/libvirt/hooks/lxc guest_name prepare begin -
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| The second location, available :since:`Since 0.8.0` , occurs after libvirt
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has finished labeling all resources, but has not yet started the guest,
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called as:
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::
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/etc/libvirt/hooks/lxc guest_name start begin -
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| The third location, :since:`0.9.13` , occurs after the LXC process has
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successfully started up:
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::
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/etc/libvirt/hooks/lxc guest_name started begin -
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- | When a LXC guest is stopped, the lxc hook script is called in two
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locations, to match the startup. First, :since:`since 0.8.0` , the hook is
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called before libvirt restores any labels:
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::
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/etc/libvirt/hooks/lxc guest_name stopped end -
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| Then, after libvirt has released all resources, the hook is called again,
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:since:`since 0.9.0` , to allow any additional resource cleanup:
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::
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/etc/libvirt/hooks/lxc guest_name release end -
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- :since:`Since 0.9.13` , the lxc hook script is also called when the libvirtd
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daemon restarts and reconnects to previously running LXC processes. If the
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script fails, the existing LXC process will be killed off. It is called as:
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::
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/etc/libvirt/hooks/lxc guest_name reconnect begin -
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/etc/libvirt/hooks/libxl
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^^^^^^^^^^^^^^^^^^^^^^^^
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- | Before a Xen guest is started using libxl driver, the libxl hook script is
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called in three locations; if any location fails, the guest is not started.
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The first location, :since:`since 2.1.0` , is before libvirt performs any
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resource labeling, and the hook can allocate resources not managed by
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libvirt. This is called as:
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::
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/etc/libvirt/hooks/libxl guest_name prepare begin -
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| The second location, available :since:`Since 2.1.0` , occurs after libvirt
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has finished labeling all resources, but has not yet started the guest,
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called as:
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::
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/etc/libvirt/hooks/libxl guest_name start begin -
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| The third location, :since:`2.1.0` , occurs after the domain has
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successfully started up:
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::
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/etc/libvirt/hooks/libxl guest_name started begin -
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- | When a libxl-handled Xen guest is stopped, the libxl hook script is called
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in two locations, to match the startup. First, :since:`since 2.1.0` , the
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hook is called before libvirt restores any labels:
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::
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/etc/libvirt/hooks/libxl guest_name stopped end -
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| Then, after libvirt has released all resources, the hook is called again,
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:since:`since 2.1.0` , to allow any additional resource cleanup:
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::
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/etc/libvirt/hooks/libxl guest_name release end -
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- :since:`Since 2.1.0` , the libxl hook script is also called at the beginning
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of incoming migration. It is called as:
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::
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/etc/libvirt/hooks/libxl guest_name migrate begin -
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with domain XML sent to standard input of the script. In this case, the
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script acts as a filter and is supposed to modify the domain XML and print it
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out on its standard output. Empty output is identical to copying the input
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XML without changing it. In case the script returns failure or the output XML
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is not valid, incoming migration will be canceled. This hook may be used,
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e.g., to change location of disk images for incoming domains.
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- :since:`Since 6.5.0` , you can also place several hook scripts in the
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directory ``/etc/libvirt/hooks/libxl.d/``. They are executed in alphabetical
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order after main script. In this case each script also acts as filter and can
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modify the domain XML and print it out on its standard output. This script
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output is passed to standard input next script in order. Empty output from
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any script is also identical to copying the input XML without changing it. In
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case any script returns failure common process will be aborted, but all
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scripts from the directory will are executed.
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- :since:`Since 2.1.0` , the libxl hook script is also called when the libvirtd
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daemon restarts and reconnects to previously running Xen domains. If the
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script fails, the existing Xen domains will be killed off. It is called as:
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::
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/etc/libvirt/hooks/libxl guest_name reconnect begin -
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/etc/libvirt/hooks/network
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^^^^^^^^^^^^^^^^^^^^^^^^^^
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- | :since:`Since 1.2.2` , before a network is started, this script is called
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as:
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::
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/etc/libvirt/hooks/network network_name start begin -
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- | After the network is started, up & running, the script is called as:
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::
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/etc/libvirt/hooks/network network_name started begin -
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- | When a network is shut down, this script is called as:
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::
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/etc/libvirt/hooks/network network_name stopped end -
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- | Later, when network is started and there's an interface from a domain to be
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plugged into the network, the hook script is called as:
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::
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/etc/libvirt/hooks/network network_name port-created begin -
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Please note, that in this case, the script is passed both network and port
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XMLs on its stdin.
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- | When network is updated, the hook script is called as:
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::
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/etc/libvirt/hooks/network network_name updated begin -
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- | When the domain from previous case is shutting down, the interface is
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unplugged. This leads to another script invocation:
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::
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/etc/libvirt/hooks/network network_name port-deleted begin -
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And again, as in previous case, both network and port XMLs are passed onto
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script's stdin.
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Script execution
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----------------
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- The "start" operation for the guest and network hook scripts, executes
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**prior** to the object (guest or network) being created. This allows the
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object start operation to be aborted if the script returns indicating
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failure.
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- The "stopped" operation for the guest and network hook scripts, executes
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**after** the object (guest or network) has stopped. If the hook script
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indicates failure in its return, the shut down of the object cannot be
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aborted because it has already been performed.
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- Hook scripts execute in a synchronous fashion. Libvirt waits for them to
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return before continuing the given operation.
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This is most noticeable with the guest or network start operation, as a
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lengthy operation in the hook script can mean an extended wait for the guest
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or network to be available to end users.
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- For a hook script to be utilised, it must have its execute bit set (e.g.
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chmod o+rx *qemu*), and must be present when the libvirt daemon is started.
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- If a hook script is added to a host after the libvirt daemon is already
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running, it won't be used until the libvirt daemon next starts.
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QEMU guest migration
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--------------------
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Migration of a QEMU guest involves running hook scripts on both the source and
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destination hosts:
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#. At the beginning of the migration, the *qemu* hook script on the
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**destination** host is executed with the "migrate" operation.
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#. Before QEMU process is spawned, the two operations ("prepare" and "start")
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called for domain start are executed on **destination** host.
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#. If both of these hook script executions exit successfully (exit status 0),
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the migration continues. Any other exit code indicates failure, and the
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migration is aborted.
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#. The QEMU guest is then migrated to the destination host.
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#. Unless an error occurs during the migration process, the *qemu* hook script
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on the **source** host is then executed with the "stopped" and "release"
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operations to indicate it is no longer running on this host. Regardless of
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the return codes, the migration is not aborted as it has already been
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performed.
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Calling libvirt functions from within a hook script
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---------------------------------------------------
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**DO NOT DO THIS!**
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A hook script must not call back into libvirt, as the libvirt daemon is already
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waiting for the script to exit.
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A deadlock is likely to occur.
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Return codes and logging
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------------------------
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If a hook script returns with an exit code of 0, the libvirt daemon regards this
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as successful and performs no logging of it.
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However, if a hook script returns with a non zero exit code, the libvirt daemon
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regards this as a failure, logs its return code, and additionally logs anything
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on stderr the hook script returns.
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For example, a hook script might use this code to indicate failure, and send a
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text string to stderr:
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::
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echo "Could not find required XYZZY" >&2
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exit 1
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The resulting entry in the libvirt log will appear as:
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::
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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
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HOME=/root USER=root LOGNAME=root /etc/libvirt/hooks/qemu qemu prepare begin -) unexpected exit status 1: Could not find required XYZZY
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