分类：libvirt笔记

libvirt库通过以下三个环境变量配置log：

The library configuration of logging is through 3 environment variables allowing to control the logging behaviour:

LIBVIRT_DEBUG: it can take the four following values:

1 or “debug”: asking the library to log every message emitted, though the filters can be used to avoid filling up the output

2 or “info”: log all non-debugging information

3 or “warn”: log warnings and errors, that’s the default value

4 or “error”: log only error messages

LIBVIRTLOGFILTERS: defines logging filters

LIBVIRTLOGOUTPUTS: defines logging outputs

Note that, for example, setting LIBVIRT_DEBUG= is the same as unset. If you specify an invalid value, it will be ignored with a warning. If you have an error in a filter or output string, some of the settings may be applied up to the point at which libvirt encountered the error.

libvirtd daemon程序也有三个类似的配置项（存储在配置文件libvirtd.conf）：

log_level: accepts the following values:

4: only errors

3: warnings and errors

2: information, warnings and errors

1: debug and everything

log_filters: defines logging filters

log_outputs: defines logging outputs

对于libvirtd程序来讲，log配置项的优先级如下：

When starting the libvirt daemon, any logging environment variable settings will override settings in the config file. Command line options take precedence over all. If no outputs are defined for libvirtd, it will try to use

0.10.0 or later: systemd journal, if /run/systemd/journal/socket exists 0.9.0 or later: file /var/log/libvirt/libvirtd.log if running as a daemon before 0.9.0: syslog if running as a daemon all versions: to stderr stream if running in the foreground

参考资料：
Logging in the library and the daemon。

Hypervisor connection是libvirt一个核心概念（以下内容摘自这里）：

A connection is the primary or top level object in the libvirt API and Python libvirt module. An instance of this object is required before attempting to use almost any of the classes or methods. A connection is associated with a particular hypervisor, which may be running locally on the same machine as the libvirt client application, or on a remote machine over the network. In all cases, the connection is represented by an instance of the virConnect class and identified by a URI. The URI scheme and path defines the hypervisor to connect to, while the host part of the URI determines where it is located.

An application is permitted to open multiple connections at the same time, even when using more than one type of hypervisor on a single machine. For example, a host may provide both KVM full machine virtualization and LXC container virtualization. A connection object may be used concurrently across multiple threads. Once a connection has been established, it is possible to obtain handles to other managed objects or create new managed objects.

以下代码测试Xen连接：

#!/usr/bin/python
from __future__ import print_function
import sys
import libvirt

conn = libvirt.open('xen:///')
if conn == None:
        print('Failed to open connection to xen:///', file=sys.stderr)
        exit(1)
else:
        print('Open connection success', file=sys.stdout)
        conn.close()
        exit(0)

Now we can define the goal of libvirt: to provide a common and stable layer sufficient to securely manage domains on a node, possibly remote.

在libvirt中，node即指物理机器，domain可以理解为虚拟机。