源码版本：H版
nova通过nova/virt/driver.py中的ComputeDriver对底层虚拟化技术进行抽象，不同的虚拟化技术在nova/virt下有不同的目录，里面均有driver.py文件，通过继承ComputeDriver类来实现自己的Driver类。nova可以通过对Driver类进行统一接口的调用实现底层虚拟技术的管理。下面具体谈谈nova对libvirt的使用：
一、libvirt基础
参考：http://www.cnblogs.com/littlebugfish/p/4231996.html
二、nova对libvirt的使用架构
说明：其中libvirt-python仅仅是对libvirt的Python绑定，并没有改变功能，对API的改动也不大。
三、nova创建虚拟机时对libvirt的调度
nova/virt/libvirt/driver.py
LibvirtDriver类:
def spawn(self, context, instance, image_meta, injected_files,
admin_password, network_info=None, block_device_info=None):
"""获取disk配置信息"""
disk_info = blockinfo.get_disk_info(CONF.libvirt_type,
instance,
block_device_info,
image_meta)
"""处理镜像，见下文第1节"""
self._create_image(context, instance,
disk_info['mapping'],
network_info=network_info,
block_device_info=block_device_info,
files=injected_files,
admin_pass=admin_password)
"""将当前的参数配置转成创建虚拟机需要用到的xml文件"""
xml = self.to_xml(context, instance, network_info,
disk_info, image_meta,
block_device_info=block_device_info,
write_to_disk=True)
"""调用libvirt创建并启动虚拟机，见下文第2节"""
self._create_domain_and_network(context, xml, instance, network_info,
block_device_info)
...
1、处理镜像
def _create_image(self, context, instance,
disk_mapping, suffix='',
disk_images=None, network_info=None,
block_device_info=None, files=None,
admin_pass=None, inject_files=True):
...
def image(fname, image_type=CONF.libvirt_images_type):
return self.image_backend.image(instance,
fname + suffix, image_type)
...
"""初始化镜像文件引用"""
if not disk_images:
disk_images = {'image_id': instance['image_ref'],
'kernel_id': instance['kernel_id'],
'ramdisk_id': instance['ramdisk_id']}
"""从kernel创建磁盘镜像，一般不采用"""
if disk_images['kernel_id']:
fname = imagecache.get_cache_fname(disk_images, 'kernel_id')
... inst_type = flavors.extract_flavor(instance)
"""从image创建启动磁盘镜像"""
if not booted_from_volume:
root_fname = imagecache.get_cache_fname(disk_images, 'image_id')
size = instance['root_gb'] * 1024 * 1024 * 1024
if size == 0 or suffix == '.rescue':
size = None
image('disk').cache(fetch_func=libvirt_utils.fetch_image,
context=context,
filename=root_fname,
size=size,
image_id=disk_images['image_id'],
user_id=instance['user_id'],
project_id=instance['project_id']) """创建临时磁盘镜像和swap磁盘镜像，过程基本同创建启动磁盘镜像类似"""
... """配置驱动"""
if configdrive.required_by(instance):
LOG.info(_('Using config drive'), instance=instance)
extra_md = {}
... """文件注入"""
elif inject_files and CONF.libvirt_inject_partition != -2:
if booted_from_volume:
LOG.warn(_('File injection into a boot from volume '
'instance is not supported'), instance=instance)
... if CONF.libvirt_type == 'uml':
libvirt_utils.chown(image('disk').path, 'root')
这里重点分析从image创建启动磁盘镜像的过程 。接着分析代码如下：
image('disk').cache(fetch_func=libvirt_utils.fetch_image,
context=context,
filename=root_fname,
size=size,
image_id=disk_images['image_id'],
user_id=instance['user_id'],
project_id=instance['project_id'])首先来看image函数，它本身在_create_image函数中进行定义，如下：
def image(fname, image_type=CONF.libvirt_images_type):
return self.image_backend.image(instance,
fname + suffix, image_type)
主要的作用是根据配置文件在nova.virt.libvirt.imagebackend模块中选择适当对象进行构造并返回，通常为Qcow2对象。
接着来看cache函数。Qcow2类没有定义该函数，但是由于在nova.virt.libvirt.imagebackend模块中Qcow2类继承自Image类，所以自然调用Image类中定义的该函数。两个类的关系如下图所示：
具体代码如下：
nova/virt/libvirt/imagebackend.py
Image类：
def cache(self, fetch_func, filename, size=None, *args, **kwargs):
"""构造获取base镜像的函数"""
@utils.synchronized(filename, external=True, lock_path=self.lock_path)
def call_if_not_exists(target, *args, **kwargs):
if not os.path.exists(target):
fetch_func(target=target, *args, **kwargs)
elif CONF.libvirt_images_type == "lvm" and \
'ephemeral_size' in kwargs:
fetch_func(target=target, *args, **kwargs)
base_dir = os.path.join(CONF.instances_path, CONF.base_dir_name)
if not os.path.exists(base_dir):
fileutils.ensure_tree(base_dir)
base = os.path.join(base_dir, filename)
"""如果不存在镜像或不存在base镜像的话"""
if not self.check_image_exists() or not os.path.exists(base):
self.create_image(call_if_not_exists, base, size,
*args, **kwargs)
...
由于创建虚拟机时建立的启动磁盘镜像为qcow2格式，所以需要base镜像，镜像格式可参考：http://docs.openstack.org/image-guide/content/ch_introduction.html。但此时还没有启动磁盘镜像，所以需要调用Qcow2类的create_image函数进行创建，该函数代码如下：
nova/virt/libvirt/imagebackend.py
Qcow2类：
def create_image(self, prepare_template, base, size, *args, **kwargs):
@utils.synchronized(base, external=True, lock_path=self.lock_path)
def copy_qcow2_image(base, target, size):
libvirt_utils.create_cow_image(base, target)
if size:
disk.extend(target, size, use_cow=True) """没有base镜像的话先下载base镜像"""
if not os.path.exists(base):
prepare_template(target=base, max_size=size, *args, **kwargs)
else:
self.verify_base_size(base, size)
...
"""有base镜像后创建qcow2镜像"""
if not os.path.exists(self.path):
with fileutils.remove_path_on_error(self.path):
copy_qcow2_image(base, self.path, size)
假设此时连base镜像也没有，则需要先下载base镜像。从上面可以看出，prepare_template 函数即为cache 函数中传入的call_if_not_exists 函数，call_if_not_exists 函数里面接着调用fetch_func 函数，而fetch_func在_create_image 函数中已经表明为fetch_func=libvirt_utils.fetch_image，所以继续如下分析：
nova/virt/libvirt/utils.py
def fetch_image(context, target, image_id, user_id, project_id, max_size=0):
"""images为nova/virt/images.py"""
images.fetch_to_raw(context, image_id, target, user_id, project_id,
max_size=max_size)
nova/virt/images.py
def fetch_to_raw(context, image_href, path, user_id, project_id, max_size=0):
path_tmp = "%s.part" % path
fetch(context, image_href, path_tmp, user_id, project_id,
max_size=max_size)
... def fetch(context, image_href, path, _user_id, _project_id, max_size=0):
"""其中glance为nova/image/glance.py模块"""
(image_service, image_id) = glance.get_remote_image_service(context,
image_href)
with fileutils.remove_path_on_error(path):
image_service.download(context, image_id, dst_path=path)
nova/image/glance.py
GlanceImageService类：
def download(self, context, image_id, data=None, dst_path=None):
...
try:
"""使用glanceclient获取镜像数据"""
image_chunks = self._client.call(context, 1, 'data', image_id)
except Exception:
_reraise_translated_image_exception(image_id)
...
接着来看当有了base镜像后，但此时仍然没有启动磁盘镜像，所以需要接着创建启动磁盘镜像，回到create_image函数中进行分析，涉及的代码如下：
nova/virt/libvirt/imagebackend.py
Qcow2类：
def create_image(self, prepare_template, base, size, *args, **kwargs):
@utils.synchronized(base, external=True, lock_path=self.lock_path)
def copy_qcow2_image(base, target, size):
libvirt_utils.create_cow_image(base, target)
if size:
disk.extend(target, size, use_cow=True)
...
"""有base镜像后创建qcow2镜像"""
if not os.path.exists(self.path):
with fileutils.remove_path_on_error(self.path):
copy_qcow2_image(base, self.path, size)
其中主要调用copy_qcow2_image函数，而copy_qcow2_image函数又调用create_cow_image函数，接着分析代码：
nova/virt/libvirt/utils.py
def create_cow_image(backing_file, path, size=None):
"""构建命令创建qcow2镜像，其中设置base镜像为_base目录下相应的镜像,即backing_file"""
base_cmd = ['qemu-img', 'create', '-f', 'qcow2']
cow_opts = []
if backing_file:
cow_opts += ['backing_file=%s' % backing_file]
base_details = images.qemu_img_info(backing_file)
else:
base_details = None
if base_details and base_details.cluster_size is not None:
cow_opts += ['cluster_size=%s' % base_details.cluster_size]
if base_details and base_details.encryption:
cow_opts += ['encryption=%s' % base_details.encryption]
if size is not None:
cow_opts += ['size=%s' % size]
if cow_opts:
csv_opts = ",".join(cow_opts)
cow_opts = ['-o', csv_opts]
cmd = base_cmd + cow_opts + [path]
execute(*cmd)
2、创建并启动虚拟机
继续分析spawn函数中_create_domain_and_network函数的调用，具体代码如下：
nova/virt/libvirt/driver.py
LibvirtDriver类:
def _create_domain_and_network(self, xml, instance, network_info,
block_device_info=None, power_on=True,
context=None, reboot=False):
...
self.plug_vifs(instance, network_info)
self.firewall_driver.setup_basic_filtering(instance, network_info)
self.firewall_driver.prepare_instance_filter(instance, network_info)
domain = self._create_domain(xml, instance=instance, power_on=power_on)
self.firewall_driver.apply_instance_filter(instance, network_info)
return domain def _create_domain(self, xml=None, domain=None,
instance=None, launch_flags=0, power_on=True):
...
if xml:
try:
"""self._conn为libvirt.virConnect，所以这里实际上是调用libvirt定义虚拟机"""
domain = self._conn.defineXML(xml)
except Exception as e:
LOG.error(_("An error occurred while trying to define a domain"
" with xml: %s") % xml)
raise e if power_on:
try:
"""调用libvirt启动虚拟机"""
domain.createWithFlags(launch_flags)
except Exception as e:
with excutils.save_and_reraise_exception():
LOG.error(_("An error occurred while trying to launch a "
"defined domain with xml: %s") %
domain.XMLDesc(0))
...
return domain