nova-virt与libvirt
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
