OpenStack 创建快照(Create Snapshot)过程源码分析
1. 背景：
一直以为OpenStack的创建快照的操作是在线创建快照（live snapshot）, 并且应该是增量的快照，即利用virsh或者qemu的live snapshot来实现的：
virsh snapshot-create-as --live ....
后来发现快照和原始镜像之间并没有依赖关系，感觉OpenStack还做的挺好的，自动解决了增量快照和原始镜像之间的依赖关系;
但是后来又发现做快照的时候虚拟机竟然会shutoff, 就感觉不对了，于是分析了下源码。
2. 结论：目前OpenStack默认的快照方式都是cold snapshot, 首先先关机，其次执行如下命令生成一个镜像文件，再次开机，最后再调用glance api将镜像上传。
qemu-img convert -f qcow2 -O qcow2
所以目前并不是真正意义的快照，其实和关闭虚拟机，拷贝一份，再上传没有本质区别。
一直以为OpenStack的创建快照的操作是在线创建快照（live snapshot）, 并且应该是增量的快照，即利用virsh或者qemu的live snapshot来实现的：
virsh snapshot-create-as --live ....
后来发现快照和原始镜像之间并没有依赖关系，感觉OpenStack还做的挺好的，自动解决了增量快照和原始镜像之间的依赖关系;
但是后来又发现做快照的时候虚拟机竟然会shutoff, 就感觉不对了，于是分析了下源码。
2. 结论：目前OpenStack默认的快照方式都是cold snapshot, 首先先关机，其次执行如下命令生成一个镜像文件，再次开机，最后再调用glance api将镜像上传。
qemu-img convert -f qcow2 -O qcow2
所以目前并不是真正意义的快照，其实和关闭虚拟机，拷贝一份，再上传没有本质区别。
3. 源代码流程分析
3.1 nova/compute/api.py
# NOTE(melwitt): We don't check instance lock for snapshot because lock is
# intended to prevent accidental change/delete of instances
@wrap_check_policy
@check_instance_cell
@check_instance_state(vm_state=[vm_states.ACTIVE, vm_states.STOPPED,
vm_states.PAUSED, vm_states.SUSPENDED])
def snapshot(self, context, instance, name, extra_properties=None):
"""Snapshot the given instance.
:param instance: nova.db.sqlalchemy.models.Instance
:param name: name of the snapshot
:param extra_properties: dict of extra image properties to include
when creating the image.
:returns: A dict containing image metadata
"""
#调用glance api创建image entry，为后将snapshot上传为镜像做准备，虽然镜像和snapshot在可以上传到glance作为镜像启动虚拟机，
#但是为了区分二者的不同，glance将镜像和snapshot标记卫不同的类型：type=image 和 type=snapshot
image_meta = self._create_image(context, instance, name,
'snapshot',
extra_properties=extra_properties)
# NOTE(comstud): Any changes to this method should also be made
# to the snapshot_instance() method in nova/cells/messaging.py
# 将任务状态(task state) 设置为:image_snapshot_pending
instance.task_state = task_states.IMAGE_SNAPSHOT_PENDING
instance.save(expected_task_state=[None])
#通过rpc调用nova/compute/rpcapi.py的snapshot_instance函数
self.compute_rpcapi.snapshot_instance(context, instance,
image_meta['id
3.2 nova/compute/rpcapi.py
#梳理下流程： （1）用户发起create snapshot的请求; （2）nova-api服务接收到这个请求并进行前期处理，即3.1中代码的处理流程;
（3）真正的snapshot操作是需要在nova-compute节点上执行的，所以nova-api需要向nova-compute发送message
#由于OpenStack环境中会有很多个nova-compute,所以需要通过server=_compute_host(None, instance)来获取虚拟机所在的host，并向其发送message。
def snapshot_instance(self, ctxt, instance, image_id):
version = '3.0'
cctxt = self.client.prepare(server=_compute_host(None, instance),
version=version)
cctxt.cast(ctxt, 'snapshot_instance',
instance=instance,
image_id=image_id)
3.3 nova/virt/libvirt/driver.py
def snapshot(self, context, instance, image_id, update_task_state):
"""Create snapshot from a running VM instance.
This command only works with qemu 0.14+
"""
try:
virt_dom = self._get_domain(instance)
except exception.InstanceNotFound:
raise exception.InstanceNotRunning(instance_id=instance['uuid'])
base_image_ref = instance['image_ref']
base = compute_utils.get_image_metadata(
context, self._image_api, base_image_ref, instance)
snapshot = self._image_api.get(context, image_id)
disk_path = libvirt_utils.find_disk(virt_dom)
source_format = libvirt_utils.get_disk_type(disk_path)
image_format = CONF.libvirt.snapshot_image_format or source_format
# NOTE(bfilippov): save lvm and rbd as raw
if image_format == 'lvm' or image_format == 'rbd':
image_format = 'raw'
metadata = self._create_snapshot_metadata(base,
instance,
image_format,
snapshot['name'])
snapshot_name = uuid.uuid4().hex
state = LIBVIRT_POWER_STATE[virt_dom.info()[0]]
# NOTE(rmk): Live snapshots require QEMU 1.3 and Libvirt 1.0.0.
# These restrictions can be relaxed as other configurations
# can be validated.
# NOTE(dgenin): Instances with LVM encrypted ephemeral storage require
# cold snapshots. Currently, checking for encryption is
# redundant because LVM supports only cold snapshots.
# It is necessary in case this situation changes in the
# future.
#这里需要注意，解释了为啥现在是cold snapshot而不是live snapshot:
# 有人提过live snapshot的bug，社区认为live snapshot目前不稳定，所以默认条件下采用cold snapshot，并且是通过硬编码来实现的
# 看下面这个判断条件，成立的时候将live_snapshot = true,其中MIN_LIBVIRT_LIVESNAPSHOT_VERSION=1.3.0, 其实现在libvirt的最新版本
# 才到1.2.11， 所以这个live_snapshot的条件不满足，就变成了cold_snapshot
if (self._host.has_min_version(MIN_LIBVIRT_LIVESNAPSHOT_VERSION,
MIN_QEMU_LIVESNAPSHOT_VERSION,
REQ_HYPERVISOR_LIVESNAPSHOT)
and source_format not in ('lvm', 'rbd')
and not CONF.ephemeral_storage_encryption.enabled):
live_snapshot = True
# Abort is an idempotent operation, so make sure any block
# jobs which may have failed are ended. This operation also
# confirms the running instance, as opposed to the system as a
# whole, has a new enough version of the hypervisor (bug 1193146).
try:
virt_dom.blockJobAbort(disk_path, 0)
except libvirt.libvirtError as ex:
error_code = ex.get_error_code()
if error_code == libvirt.VIR_ERR_CONFIG_UNSUPPORTED:
live_snapshot = False
else:
pass
else:
live_snapshot = False
# NOTE(rmk): We cannot perform live snapshots when a managedSave
# file is present, so we will use the cold/legacy method
# for instances which are shutdown.
if state == power_state.SHUTDOWN:
live_snapshot = False
# NOTE(dkang): managedSave does not work for LXC
#注意这里，如果live_snashot目前是false,所以在做snapshot之前先要执行：
#（1）_detach_pci_devices， 卸载虚拟机挂载的pci设备，比如数据盘
#（2） self._detach_sriov_ports， 卸载虚拟机挂载的SRIOV设备，比如支持SRIOV的网卡设备
if CONF.libvirt.virt_type != 'lxc' and not live_snapshot:
if state == power_state.RUNNING or state == power_state.PAUSED:
self._detach_pci_devices(virt_dom,
pci_manager.get_instance_pci_devs(instance))
self._detach_sriov_ports(instance, virt_dom)
virt_dom.managedSave(0)
#判断虚拟机的后端存储是什么，不同的后端存储做snapshot是不同的，本地文件系统的化，默认qcow2
snapshot_backend = self.image_backend.snapshot(instance,
disk_path,
image_type=source_format)
if live_snapshot:
LOG.info(_LI("Beginning live snapshot process"),
instance=instance)
else:
LOG.info(_LI("Beginning cold snapshot process"),
instance=instance)
#更新任务的状态为：image_pending_upload, 大家都知道做完snapshot要上传
update_task_state(task_state=task_states.IMAGE_PENDING_UPLOAD)
#目前做快照的过程是：
#（1）现在../data/nova/instance/snapshots目录下生成临时目录，比如nova/instances/snapshots/tmptHr585
#（2）然后将快照生成到这个目录，具体参见snapshot_backend.snapshot_extract(out_path, image_format)这个函数
#（3）生成完成后，通过glance api上传，具体参见 self._image_api.update
snapshot_directory = CONF.libvirt.snapshots_directory
fileutils.ensure_tree(snapshot_directory)
with utils.tempdir(dir=snapshot_directory) as tmpdir:
try:
out_path = os.path.join(tmpdir, snapshot_name)
if live_snapshot:
# NOTE(xqueralt): libvirt needs o+x in the temp directory
os.chmod(tmpdir, 0o701)
self._live_snapshot(context, instance, virt_dom, disk_path,
out_path, image_format, base)
else:
#这个函数实际执行了一条命令： qemu-img convert -f qcow2 -O qcow2 disk_path out_path,算是生成了快照
snapshot_backend.snapshot_extract(out_path, image_format)
finally:
new_dom = None
# NOTE(dkang): because previous managedSave is not called
# for LXC, _create_domain must not be called.
if CONF.libvirt.virt_type != 'lxc' and not live_snapshot:
if state == power_state.RUNNING:
new_dom = self._create_domain(domain=virt_dom) ##恢复做快照之前虚拟机的状态
elif state == power_state.PAUSED:
new_dom = self._create_domain(domain=virt_dom,
launch_flags=libvirt.VIR_DOMAIN_START_PAUSED)
if new_dom is not None:
self._attach_pci_devices(new_dom,
pci_manager.get_instance_pci_devs(instance))
self._attach_sriov_ports(context, instance, new_dom)
LOG.info(_LI("Snapshot extracted, beginning image upload"),
instance=instance)
# Upload that image to the image service
update_task_state(task_state=task_states.IMAGE_UPLOADING,
expected_state=task_states.IMAGE_PENDING_UPLOAD)
with libvirt_utils.file_open(out_path) as image_file: ###将生成的快照上传到glance
self._image_api.update(context,
image_id,
metadata,
image_file)
LOG.info(_LI("Snapshot image upload complete"),
instance=instance)
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