OpenStack之虚机热迁移的代码详细解析

话说虚机迁移分为冷迁移以及热迁移，所谓热迁移用度娘的话说即是：热迁移（Live Migration，又叫动态迁移、实时迁移），即虚机保存/恢复(Save/Restore)：将整个虚拟机的运行状态完整保存下来，同时可以快速的恢复到原有硬件平台甚至是不同硬件平台上。恢复以后，虚机仍旧平滑运行，用户不会察觉到任何差异。OpenStack的虚机迁移是基于Libvirt实现的，下面来看看Openstack虚机热迁移的具体代码实现。

首先，由API入口进入到nova/api/openstack/compute/contrib/admin_actions.py

@wsgi.action('os-migrateLive')
def _migrate_live(self, req, id, body):
"""Permit admins to (live) migrate a server to a new host."""
context = req.environ["nova.context"]
authorize(context, 'migrateLive')

try:
block_migration = body["os-migrateLive"]["block_migration"]
disk_over_commit = body["os-migrateLive"]["disk_over_commit"]
host = body["os-migrateLive"]["host"]
except (TypeError, KeyError):
msg = _("host, block_migration and disk_over_commit must "
"be specified for live migration.")
raise exc.HTTPBadRequest(explanation=msg)

try:
block_migration = strutils.bool_from_string(block_migration,
strict=True)
disk_over_commit = strutils.bool_from_string(disk_over_commit,
strict=True)
except ValueError as err:
raise exc.HTTPBadRequest(explanation=str(err))

try:
instance = self.compute_api.get(context, id, want_objects=True)self.compute_api.live_migrate(context, instance, block_migration,
disk_over_commit, host)
except (exception.ComputeServiceUnavailable,
exception.InvalidHypervisorType,
exception.UnableToMigrateToSelf,
exception.DestinationHypervisorTooOld,
exception.NoValidHost,
exception.InvalidLocalStorage,
exception.InvalidSharedStorage,
exception.MigrationPreCheckError) as ex:
raise exc.HTTPBadRequest(explanation=ex.format_message())
except exception.InstanceNotFound as e:
raise exc.HTTPNotFound(explanation=e.format_message())
except exception.InstanceInvalidState as state_error:
common.raise_http_conflict_for_instance_invalid_state(state_error,
'os-migrateLive')
except Exception:
if host is None:
msg = _("Live migration of instance %s to another host "
"failed") % id
else:
msg = _("Live migration of instance %(id)s to host %(host)s "
"failed") % {'id': id, 'host': host}
LOG.exception(msg)
# Return messages from scheduler
raise exc.HTTPBadRequest(explanation=msg)

return webob.Response(status_int=202)

   这里第一行可以看到是与API文档的第二行照应的：
  

{
"os-migrateLive": {
"host": "0443e9a1254044d8b99f35eace132080",
"block_migration": false,
"disk_over_commit": false
}
}

好了，源码中其实执行迁移工作的就是第26、27行的一条语句：

self.compute_api.live_migrate(context, instance, block_migration,
disk_over_commit, host)

由这句进入到nova/compute/api.py中，源码如下：

@check_instance_cell
@check_instance_state(vm_state=[vm_states.ACTIVE])
def live_migrate(self, context, instance, block_migration,
disk_over_commit, host_name):
"""Migrate a server lively to a new host."""
LOG.debug(_("Going to try to live migrate instance to %s"),
host_name or "another host", instance=instance)

instance.task_state = task_states.MIGRATING
instance.save(expected_task_state=[None])

self.compute_task_api.live_migrate_instance(context, instance,
host_name, block_migration=block_migration,
disk_over_commit=disk_over_commit)

第2行是一个装饰器，用于在进入API方法之前，检测虚拟机和/或任务的状态， 如果实例处于错误的状态，将会引发异常；接下来实时迁移虚机到新的主机，并将虚机状态置于“migrating”,然后由12行进入nova/conductor/api.py

def live_migrate_instance(self, context, instance, host_name,
block_migration, disk_over_commit):
scheduler_hint = {'host': host_name}
self._manager.migrate_server(
context, instance, scheduler_hint, True, False, None,
block_migration, disk_over_commit, None)

将主机名存入字典scheduler_hint中，然后调用nova/conductor/manager.py方法migrate_server,

def migrate_server(self, context, instance, scheduler_hint, live, rebuild,
flavor, block_migration, disk_over_commit, reservations=None):
if instance and not isinstance(instance, instance_obj.Instance):
# NOTE(danms): Until v2 of the RPC API, we need to tolerate
# old-world instance objects here
attrs = ['metadata', 'system_metadata', 'info_cache',
'security_groups']
instance = instance_obj.Instance._from_db_object(
context, instance_obj.Instance(), instance,
expected_attrs=attrs)
if live and not rebuild and not flavor:
self._live_migrate(context, instance, scheduler_hint,
block_migration, disk_over_commit)
elif not live and not rebuild and flavor:
instance_uuid = instance['uuid']
with compute_utils.EventReporter(context, self.db,
'cold_migrate', instance_uuid):
self._cold_migrate(context, instance, flavor,
scheduler_hint['filter_properties'],
reservations)
else:
raise NotImplementedError()

由于在nova/conductor/api.py中传过来的参数是

self._manager.migrate_server(
context, instance, scheduler_hint, True, False, None,
block_migration, disk_over_commit, None)

因此live是True，rebuild是Flase,flavor是None，执行第12、13行代码：

if live and not rebuild and not flavor:
self._live_migrate(context, instance, scheduler_hint,
block_migration, disk_over_commit)
　_live_migrate代码如下：
def _live_migrate(self, context, instance, scheduler_hint,
block_migration, disk_over_commit):
destination = scheduler_hint.get("host")
try:
live_migrate.execute(context, instance, destination,
block_migration, disk_over_commit)
except (exception.NoValidHost,
exception.ComputeServiceUnavailable,
exception.InvalidHypervisorType,
exception.InvalidCPUInfo,
exception.UnableToMigrateToSelf,
exception.DestinationHypervisorTooOld,
exception.InvalidLocalStorage,
exception.InvalidSharedStorage,
exception.HypervisorUnavailable,
exception.MigrationPreCheckError) as ex:
with excutils.save_and_reraise_exception():
#TODO(johngarbutt) - eventually need instance actions here
request_spec = {'instance_properties': {
'uuid': instance['uuid'], },
}
scheduler_utils.set_vm_state_and_notify(context,
'compute_task', 'migrate_server',
dict(vm_state=instance['vm_state'],
task_state=None,
expected_task_state=task_states.MIGRATING,),
ex, request_spec, self.db)
except Exception as ex:
LOG.error(_('Migration of instance %(instance_id)s to host'
' %(dest)s unexpectedly failed.'),
{'instance_id': instance['uuid'], 'dest': destination},
exc_info=True)
raise exception.MigrationError(reason=ex)

首先，第三行中将主机名赋给destination,然后执行迁移，后面的都是异常的捕捉，执行迁移的代码分为两部分，先看第一部分，在nova/conductor/tasks/live_migrate.py的184行左右：

def execute(context, instance, destination,
block_migration, disk_over_commit):
task = LiveMigrationTask(context, instance,
destination,
block_migration,
disk_over_commit)
#TODO(johngarbutt) create a superclass that contains a safe_execute call
return task.execute()

先创建包含安全执行回调的超类，然后返回如下函数也即执行迁移的第二部分代码，在54行左右：

def execute(self):
self._check_instance_is_running()
self._check_host_is_up(self.source)

if not self.destination:
self.destination = self._find_destination()
else:
self._check_requested_destination()

#TODO(johngarbutt) need to move complexity out of compute manager
return self.compute_rpcapi.live_migration(self.context,
host=self.source,
instance=self.instance,
dest=self.destination,
block_migration=self.block_migration,
migrate_data=self.migrate_data)
#TODO(johngarbutt) disk_over_commit?

这里有三部分内容：

如果目前主机不存在，则由调度算法选取一个目标主机，并且进行相关的检测，确保能够进行实时迁移操作；

如果目标主机存在，则直接进行相关的检测操作，确保能够进行实时迁移操作；

执行迁移操作。

前两部分不再赘述，直接看第三部分代码，在nova/compute/rpcapi.py中：

def live_migration(self, ctxt, instance, dest, block_migration, host,
migrate_data=None):
# NOTE(russellb) Havana compat
version = self._get_compat_version('3.0', '2.0')
instance_p = jsonutils.to_primitive(instance)
cctxt = self.client.prepare(server=host, version=version)
cctxt.cast(ctxt, 'live_migration', instance=instance_p,
dest=dest, block_migration=block_migration,
migrate_data=migrate_data)

热迁移开始执行：

def live_migration(self, context, instance, dest,
post_method, recover_method, block_migration=False,
migrate_data=None):
"""Spawning live_migration operation for distributing high-load.

:param context: security context
:param instance:
nova.db.sqlalchemy.models.Instance object
instance object that is migrated.
:param dest: destination host
:param post_method:
post operation method.
expected nova.compute.manager.post_live_migration.
:param recover_method:
recovery method when any exception occurs.
expected nova.compute.manager.recover_live_migration.
:param block_migration: if true, do block migration.
:param migrate_data: implementation specific params

"""

greenthread.spawn(self._live_migration, context, instance, dest,
post_method, recover_method, block_migration,
migrate_data)

这个方法中建立一个绿色线程来运行方法_live_migration，来执行实时迁移； 主要是调用libvirt python接口方法virDomainMigrateToURI，来实现从当前主机迁移domain对象到给定的目标主机；

spawn：建立一个绿色线程来运行方法“func(*args, **kwargs)”，这里就是来运行方法_live_migration；

 _live_migration：执行实时迁移； 主要是调用libvirt python接口方法virDomainMigrateToURI，来实现从当前主机迁移domain对象到给定的目标主机；

接着在绿色线程中调用_live_migration方法：

def _live_migration(self, context, instance, dest, post_method,
recover_method, block_migration=False,
migrate_data=None):
"""Do live migration.

:param context: security context
:param instance:
nova.db.sqlalchemy.models.Instance object
instance object that is migrated.
:param dest: destination host
:param post_method:
post operation method.
expected nova.compute.manager.post_live_migration.
:param recover_method:
recovery method when any exception occurs.
expected nova.compute.manager.recover_live_migration.
:param block_migration: if true, do block migration.
:param migrate_data: implementation specific params
"""

# Do live migration.
try:if block_migration:
flaglist = CONF.libvirt.block_migration_flag.split(',')else:
flaglist = CONF.libvirt.live_migration_flag.split(',')
flagvals = [getattr(libvirt, x.strip()) for x in flaglist]
logical_sum = reduce(lambda x, y: x | y, flagvals)dom = self._lookup_by_name(instance["name"])
dom.migrateToURI(CONF.libvirt.live_migration_uri % dest,
logical_sum,
None,
CONF.libvirt.live_migration_bandwidth)

except Exception as e:
with excutils.save_and_reraise_exception():
LOG.error(_("Live Migration failure: %s"), e,
instance=instance)
recover_method(context, instance, dest, block_migration)

# Waiting for completion of live_migration.timer = loopingcall.FixedIntervalLoopingCall(f=None)

if block_migration:
flaglist = CONF.libvirt.block_migration_flag.split(',')

这个获取块迁移标志列表，block_migration_flag：这个参数定义了为块迁移设置迁移标志。 

else:
flaglist = CONF.libvirt.live_migration_flag.split(',')
flagvals = [getattr(libvirt, x.strip()) for x in flaglist]
logical_sum = reduce(lambda x, y: x | y, flagvals)

这部分获取实时迁移标志列表，live_migration_flag这个参数定义了实时迁移的迁移标志。

dom = self._lookup_by_name(instance["name"])

根据给定的实例名称检索libvirt域对象。

timer = loopingcall.FixedIntervalLoopingCall(f=None)

获取等待完成实时迁移的时间。

热迁移代码部分至此结束。

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持脚本之家。 

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