Internal snapshot: A type of snapshot, where a single QCOW2 file will hold both the ‘saved state’ and the ‘delta’ since that saved point. ‘Internal snapshots’ are very handy because it’s only a single file where all the snapshot info. is captured, and easy to copy/move around the machines. （管理方便）
External snapshot: Here, the ‘original qcow2 file’ will be in a ‘read-only’ saved state, and the new qcow2 file(which will be generated once snapshot is created) will be the delta for the changes. So, all the changes will now be written to this delta file. ‘External Snapshots’ are useful for performing backups. Also, external snapshot creates a qcow2 file with the original file as its backing image, and the backing file can be /read/ in parallel with the running qemu.（子快照依赖于父快照，占用磁盘空间小，适用于备份场景）:
.-----------. .-----------. .------------. .------------. .------------.
| | | | | | | | | |
| RootBase |<--- Overlay-1 |<--- Overlay-1A <--- Overlay-1B <--- Overlay-1C |
| | | | | | | | | (Active) |
'-----------' '-----------' '------------' '------------' '------------'
^
|
| .-----------. .------------.
| | | | |
'-------| Overlay-2 |<---| Overlay-2A |
| | | (Active) |
'-----------' '------------'
在openstack中只提供Internal snapshot方式，并支持live snapshot和cold snapshot。
在 nova/virt/libvirt/driver.py
if state == power_state.SHUTDOWN:
live_snapshot = False
# NOTE(dkang): managedSave does not work for LXC
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))
virt_dom.managedSave(0)
snapshot_backend = self.image_backend.snapshot(disk_path,
image_type=source_format)
if live_snapshot:
LOG.info(_("Beginning live snapshot process"),
instance=instance)
else:
LOG.info(_("Beginning cold snapshot process"),
instance=instance)
update_task_state(task_state=task_states.IMAGE_PENDING_UPLOAD)
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(virt_dom, disk_path, out_path,
image_format)
else:
snapshot_backend.snapshot_extract(out_path, image_format)
虚拟机如果是运行状态，快照是很慢的，而且宿主机需要有足够的空间(至少要比vm的root disk大才行)。
虚拟机如果是关闭状态，快照是非常快的，他只是vm的disk文件做了格式转换，转成qcow2。