blockpull
Syntax:
blockpull domain path [bandwidth] [--bytes] [base]
[--wait [--verbose] [--timeout seconds] [--async]]
[--keep-relative]
Populate(充满) a disk from its backing image chain.
By default, this command flattens(整合) the entire（完整的） chain;
but if base is specified, containing the name of one of the backing files in the chain,
then that file becomes the new backing file and only the intermediate portion of the chain is pulled.
Once all requested data from the backing image chain has been pulled, the disk no longer depends on that portion of the backing chain.
但如果指定了base，其中包含链中的一个备份文件的名称，然后，该文件成为新的备份文件，只提取链的中间部分。
一旦从后备图像链中提取了所有请求的数据，磁盘就不再依赖后备链中的那一部分
By default, this command returns as soon as possible, and data for the entire disk is pulled in the background;
默认情况下，这个命令会尽快返回，整个磁盘的数据会在后台提取
the progress of the operation can be checked with blockjob.
However, if --wait is specified, then this command will block until the operation completes（只要命令完成，才返回到用户的控制界面）, or cancel the operation if the optional timeout in seconds elapses（流逝） or SIGINT is sent (usually with Ctrl-C).
Using --verbose along with --wait will produce periodic(周期性) status updates.
If job cancellation is triggered, --async will return control to the user as fast as possible,
otherwise the command may continue to block a little while longer until the job is done cleaning up.
如果作业取消被触发，--async将尽快将控制权返回给用户，
否则，命令可能会继续阻塞一段时间，直到清理工作完成
Using the --keep-relative flag will keep the backing chain names relative(相对的；有关系的；成比例的).
使用--keep-relative标志将使后备链名保持相对
path specifies fully-qualified path of the disk;
path指定磁盘的全限定路径;
it corresponds to a unique target name (<target dev='name'/>) or source file (<source file='name'/>) for one of the disk devices attached to domain (see also domblklist for listing these names).
bandwidth specifies copying bandwidth limit in MiB/s.
For further information on the bandwidth argument see the corresponding section for the blockjob command.