The pipe between the two (|) feeds the standard output from the first tar into the standard input of the second, effectively copying the files into, and then out of, a non-existent archive file.
两者之间的管道 (|) 将第一个 tar 的标准输出传入到第二个 tar 的标准输入，并且高效地将文件复制到一个并不存在的存档文件，然后再从其中提取文件。
2
The second type allocates space out of the original file system, this is known as an internal snapshot.