Hi, the physical backup strategy suggested by official mongo's document is to use
lvm, the filesystem level backup, which can be found here(https://docs.mongodb.com/manual/core/backups/).
It makes the process of initializing the machine more complex.
Physical backup is necessary because we can find a more convinent way to do initialSync,
No matter how fast the initialSync process can be after mongo 3.4, it can not be as fast as
physical backup.
I know there is a hotbackup api in wiredtiger layer(http://source.wiredtiger.com/2.5.0/backup.html), and percona also provides mongo layer's hotBackup command.
After some considerations, I find it not difficult to implement the command.
The best way is to forward the backup bytes onto a stream, the stream may be a tcp connection to the target machine, or a fd to the local disk. But mongo is only a service and does not have a sdk for users, so it may be considered futher.
What can be provided easily is what percona did, backuping up onto the local disk.

I think this improment is valuable and if you like, I'm glad to make this contribution.