Why mongodb doc use block level copy to do a snapshot backup?

[崔鑫]

Hi all,

I try to do a mongodb backup file by lvm snapshot. 

As the doc(https://docs.mongodb.com/manual/tutorial/backup-with-filesystem-snapshots/#lvm-backup-operation) said, it is suggest to use a  block level copy.

Could I know the block leve copy's reason and could I use the cp command to do a filesystem level copy?