[CentOS] Copying a live system

Tue May 3 13:55:11 UTC 2016
Timothy Murphy <gayleard at eircom.net>

I want to backup my current CentOS-7.2 system to another drive.
Is it safe to copy the system while it is running?
Eg by
mount /dev/sdb5 /mnt
rsync -HPaxvz /. /mnt/
I've found contradictory advice on the web.

Timothy Murphy  
gayleard /at/ eircom.net
School of Mathematics, Trinity College, Dublin