[CentOS] Backup live system

Though I've worked with enterprise systems, I'm not familiar with FOOS 
backup software.  Which of those recommended would allow me to backup a 
system while users are active on it?  If it matters the system uses LVM. 
    I'd also like to be able to avoid needing the network if possible. 
That is, I'd plug in a disk into a USB port and backup the system onto 
that... again, while the system is live.

Thanks much.

-- 
War is a failure of the imagination.
         --William Blake