Can you do a btrfs snapshot of a live system?

I am planning on giving btrfs a try soon, and what I want to know, and can't seem to find online is this:

Can you do a btrfs snapshot on a running system if the snapshot is not just of some small directory but of the entire system (/var, /usr, /bin, /sbin, ... everything), or do you need to boot into a live CD to do a btrfs snapshot while the system is offline?

If it is technically possible to do a snapshot of a running system, I assume you still need to shut the system down to use that snapshot. Would booting a snapshot (subvolume) of a system that was running at the time the snapshot was taken be very similar to a hard boot, since software is not aware when a snapshot is about to take place, so programs might be writing something to /var at the time, and next thing they know, the program is starting up again with data half written because the system restarted?