btrfs snapshots (of root file system)?

[hw <hw@adminart.net>]

Hi,

with btrfs, how do I make a snapshot of the root file system?  The
purpose is to update software and being able to go back to a previous
state if necessary.

There doesn't seem to be a command to create snapshots but only
subvolumes?  How does a subvolume turn into a snapshot?  (The root
file system is, of course, not on a subvolume.)

How do I merge snapshots?  IIRC, when you remove a ZFS snapshot, the
older state is merged to the state the snapshot is in.  Apparently
btrfs can only delete snapshots --- and it seems like a bad idea to
delete the root file system.  How would I boot from it when it's been
deleted?

Can I make a snapshot on a different volume?  The manpage doesn't say
that the destination of a subvolume must be on the same volume, and in
any case, I should be able to do that.