[gentoo-user] LVM, crypto, and backup

I want to set up two portable 1TB drives so users have their own LVM
partitions to mount as crypto drives.  These partitions would have to
be mounted manually with the passpharse supplied by each user (this is
a family setup, just a few users).  But I want the system to be able
to backup one 1TB drive to the other for offline backup.

1.  What crypto system to use?

2.  I have planned this with each user having their own LVM partition,
    but would it be possible to use one passphrase and make the entire
    drive crypto, then run LVM on top of that?  This is a curiousity
    question more than anything.  It might be handy for a system disk
    someday, but not now.

3.  What is the right way to backup one 1TB drive to the other?  dd
    would probably work, but that sounds rather crude, not to mention
    it would have to copy the entire 1TB over USB.  It would be a lot
    faster to only backup what is needed.  I want to backup partitions
    automatically once a week, without knowing the passphrase or
    requiring the partitions to be mounted.  If each LVM partition is
    only expanded as necessary, unused space in each partition would
    be kept to a minimum.  Or is it possible for the backup to only
    backup as much of each partition as is used, without knowing the
    passphrase or having it mounted?  I suspect not, but I don't know.

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