confused about BDB directory layout in imapd

[Andreas]

There are these files/directories in cyrus-imapd:

/var/lib/imap/deliver.db
/var/lib/imap/mailboxes.db
/var/lib/imap/tls_sessions.db
/var/lib/imap/db/*

The /var/lib/imap/db/ directory, however, seems to hold no database:
-rw-------    1 cyrus    mail         8192 2004-01-08 18:02 __db.001
-rw-------    1 cyrus    mail       270336 2004-01-08 18:02 __db.002
-rw-------    1 cyrus    mail        98304 2004-01-08 18:02 __db.003
-rw-------    1 cyrus    mail     18563072 2004-01-08 18:02 __db.004
-rw-------    1 cyrus    mail        32768 2004-01-08 18:02 __db.005
-rw-------    1 cyrus    mail      1637827 2004-01-12 09:48 log.0000000003

Now, some practical questions:

- if I run db_recover in /var/lib/imap/db, which database will be recovered?
- if I run db_recover in /var/lib/imap (which holds deliver, mailboxes and tls_sessions),
  again, which database will be recovered?
- why can't I run db_checkpoint? Where should I? Or should I stick with the ctl_cyrusdb tool?