A Durus (FileStorage) database is just a file.
New data is written only at the end of the file.
You can backup the database by copying the file at
any time.

You could maintain a mirror of a Durus database by copying
the Durus file at regular intervals, but this may be too
slow and it may use more bandwidth than you would like.

The latest release of rsync (I'm using 3.0.2) includes
an "--append" option that seems to be excellent for mirroring
Durus files.  It transmits only the new tail of the file,
and skips verifying the contents of the part of the file that
has already been transmitted.  The "--append-verify" option
does the same thing, but it does verify whole file.
I think you could run rsync frequently with --append,
and perhaps occasionally with --append-verify to maintain
a nice backup of a Durus database, and with minimal
bandwidth.

Are there alternatives that are just as good or better?