[root@messaging ~]# erl -sname ejabberd@localhost -mnesia dir \"/usr/local/var/lib/ejabberd\"
Erlang/OTP 21 [erts-10.2.4] [source] [64-bit] [smp:1:1] [ds:1:1:10] [async-threads:1] [hipe]

Eshell V10.2.4  (abort with ^G)
(ejabberd@localhost)1> mnesia:start().
ok
(ejabberd@localhost)2> mnesia:info().
---> Processes holding locks <---
---> Processes waiting for locks <---
---> Participant transactions <---
---> Coordinator transactions <---
---> Uncertain transactions <---
---> Active tables <---
schema         : with 1        records occupying 417      words of mem
===> System info in version "4.15.5", debug level = none <===
opt_disc. Directory "/usr/local/var/lib/ejabberd" is NOT used.
use fallback at restart = false
running db nodes   = [ejabberd@localhost]
stopped db nodes   = []
master node tables = []
remote             = []
ram_copies         = [schema]
disc_copies        = []
disc_only_copies   = []
[{ejabberd@localhost,ram_copies}] = [schema]
2 transactions committed, 0 aborted, 0 restarted, 0 logged to disc
0 held locks, 0 in queue; 0 local transactions, 0 remote
0 transactions waits for other nodes: []
ok
(ejabberd@localhost)3>
(ejabberd@localhost)3> mnesia:backup("/tmp/aa.backup").
ok