Restore session failed as follows .
[Normal] From: RMA@Host_A "LIB" Time: 2020/02/07 11:23:14
STARTING Media Agent "LIB"
[Normal] From: RSM@Host_C "" Time: 2020/02/07 11:24:54
Opening MultiPath library control named 'LIB' on path 'Host_C;/dev/sg679'.
[Critical] From: RMA@Host_A "LIB" Time: 2020/02/07 11:24:54
Cannot open exchanger control device(Interprocess communication problem.)
The Media Agent System has 2 Network Interface Cards .
The MA system was configured as Host_B in the DP Cell that is for Backup LAN , but the hostname is Host_A .
Cell Manager has a certification of the MA system as Host_B .
So, RSM can connect to the MA system to start up RMA .
RMA will start up UMA, and then UMA needs to confirm a certification of RMA as Host_A because the request came from Host_A .
However, CM doesn't have a certification of Host_A because the system was recognized as Host_B in the DP Cell .
As a result, UMA is not allowed to connect to RMA , then the restore session failed .
Workaround / Fix
Adding Host_A as an alias name of Host_B in /etc/hosts file on CM solved the issue .