Our barman.log occasionally shows such a message:
2018-02-02 06:36:44,588 [3377] barman.server ERROR: Check 'ssh output clean' failed for server 'pg'
However, when we execute barman check we get no failures, but if this happens during a backup, the operation is interrupted and we need to repeat it from scratch.
Make sure that when you connect to the Barman remote server via SSH, as specified in the ssh_command option, you do not receive any output.
Sometimes, temporary and sporadic network issues can cause this issue to happen. In this case, you need to investigate and verify your network reliability.
Supposing your ssh_command option in Barman is set to ssh postgres@pg, execute the following command and verify that no output is returned:
ssh postgres@ariddbpg01.ams true
Since version 2.0, Barman requires that no extra output is returned by a remote command. For implementation details, please refer to commit "Add a check for spurious messages in ssh_command output" (14 July 2016).
The reason behind this is that Barman needs to parse the output of remotely executed commands in order to operate.