8.2.1. Flow-control and multiple destinations The syslog-ng Premium Edition 7 Administrator Guide
Using flow-control on a source has an important side-effect if the messages of the source are sent to multiple destinations. If flow-control is in use and one of the destinations cannot accept the messages, the other destinations do not receive any messages either, because syslog-ng stops reading the source. For example, if messages from a source are sent to a remote server and also stored locally in a file, and the network connection to the server becomes unavailable, neither the remote server nor the local file will receive any messages.
Creating separate log paths for the destinations that use the same flow-controlled source does not avoid the problem.
If you use flow-control and reliable disk-based buffering together with multiple destinations, the flow-control starts slowing down the source only when: