Rsyslog provides the “queue.size” parameter to set a limit on the number of messages a queue can keep in memory. This is primarily meant to support peak traffic.
Note that this counter is given in number of messages, not bytes. A frequent mistake is to think in bytes and select very large values (e.g. 7 million frequently seen, maybe due to a web tutorial somewhere). If queues are that large there is a chance the rsyslog will be aborted by out of memory condition when the queue gets fuller and fuller.
An example. You send data to a remote syslog server. You define a very large queue on it. Usually, the queue keeps very slow. But when the system goes offline, the queue fills up. This will lead to sharply increasing memory usage. Depending on all circumstances this may not be a problem – or it may be! The likelihood of becoming problematic, and harder to reproduce, increases with the number of queues defined.
To avoid such misunderstandings, rsyslog starting at 8.1905.0 emits a warning message. It has probably lead you to this page. If the queue size is correct, you can ignore the warning message. You can also filter it out via regular rules, if you like. But if you did not intend to define such a large queue, please reconsider the value.
Note: rsyslog considers queues larger than 500,000 messages to be overly large – there seldom is a good reason to use sizes in excess of that.