According to their documentation, the data storage for log events is Elasticsearch and if the Elasticsearch data is lost then the logs are gone. Consideribg that ES is not a database and may lose data this sounds a bit scary to me.
1. Whatever mechanisim you're using to send data to Graylog, you send that data to S3 as well. You can then reload Graylog at anytime with S3 data.
2. Backup your Elasticsearch nodes to S3
I should've mentioned I run this in AWS. Sorry about that!
they had a major data corruption bug last year but have taken measures to correct it. I asked if elasticsearch could be a source of truth at elasticon and they didn't say yes, but they indicated that you "could do it" and it is a goal