One question I often get from people when starting to document postmortems in an organization is "should I write a postmortem?" There are many different ways that organizations decide on when to write a postmortem, but I have a simple rule I like to follow—if someone asks for a postmortem, you should write one. In a large organization, this works great, because someone will often ask, "What was that incident last night all about?" or "Will there be a postmortem for last week's outage?" These are cues that a document should be created.
In a smaller organization, it is more difficult because, often, it will be you deciding whether you should document the incident. To determine whether you should, you could come up with an internal metric. For example, saying, "If it took us more than 10 minutes to recover, we should write a postmortem." You can also keep it casual and only postmortem things that feel abnormal.
This can be dangerous though. Over the years...