Stefan Fercot: pgBackRest and PostgreSQL failover: why archive_mode matters

A recent question on the community channels described a difficult situation: a standby had been promoted with archive_mode=off, and restarting the new primary was something the team wanted to avoid. Could they enable archiving on a downstream standby and take a pgBackRest backup from there instead? Their attempt failed with archive_mode must be enabled, even with archive-mode-check=n. Removing the checks from pgBackRest’s source code allowed their proof of concept to succeed, but was that enough to trust the approach? My recommendation was to return to a supported configuration, either through a restart or a controlled switchover. But I wanted to take a closer look and see whether archive-mode-check=n might allow the standby-based approach. So, in this post, we’ll try to reproduce the situation, examine what pgBackRest checks, and see why a successful backup and restore don’t tell the whole recovery story.