Thread

Re: Startup PANIC on standby promotion due to zero-filled WAL segment

Alena Vinter <dlaaren8@gmail.com> — 2025-12-23T09:33:30Z

Hi Michael,

Thanks for the review. To clarify: TLI 1 does not diverge — it is fully
replicated to the standby before the timeline switch. The test then
intentionally slows down replication on TLI 2 (e.g., by delaying WAL
shipping), reproducing the scenario I illustrated. As far as I’m aware,
`fsync` is `on` by default, and the test does not modify it — so no WAL
records are lost due to unsafe flushing.

The core issue is that the new timeline’s segment is zero-initialized
instead of copying the same segment from the previous timeline (as done in
crash-recovery startup).  As a result, startup cannot finish recovery due
to non-replicated end of WAL causing failures like “invalid magic number”.

---
Alena Vinter