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Last week, I faced an interesting problem at work. I received a critical warning from AWS regarding our PostgreSQL RDS cluster, it was approaching transaction ID wraparound, something that can cause a full database outage if not handled in time.
This was a good reminder of how important it is to understand the internals of the systems we rely on every day.
For those unfamiliar, PostgreSQL uses a 32-bit transaction counter (XID) that wraps around after ~2.1 billion transactions. If old rows aren’t vacuumed and frozen in time, PostgreSQL can no longer determine which rows are visible, forcing it to stop accepting new writes to prevent data corruption.
In our case, it turned out that autovacuum wasn’t keeping up due to high write throughput and a few misconfigured thresholds.
What helped:
👉🏻 Identifying large tables with unvacuumed rows.
👉🏻 Manually running 𝖵𝖠𝖢𝖴𝖴𝖬.
👉🏻 Tuning autovacuum workers settings and vacuum cost limits.
It’s one of those things that often goes unnoticed until it becomes urgent. If you're running PostgreSQL on RDS (or self-managed), keep an eye on 𝖺𝗀𝖾(𝖽𝖺𝗍𝖿𝗋𝗈𝗓𝖾𝗇𝗑𝗂𝖽) and ensure autovacuum is running efficiently.
Invisible debt in infrastructure has a way of surfacing at the worst possible time. This one got our attention early, worth sharing in case it helps others avoid a painful surprise.
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