What is WAL backpressure, and why does ClickHouse Managed Postgres need it?

WAL archiving in PostgreSQL copies completed write-ahead log segments to object storage for point-in-time recovery, but an archiving backlog can fill the WAL disk and trigger a database PANIC if writes outpace uploads. ClickHouse Managed Postgres addresses this risk with an automated data-plane mechanism that checks pending WAL segments every 15 seconds and progressively limits client write bandwidth through cgroup v2 I/O controls, while leaving reads and the archival, checkpointing, and logging paths uncapped so recovery can proceed at full speed. In a test using a deliberately rate-limited archiver, write caps activated as the backlog crossed 100, 500, and 1,000 segments, reducing pgbench throughput but keeping disk use below 33%; after archival throughput was restored, the uncapped archiver drained the 53 GiB peak backlog and the throttle automatically lifted once the queue cleared. The approach is intended to buy time during archival failures without requiring control-plane access or manual intervention, although its effectiveness varies with workload characteristics because some WAL generation may continue through uncapped internal processes.

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