How to Simulate Disk-Full Failures to Build Reliable WAL Health Checks

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A technical guide highlights a critical gap in service health monitoring: a process can report as healthy even while write-ahead log (WAL) writes are failing due to a full disk (ENOSPC error). The article demonstrates how to intentionally fill a disposable test volume using standard Linux tools to observe how systems behave under disk exhaustion. Key findings show that log rotation fails, retries amplify system load, and simply deleting a file does not guarantee durable writes have resumed. The guide recommends tiered thresholds — warning at 20% free space and rejecting new work at 10% — along with separating liveness probes from readiness probes that verify actual durable writes. Engineers are advised to run integrity and checkpoint checks before restoring a service to readiness, and to avoid running such drills on production or shared volumes.

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