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Logging Added 11 February 2026

Fluentd: Buffer overflow

Fluentd's output buffer is full because the destination cannot accept data fast enough. Logs may be dropped.

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

Quick fix
# Increase buffer size in fluentd.conf
<buffer>
  @type file
  path /var/log/fluentd/buffer
  total_limit_size 5GB
  chunk_limit_size 32MB
  overflow_action throw_exception
</buffer>
# Scale destination or add retry
retry_max_times 10
retry_wait 5s

How to diagnose Logging errors

Logging failures are dangerous because they are silent: the application keeps running while its telemetry disappears. The recurring causes are buffer overflow under backpressure (the destination cannot keep up), permissions on log files or directories, and ingestion rate limits at the cloud provider. Monitoring the logging pipeline itself is the only reliable way to notice.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

  1. Check the collector's own logs first: Fluentd, Logstash and the CloudWatch agent all log their own failures, usually to a separate destination.
  2. Look for backpressure metrics: buffer queue length, retry counts, and dropped-record counters. A full buffer means the destination, not the collector, is the bottleneck.
  3. Verify write permissions and disk space on the buffer path. A full disk silently stops most collectors.
  4. For cloud ingestion, check the API rate limit for the log group or stream and batch more aggressively rather than retrying harder.
  5. Add a heartbeat log line and alert on its absence. This is the only way to detect a pipeline that has stopped without erroring.

Tools worth reaching for

  • collector self-logs
  • buffer/queue metrics
  • df -h
  • aws logs describe-log-streams
  • logger / fluent-cat for test events

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

docs.fluentd.org

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