CassandraFlushWriterBlockedTasks

CassandraFlushWriterBlockedTasks #

Some Cassandra flush writer tasks are blocked

Alert Rule
alert: CassandraFlushWriterBlockedTasks
annotations:
  description: |-
    Some Cassandra flush writer tasks are blocked
      VALUE = {{ $value }}
      LABELS = {{ $labels }}    
  runbook: https://srerun.github.io/prometheus-alerts/runbooks/criteo-cassandra-exporter/cassandraflushwriterblockedtasks/
  summary: Cassandra flush writer blocked tasks (instance {{ $labels.instance }})
expr: cassandra_stats{name="org:apache:cassandra:metrics:threadpools:internal:memtableflushwriter:currentlyblockedtasks:count"}
  > 0
for: 2m
labels:
  severity: warning

Here is the runbook for the Prometheus alert rule:

Meaning #

The CassandraFlushWriterBlockedTasks alert is triggered when the number of blocked tasks in the MemTableFlushWriter thread pool in Cassandra is greater than 0 for more than 2 minutes. This indicates that some Cassandra flush writer tasks are blocked, which can lead to performance issues and data inconsistencies.

Impact #

The impact of this alert is moderate to severe, as blocked flush writer tasks can:

  • Cause data to be delayed or lost
  • Lead to increased latency and slower query performance
  • Impact the overall health and stability of the Cassandra cluster

Diagnosis #

To diagnose the root cause of the blocked flush writer tasks, follow these steps:

  1. Check the Cassandra logs for errors or warnings related to the MemTableFlushWriter thread pool.
  2. Verify that the Cassandra node is not experiencing high CPU or disk usage.
  3. Check for any network issues or connectivity problems that may be causing tasks to be blocked.
  4. Review the Cassandra configuration and ensure that the MemTableFlushWriter thread pool is properly sized and configured.
  5. Check the disk space usage and ensure that there is enough available space for the flush writer tasks to complete.

Mitigation #

To mitigate the blocked flush writer tasks, follow these steps:

  1. Check the Cassandra logs and error messages to identify the root cause of the blockage.
  2. Restart the Cassandra node to clear any stuck tasks and allow the flush writer to recover.
  3. Increase the MemTableFlushWriter thread pool size to handle the load.
  4. Adjust the Cassandra configuration to improve performance and reduce the likelihood of blockages.
  5. Monitor the Cassandra cluster for any signs of network or disk issues and take corrective action as needed.

Note: This runbook is a general guide and may need to be tailored to your specific use case and environment.

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