CassandraFlushWriterBlockedTasks #

Some Cassandra flush writer tasks are blocked - {{ $labels.cassandra_cluster }}

alert: CassandraFlushWriterBlockedTasks
annotations:
  description: |-
    Some Cassandra flush writer tasks are blocked - {{ $labels.cassandra_cluster }}
      VALUE = {{ $value }}
      LABELS = {{ $labels }}    
  runbook: https://srerun.github.io/prometheus-alerts/runbooks/instaclustr-cassandra-exporter/cassandraflushwriterblockedtasks/
  summary: Cassandra flush writer blocked tasks (instance {{ $labels.instance }})
expr: cassandra_thread_pool_blocked_tasks{pool="MemtableFlushWriter"} > 15
for: 2m
labels:
  severity: warning

Here is the runbook for the CassandraFlushWriterBlockedTasks alert:

Meaning #

The CassandraFlushWriterBlockedTasks alert is triggered when the number of blocked tasks in the MemtableFlushWriter thread pool exceeds 15 for more than 2 minutes. This indicates that Cassandra is experiencing issues with flushing memtables to disk, which can lead to performance degradation, increased memory usage, and potentially even node crashes.

Impact #

The impact of this alert is high, as blocked flush writer tasks can cause:

• Increased memory usage, leading to OutOfMemory errors
• Performance degradation, resulting in slower query responses
• Node instability, potentially leading to node crashes
• Data loss, in extreme cases where the node crashes before flushing data to disk

Diagnosis #

To diagnose the root cause of this issue, perform the following steps:

1. Check the Cassandra node’s system logs for any errors or exceptions related to disk I/O or memtable flushing.
2. Verify that the disk has sufficient free space and is not experiencing high latency.
3. Check the MemtableFlushWriter thread pool metrics to identify the trend and pattern of blocked tasks.
4. Investigate any recent configuration changes or upgrades to Cassandra or the underlying infrastructure.
5. Review the Node’s metrics to identify any signs of resource starvation (e.g., high CPU usage, low available memory).

Mitigation #

To mitigate the issue, perform the following steps:

1. Restart the Cassandra node to clear the blocked tasks and allow the flush writer to catch up.
2. Check and adjust the disk configuration to ensure it can handle the write load.
3. Adjust the memtableflushwriter_thread_count and/or memtableflushwriter_queue_timeout configuration options to improve flushing performance.
4. Consider adding more resources (e.g., increasing the node count, increasing the disk capacity) to improve overall system performance.
5. If the issue persists, consider engaging Cassandra experts or the support team for further assistance.