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Runbook #5 – Troubleshooting JVM, Repair, and Hints Errors

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Cassandra is a data repair technique that stores temporary hints on their local filesystem for later application to the unavailable replica. Symptoms of hints failures are not obvious but it may cause startup issues due to a corrupt hints file. A corrupt hints file can be recovered by syncing it from other replicas using nodetool repair. Cassandra runs within a Java Virtual Machine (JVM) and can cause the JVM to run out of memory (OOM) if Cassandra has used up all the memory allocated to it. This can be caused by large result sets, a high application load, or a large row cache. Symptoms of the JVM running out of memory include large GC pauses and an unresponsive node. Diagnosis involves checking the system.log for specific error messages related to the JVM running out of memory.

Excerpted from the text – “A corrupt hints file cannot be recovered directly. We will try to recover that data by syncing it from the other replicas by running nodetool repair.”

 

Questions the Runbook Answers:

  1. What is a hints file in Cassandra, and why is it important for reducing data inconsistency?

  2. How can you diagnose a corrupt hints file in Cassandra and what are the common reasons for a hints file getting corrupted?

  3. What are the symptoms and diagnosis of the JVM running out of memory in Cassandra, and what are the common reasons for this issue to occur?

  4. What are the consequences of not fixing a corrupt hints file in Cassandra?

  5. How can we prevent Cassandra from running out of memory and throwing an OOM error?

  6. What are some common causes of large result sets in Cassandra?

  7. Is it possible to increase the size of the fixed-size heap allocated by the JVM for Cassandra?

  8. Are there any tools available for monitoring and diagnosing performance issues in a Cassandra cluster?

However, like any complex system, Cassandra can experience issues and failures that need to be diagnosed and fixed. One common issue is the use of hints to repair data inconsistencies across nodes. Hints are temporary data stored on a node’s local filesystem that provide information on how to repair data inconsistencies on another node that is currently unavailable. If a hints file becomes corrupted, it can cause startup issues and may require syncing from other replicas using the nodetool repair command.

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