Elasticsearch Elasticsearch Excessive Replicas on Hot Nodes

By Opster Team

Updated: Jul 4, 2023

| 2 min read

What does this mean?

Having too many replicas for certain indices on the hot tier of your Elasticsearch cluster can lead to performance and stability issues, as well as increased storage costs.

This issue is monitored by Opster AutoOps in real-time with personalized recommendations provided for your own system. You can also configure notifications to avoid this issue forming in the future.

Why does this occur?

This occurs when the number of replicas for some indices is higher than necessary on hot nodes. Replicas are used to ensure data availability and fault tolerance, but having too many replicas can introduce additional resource usage, which can negatively impact indexing performance and stability.

Possible impact and consequences of excessive replicas

The possible consequences include:

  1. Reduced performance: Having too many replicas can lead to increased resource usage, which can negatively impact the performance of your Elasticsearch cluster.
  2. Increased storage costs: More replicas mean more storage space is required, leading to higher storage costs.
  3. Lower indexing throughput: Excessive replicas can cause indexing operations to slow down, reducing the overall throughput of your cluster.
  4. Cluster instability: The additional resource usage caused by too many replicas can lead to instability in your Elasticsearch cluster.

How to resolve

To resolve the issue of excessive replicas on hot nodes, you can take the following steps:

1. Reduce replicas to 1 on hot data nodes: When a cluster contains indices with more than one replica shard, reducing the number of replicas can potentially impact performance, leading to an increase in search latency since fewer nodes are available to participate in the search operation. Reducing the number of replicas presents a tradeoff between cost and performance. It is worth noting that this tradeoff may not always apply, but it should be considered as a possibility in order to save on storage costs.

To reduce the number of replicas, you can use the following command:

PUT /<index_name>/_settings
{
  "index" : {
    "number_of_replicas" : 1
  }
}

Replace `<index_name>` with the name of the index located on hot nodes for which replicas should be reduced.

2. Monitor and adjust replica settings: After reducing the number of replicas, monitor your Elasticsearch cluster’s performance and stability. If necessary, adjust the replica settings further to find the optimal balance between cost and performance.

3. Implement index lifecycle management (ILM) policies: ILM policies can help you automate the process of managing replicas based on the age and size of your indices. This can help ensure that the appropriate number of replicas are maintained throughout the lifecycle of your indices.

Conclusion

By understanding the issue of excessive replicas on hot nodes in Elasticsearch and taking the appropriate steps to resolve it, you can improve the performance, stability, and cost-efficiency of your Elasticsearch cluster.

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