The cluster.routing.allocation.cluster_concurrent_rebalance setting in Elasticsearch controls the number of concurrent shard rebalancing operations allowed cluster-wide. This setting plays a crucial role in managing the cluster's performance during shard reallocation processes.
Description
- Default Value: 2
- Possible Values: Any positive integer
- Recommendations: The optimal value depends on your cluster size, hardware capabilities, and workload. For larger clusters or those with powerful hardware, you may consider increasing this value to speed up rebalancing. However, be cautious as higher values can increase system load.
This setting limits the number of shards that can be moved simultaneously across the entire cluster. It helps prevent excessive load on the cluster during rebalancing operations, which could otherwise impact performance.
Example
To change the cluster.routing.allocation.cluster_concurrent_rebalance setting using the cluster settings API:
PUT _cluster/settings
{
"persistent": {
"cluster.routing.allocation.cluster_concurrent_rebalance": 3
}
}
This example increases the concurrent rebalance operations to 3. You might want to do this if you have a large cluster with powerful nodes and want to speed up the rebalancing process. The effect will be faster rebalancing but potentially higher resource utilization during this process.
Common Issues or Misuses
- Setting the value too high can lead to excessive CPU and network usage during rebalancing, potentially impacting cluster performance.
- Setting the value too low in large clusters can significantly slow down the rebalancing process, leading to prolonged periods of uneven shard distribution.
Do's and Don'ts
- Do monitor your cluster's performance when adjusting this setting.
- Do consider your hardware capabilities and cluster size when setting this value.
- Don't set this value excessively high without careful testing.
- Don't ignore this setting in large clusters where rebalancing operations are frequent.
Frequently Asked Questions
Q: How does this setting differ from index.recovery.max_concurrent_file_chunks?
A: While cluster.routing.allocation.cluster_concurrent_rebalance controls the number of concurrent shard movements cluster-wide, index.recovery.max_concurrent_file_chunks determines the number of file chunks that can be sent concurrently during a single shard recovery process.
Q: Can changing this setting impact ongoing searches or indexing?
A: Yes, increasing this value can lead to more concurrent shard movements, which may temporarily increase CPU and network usage, potentially affecting ongoing operations. It's best to make changes during off-peak hours.
Q: Is there a recommended value for this setting based on cluster size?
A: There's no one-size-fits-all recommendation. For small clusters (1-3 nodes), the default of 2 is often sufficient. For larger clusters, you might consider values between 2-4, but always test and monitor the impact.
Q: How does this setting interact with cluster.routing.allocation.node_concurrent_recoveries?
A: While cluster.routing.allocation.cluster_concurrent_rebalance limits concurrent rebalances cluster-wide, cluster.routing.allocation.node_concurrent_recoveries limits the number of concurrent incoming and outgoing recoveries per node. Both settings work together to manage the overall recovery and rebalancing process.
Q: Can this setting be changed dynamically?
A: Yes, this setting can be updated dynamically using the cluster settings API without requiring a cluster restart. However, changes will only affect new rebalancing operations, not ones already in progress.
