Scale in an Elasticsearch cluster - Elasticsearch Instances Management| Alibaba Cloud Documentation Center

Alibaba Cloud Elasticsearch allows you to remove data nodes from an Elasticsearch cluster to scale in it.

Precautions

After you remove data nodes from an Elasticsearch cluster, the system restarts the cluster. Therefore, before the removal, make sure that the restart does not affect your business.

Remove data nodes

Log on to the Alibaba Cloud Elasticsearch console.
In the top navigation bar, select the region where your cluster resides.
In the left-side navigation pane, click Elasticsearch Clusters. On the page that appears, find the target cluster and click its ID in the Cluster ID/Name column.
In the lower-right corner of the Basic Information page, choose Configuration Update > Remove Data Nodes.
In the Remove Data Nodes section of the page that appears, set Node Type.
Select the data nodes that you want to remove.

Notice The number of reserved nodes must be greater than two and greater than half of the existing nodes.

Migrate data.

For security purposes, make sure that the data nodes you want to remove store no data. If these data nodes store data, the system prompts you to migrate the data. After the data is migrated, no index data is stored on the data nodes and no index data will be written to them.

Click Data Migration Tool in the message that appears.

In the Migrate Data dialog box, select a data migration method.


Parameter	Description
Smart Migration	The system automatically selects data nodes for data migration.
Custom Migration	You must select the data nodes whose data you want to migrate.

Read and agree to the terms of data migration, and click OK.
Click OK.
The system then restarts the cluster. During the restart, you can view the data migration progress in the Tasks dialog box. After the cluster is restarted, the data stored on the data nodes that you want to remove is migrated.

Note During the data migration, you can click Pause in the Tasks dialog box to stop the migration.

In the lower-right corner of the Basic Information page, choose Configuration Update > Remove Data Nodes again.
In the Remove Data Nodes section of the page that appears, select the data nodes whose data is migrated and click OK.
The system then restarts the cluster. During the restart, you can view the data migration progress in the Tasks dialog box. After the cluster is restarted, the data nodes are removed from the cluster.

Roll back data migration

Data migration is time-consuming. Cluster status changes or data modifications may result in a data migration failure. You can view detailed information in the Tasks dialog box. To roll back data migration, perform the following steps:

Log on to the Kibana console of the current cluster.
For more information, see Log on to the Kibana console.
In the left-side navigation pane, click Dev Tools. On the Console tab of the page that appears, run the following command to obtain the IP addresses of the data nodes whose data is migrated:
```
GET _cluster/settings
```
If the command is successfully executed, the following result is returned:
```
{
  "transient": {
    "cluster": {
      "routing": {
        "allocation": {
          "exclude": {
            "_ip": "192.168.xx.xx,192.168.xx.xx,192.168.xx.xx"
          }
        }
      }
    }
  }
}                        
```

Roll back data.

Roll back the data on some data nodes. Use the exclude parameter to exclude the data nodes whose data you do not want to roll back.

PUT _cluster/settings
{
  "transient": {
    "cluster": {
      "routing": {
        "allocation": {
          "exclude": {
            "_ip": "192.168.xx.xx,192.168.xx.xx"
          }
        }
      }
    }
  }
}

Roll back the data on all data nodes.

PUT _cluster/settings
{
  "transient": {
    "cluster": {
      "routing": {
        "allocation": {
          "exclude": {
            "_ip": null
          }
        }
      }
    }
  }
}

Run the following command to check whether the data is rolled back:
```
GET _cluster/settings
```
If the IP addresses of the data nodes whose data is rolled back are not contained in the command output, the rollback is successful. You can also check the rollback progress based on whether shards are reallocated to the data nodes.

Note To check the status of a data migration or rollback task, run the GET _cat/shards?v command.

FAQ

What do I do if the "This operation may cause a shard distribution error or insufficient storage, CPU, or memory resources." message appears?
Cause
- Insufficient resources
  After data nodes are removed, the cluster does not have sufficient disk capacity, memory, or CPU resources to store system data or handle workloads.
- Shard allocation errors
  Based on Lucene principles, Elasticsearch does not migrate two or more replica shards of the same index on a data node to the same data node. In this case, after data nodes are removed, the number of replica shards in a cluster may be greater than or equal to the number of data nodes. This results in shard allocation errors.
Solution
- Insufficient resources
  Run the GET _cat/indices?v command to check whether the resource usage, such as the disk usage, is greater than the threshold. You must make sure that the cluster has sufficient resources to store data or process requests. If these requirements are not met, upgrade the configuration of the cluster. For more information, see Upgrade the configuration of a cluster.
- Shard allocation errors
  Run the GET _cat/indices?v command to check whether the number of replica shards in the cluster is less than the number of data nodes after data nodes are removed. If this requirement is not met, change the number of replica shards. For more information, see Index Templates. The following sample code demonstrates how to modify the index template to set the number of replica shards to 2:
```
PUT _template/template_1
{
  "template": "*",
  "settings": {
    "number_of_replicas": 2
  }
}  
```
What do I do if the "The cluster is running tasks or in an error status. Try again later." message appears?
Solution: Run the GET _cluster/health command to check the status of the cluster or go to the pages under Intelligent Maintenance to view the cause.
What do I do if the "The nodes in the cluster contain data. You must migrate the data first." message appears?
Solution: Migrate data. For more information, see Remove data nodes.
What do I do if the "The number of nodes that you reserve must be more than two and more than half of the existing nodes." message appears?
Cause: To ensure the reliability and stability of the cluster, the number of data nodes you reserve during data node removal or data migration must be greater than two and greater than half of the existing data nodes.

Solution: If the preceding requirements are not met, re-select the data nodes or upgrade the configuration of the cluster. For more information about how to upgrade the configuration of a cluster, see Upgrade the configuration of a cluster.
What do I do if the "The current Elasticsearch cluster configuration does not support this operation. Check the Elasticsearch cluster configuration first." message appears?
Solution: Run the GET _cluster/settings command to view the cluster configuration. Then, check whether the cluster configuration contains the settings that do not allow data allocation.
What do I do if data nodes fail to be removed or data fails to be migrated due to the auto_expand_replicas index setting?
Cause: Some users may use the access control feature provided by the X-Pack plug-in. In earlier Elasticsearch versions, this feature applies the "index.auto_expand_replicas" : "0-all" setting to the .security index by default. This causes errors when you migrate data or remove data nodes.
Solution:
1. Query index settings.
```
GET .security/_settings
```
  The following result is returned:
```
{
  ".security-6" : {
    "settings" : {
      "index" : {
        "number_of_shards" : "1",
        "auto_expand_replicas" : "0-all",
        "provided_name" : ".security-6",
        "format" : "6",
        "creation_date" : "1555142250367",
        "priority" : "1000",
        "number_of_replicas" : "9",
        "uuid" : "9t2hotc7S5OpPuKEIJ****",
        "version" : {
          "created" : "6070099"
        }
      }
    }
  }
}
```
2. Use one of the following methods to modify the auto_expand_replicas index setting:
  - Method 1:
```
PUT .security/_settings
{
  "index" : {
    "auto_expand_replicas" : "0-1"
  }
}
```
  - Method 2:
```
PUT .security/_settings
{
  "index" : {
    "auto_expand_replicas" : "false",
    "number_of_replicas" : "1"
  }
}
```
    Notice Set number_of_replicas based on your business requirements. Make sure that the number of replica shards enabled for each index is greater than or equal to one but no more than the number of available data nodes.