Changing the Node Specifications and Storage Type of an Elasticsearch Cluster

Prerequisites

• The cluster status is Available, and there are no ongoing tasks.
• Your CSS resource quotas are sufficient for the capacity expansion you are about to perform. You can check available resources on the Modify Configuration page.
  Figure 1 Checking available resources
  
• All mission-critical data has been backed up before a node storage type change. This is to prevent data loss.

Impact on Billing

For a pay-per-use cluster, you can see its new price when confirming the node specifications or storage type change on the console. After the change is complete, the new price will apply. For pricing details, see Cloud Search Service Price Calculator.

For a yearly/monthly cluster, you can also see the new price when you confirm the change. In the case of an upgrade, a new order is triggered when you confirm the change; and in the case of a downgrade, a refund request is triggered when you do so.

Constraints

• The node specifications and storage type cannot be changed for nodes that use local disks.
• The node specifications and storage type cannot be changed at the same time.
• The node storage type can be changed only for data nodes and cold data nodes.
• When you change the node storage type, data needs to be migrated between different nodes. The timeout for data migration per node is 48 hours. Upgrade will fail if this timeout expires. When the cluster has large quantities of data, you are advised to manually adjust the data migration rate and avoid performing the migration during peak hours.
• For a cluster without master nodes, the node storage type can be changed only if the number of data nodes plus cold data nodes is at least three.
• For a cluster with master nodes, this operation is allowed only if the cluster has at least two data nodes.
• During a node storage type change, there is always one node that is unavailable. To ensure service continuity, make sure the total number of data nodes and cold data nodes is greater than the maximum number of index replicas plus 1. For a single-AZ or dual-AZ cluster, also make sure there are at least two nodes of each type for the cluster in each of its AZs.
• During a node specifications changes, nodes are brought offline in order to make the changes. To ensure service continuity, make sure all shards have replicas.
• Make sure the disk usage is always less than 80% during the change.

Change Impact

Before changing a cluster's node specifications or storage type, it is essential to assess the potential impacts and review operational recommendations. This enables proper scheduling of the change, minimizing service interruptions.

• Performance impact (only when the node storage type is changed)

  Changing the node storage type does not interrupt services. However, data migration that occurs during this process consumes I/O performance, and taking individual nodes offline still has some impact on the overall cluster performance.

  To minimize this impact, it is advisable to adjust the data migration rate based on the cluster's traffic cycle: increase the data migration rate during off-peak hours to shorten the task duration, and decrease it before peak hours arrive to ensure optimal cluster performance. The data migration rate is determined by the indices.recovery.max_bytes_per_sec parameter. The default value of this parameter is the number of vCPUs multiplied by 32 MB. For example, for four vCPUs, the data migration rate is 128 MB. Set this parameter to a value between 40 MB and 1000 MB based on your service requirements.

  PUT /_cluster/settings  
  {  
    "transient": {  
      "indices.recovery.max_bytes_per_sec": "1000MB"  
    }  
  }

• Impact on request handling

  Taking nodes offline one at a time usually does not interrupt services. However, requests sent to offline nodes may fail. To mitigate this impact, the following measures may be taken:

  • Use a VPC endpoint or a dedicated load balancer to handle access requests to your cluster, which makes sure that requests are automatically routed to available nodes.
  • Enable an exponential backoff & retry mechanism on the client (configure three retries).
  • Perform this operation during off-peak hours.
• Impact on index replicas

  Shards that have no replicas will become unavailable when the nodes that store them are taken offline, causing service interruptions. You are advised to add replicas for all important indexes before making the change described in this topic.

• Impact on Kibana and Cerebro

  Changing the node storage type for a cluster will cause Kibana and Cerebro to be rebuilt. During this period, Kibana and Cerebro are temporarily unavailable. During a node specifications change, if Kibana and Cerebro become unavailable because the node that runs them is taken offline, refresh the web page or try to log in again, and the system will reschedule Kibana and Cerebro to an available node.

• Characteristics of this process

  Once started, a change task cannot be stopped until it succeeds or fails. A change task failure only impacts a single node, and does not interrupt services if there are data replicas, but the failed node still needs to be restored promptly.

If a change of the node specifications or storage type is urgently needed, submit a service ticket to contact technical support and perform a pre-change evaluation and the necessary checks.

Change Durations

• The following formulas can be used to estimate how long the node specifications change will take:

  Change duration (min) = 10 (min) x Total number of nodes to change + Data recovery duration (min)

  where,

  • 10 minutes indicates how long non-data recovery operations (e.g., initialization) typically take per node. It is an empirical value.
  • The total number of nodes is the sum of the number of data nodes, master nodes, client nodes, and cold data nodes in the cluster.

  Data recovery duration (min) = Total data size (MB)/[Total number of vCPUs of the data nodes x 32 (MB/s) x 60 (s)]

  where,

  • 32 MB/s indicates that each vCPU can process 32 MB of data per second. It is an empirical value.
  • The formulas above use estimates under ideal conditions. The actual data recovery speed depends on cluster load.

• The following formulas can be used to estimate how long the node storage type change will take:

  Change duration (min) = 15 (min) x Total number of nodes to change + Data migration duration (min)

  where,

  • 15 minutes indicates how long non-data migration operations (e.g., initialization) typically take per node. It is an empirical value.
  • The total number of nodes is the sum of the number of data nodes, master nodes, client nodes, and cold data nodes in the cluster.

  Data migration duration (min) = Total data size (MB)/[Total number of vCPUs of the data nodes x 32 (MB/s) x 60 (s)]

  where,

  • 32 MB/s indicates that each vCPU can process 32 MB of data per second. It is an empirical value.
  • The formulas above use estimates under ideal conditions. The actual migration speed depends on cluster load.

Changing the Node Specifications and Storage Type

1. Log in to the CSS management console.
2. In the navigation pane on the left, choose Clusters > Elasticsearch to show the Elasticsearch cluster list.
3. Check that all service data has replicas so that services will not be interrupted during the change.
   1. In the cluster list, find the target cluster, and click Access Kibana in the Operation column to log in to the Kibana console.
   2. Click Dev Tools in the navigation tree on the left.
   3. Run the GET _cat/indices?v command in Kibana.
      • If the returned rep value is greater than 0, data replicas exist. Go to the next step.
      • If the returned rep value is 0, there is no data replicas. You are advised to perform Manually Creating a Snapshot before changing node specifications or storage type for the cluster.
4. In the cluster list, select the target cluster, and choose More > Modify Configuration in the Operation column. The Modify Configuration page is displayed.
5. Click the Scale Cluster tab.
6. Click Change specifications to set parameters.

   Table 2 Changing the node specifications and storage type

   Parameter	Description
   Action	Select Change specifications.
   Resources	Shows the change of resources for this operation.
   Nodes	Configure the changes you want to make. Select a node type in the Node Type column. Select a new flavor in the Node Specifications column, or select a new storage type in the Node Storage column. The node specifications and storage type cannot be changed at the same time.

   Figure 2 Changing specifications
   
7. Click Next.
8. Confirm the information and click Submit.
9. In the displayed dialog box, confirm check items, and click OK to start the specifications change.
   • Check items for node specifications change: Verify index copies and Cluster status check.
   • Check items for node storage type change: Check cluster load.

   Table 3 Check item description

   Item	Description
   Verify index copies	By default, CSS checks for indexes that do not have any replicas created for them. You can skip this step, but the lack of index replicas may impact service availability during a node specifications change. If you select Verify index copies and the cluster has no master nodes, each index is required to have at least one replica and the cluster must have at least three nodes. If you select Verify index copies and the cluster has a master node, each index is required to have at least one replica, but there is no requirement on how many nodes the cluster must have.
   Cluster status check	By default, the cluster status is checked before specifications change. The specifications of nodes are changed one by one to ensure service availability and data security. If a cluster is overloaded and services are faulty, the request for a specifications change may fail to be delivered. In this case, you can skip cluster status check, but doing so may cause the cluster to become faulty and services become interrupted. Exercise caution when performing this operation.
   Check cluster load	During a node storage type change, data migration between nodes and the stopping and restarting of nodes will consume cluster resources, causing the cluster load to increase. A cluster load check can identify possible overload risks for a cluster and reduce the likelihood of an overload condition causing the node storage type change to fail. The cluster load check items are as follows: nodes.thread_pool.search.queue < 1000: Check that the maximum number of requests in the search queue is less than 1000. nodes.thread_pool.write.queue < 200: Check that the maximum number of requests in the write queue is less than 200. nodes.process.cpu.percent < 90: Check that the maximum CPU usage is less than 90%. nodes.os.cpu.load_average/Number of vCPUs < 80%: Check that the number of running processes plus the number of processes waiting for CPUs is less than 80% of the total number of vCPUs.

   

   If the change request fails to be submitted and a message is displayed indicating that the cluster needs to be upgraded, it means the current cluster version does not support a node storage type change. Upgrade the cluster to the latest image version and then try again. For a detailed upgrade guide, see Upgrading the Version of an Elasticsearch Cluster.

10. Click Back to Cluster List to go to the Clusters page. The Cluster Status is Configuration modified. When Cluster Status changes to Available, the cluster node specifications have been successfully changed.