A Cluster is Unavailable Due to Improper Shard Allocation

Symptom

The cluster status is Unavailable.

On the Dev Tools page of Kibana, run the GET _cluster/health command to check the cluster health status. In the output, the value of status is red and the value of unassigned_shards is not 0. Alternatively, on the Cerebro page, click overview to view the index shard allocation on each data node. If the cluster status is red and the value of unassigned shards is not 0, there are index shards that cannot be allocated in the cluster.

Figure 1 Cluster health status

Figure 2 Cerebro page

Possible Causes

Some index shards in the cluster are not properly allocated.

Procedure

Step 2: Rectify the fault.

• Incorrect shard allocation policy
  • cluster.routing.allocation.enable
    1. If the value of explanation in the output is as follows, the current allocation policy of the cluster forbids the allocation of all shards.
       Figure 3 Incorrect configuration of allocation.enable
       
    2. On the Dev Tools page of Kibana, run the following command to set enable to all to allow all shards to be allocated:

       PUT _cluster/settings
       {
         "persistent": {
           "cluster": {
             "routing": {
               "allocation.enable": "all"
             }
           }
         }
       }

       

       The index-level configuration overwrites the cluster-level configuration. The parameters are described as follows:

       • all: Default value. All types of shards can be allocated.
       • primaries: Only the primary shards can be allocated.
       • new_primaries: Only the primary shards of the newly created index can be allocated.
       • none: No shards can be allocated.
    3. Run the POST _cluster/reroute?retry_failed=true command to manually allocate shards. Wait until all index shards are allocated and the cluster status changes to Available.
  • index.routing.allocation.total_shards_per_node
    1. If the value of explanation in the output is as follows, the value of index.routing.allocation.total_shards_per_node is too small and does not meet the index shard allocation requirements.
       Figure 4 Incorrect configuration of index total_shards_per_node
       
    2. On the Dev Tools page of Kibana, run the following command to change the number of index shards that can be allocated to each node:

       PUT index_name/_settings
       {
         "index": {
           "routing": {
             "allocation.total_shards_per_node": 3
           }
         }
       }

       

       Value of index.routing.allocation.total_shards_per_node = Number of index_name index shards/(Number of data nodes - 1)

       Set this parameter to a relative large value. Assume that a cluster has 10 nodes, including five data nodes, two client nodes, and three master nodes. The number of shards of an index is 30. If total_shards_per_node is set to 4, the total number of shards that can be allocated is: 4 x 5 = 20. Not all shards cannot be allocated. To allocate all shards in this index, at least six shards should be allocated to each node. In case a data node is faulty, at least eight shards should be allocated to each node.

    3. Run the POST _cluster/reroute?retry_failed=true command to manually allocate shards. Wait until the index shards are allocated and the cluster status changes to Available.
  • cluster.routing.allocation.total_shards_per_node
    1. If the value of explanation in the output is as follows, the number of shards that can be allocated to each data node in the cluster is too small.
       Figure 5 Incorrect configuration of cluster total_shards_per_node
       
    2. The value of cluster.routing.allocation.total_shards_per_node indicates the maximum number of shards that can be allocated to each data node in a cluster. The default value of this parameter is 1000. On the Dev Tools page of Kibana, run the following command to specify the cluster.routing.allocation.total_shards_per_node parameter:

       PUT _cluster/settings
       {
         "persistent": {
           "cluster": {
             "routing": {
               "allocation.total_shards_per_node": 1000
             }
           }
         }
       }

    3. In most cases, the problem occurs because index.routing.allocation.total_shards_per_node is mistakenly set to cluster.routing.allocation.total_shards_per_node. Run the following command to specify the index.routing.allocation.total_shards_per_node parameter:

       PUT index_name/_settings
       {
         "index": {
           "routing": {
             "allocation.total_shards_per_node": 30
           }
         }
       }

       

       Both of the following parameters are used to limit the maximum number of shards that can be allocated to a single data node:

       • cluster.routing.allocation.total_shards_per_node is used to limit shard allocation at the cluster level.
       • index.routing.allocation.total_shards_per_node is used to limit shard allocation at the index level.
    4. Run the POST _cluster/reroute?retry_failed=true command to manually allocate shards. Wait until the index shards are allocated and the cluster status changes to Available.
  • index.routing.allocation.include
    1. If the value of explanation in the output is as follows, index shards can only be allocated to data nodes with the hot label. If no data nodes in the cluster are labeled with hot, shards cannot be allocated.
       Figure 6 Incorrect configuration of include
       
    2. On the Dev Tools page of Kibana, run the following command to cancel the configuration:

       PUT index_name/_settings
       {
         "index.routing.allocation.include.box_type": null
       }

    3. Run the POST _cluster/reroute?retry_failed=true command to manually allocate shards. Wait until the index shards are allocated and the cluster status changes to Available.
  • index.routing.allocation.require
    1. If the value of explanation in the output is as follows, shards can only be allocated to data nodes with specified labels. If no nodes in the cluster have such labels, the shards cannot be allocated.
       Figure 7 Incorrect configuration of require
       
    2. On the Dev Tools page of Kibana, run the following command to cancel the configuration:

       PUT index_name/_settings
       {
         "index.routing.allocation.require.box_type": null
       }

    3. Run the POST _cluster/reroute?retry_failed=true command to manually allocate shards. Wait until the index shards are allocated and the cluster status changes to Available.
• Shard lock error
  1. In the output, explanation contains [failed to obtain in-memory shard lock]. This problem usually occurs when a node is removed from a cluster for a short time and then added back to the cluster, and a thread is performing a long-term data writing to a shard, such as bulk or scroll. When the node is added to the cluster again, the master node cannot allocate the shard because the shard lock is not released.
  2. This problem does not cause fragment data loss. You only need to allocate the shard again. On the Dev Tools page of Kibana, run the POST /_cluster/reroute?retry_failed=true command to manually allocate the unallocated shard. Wait until the index shards are allocated and the cluster status changes to Available.
• Inconsistent index setting and node version
  1. The value of index and explanation in the output are as follows, indicating that the parameter configuration of an index does not match the node version.
     Figure 8 Inconsistent index configuration
     
  2. Run the GET index_name/_settings command to check the index configuration. In the output, check whether the index features match the node version.
     Figure 9 Index configuration
     

     For example, assume that a cluster version is 7.9.3. The index feature index.routing.allocation.include._tier_preference is supported by clusters of the version later than 7.10. If you use this feature in a cluster of the version earlier than 7.10, index shards cannot be allocated. As a result, the cluster is unavailable.

  3. Determine whether the inapplicable feature is mandatory for the cluster.
     • If yes, create a cluster of the required version and restore the data of the old cluster to the new cluster using the backup.
     • If no, go to the next step.
  4. Run the following command to remove the inapplicable index feature:

     PUT /index_name/_settings
     {
       "index.routing.allocation.include._tier_preference": null
     }

  5. Run the POST /_cluster/reroute?retry_failed=true command to manually allocate the unallocated shards. Wait until the index shards are allocated and the cluster status changes to Available.
• Damaged primary shard data
  1. The values of index, shard, allocate_explanation, and store_exception in the output are as follows, indicating that the data of a shard in an index is damaged.
     Figure 10 Damaged primary shard data
     
  2. When the index data is damaged or the primary backup of a shard is lost, run the following command to define an empty shard and specify the node to be allocated:

     POST /_cluster/reroute
     {
         "commands" : [
                 {
                 "allocate_empty_primary" : {
                     "index" : "index_name", 
                     "shard" : 2,
                     "node" : "node_name",
                     "accept_data_loss":true
                 }
             }
         ]
     }

     

     Data in the corresponding shard will be completely cleared. Exercise caution when performing this operation.

  3. After index shards are reallocated, the cluster status becomes Available.
• Excessive disk usage
  1. The output is as follows. The value of allocate_explanation indicates that shards of an index cannot be allocated to any data node, and the value of explanation indicates that node disk usage reaches the upper limit.
     Figure 11 Query result
     
     

     • If the disk usage of a node exceeds 85%, new shards will not be allocated to this node.
     • If the disk usage of a node exceeds 90%, the cluster attempts to migrate the shards of this node to other data nodes with low disk usage. If data cannot be migrated, the system forcibly sets the read_only_allow_delete attribute for each index in the cluster. In this case, data cannot be written to indexes, and the indexes can only be read or deleted.
     • A node may be disconnected due to high disk usage. After the node automatically recovers, once the cluster is overloaded, the cluster may fail to respond when you call the Elasticsearch API to query the cluster status. If the cluster status cannot be updated in a timely manner, the cluster becomes Unavailable.
  2. Increase the available disk capacity of the cluster.
     • On the Dev Tools page of Kibana, run the DELETE index_name command to clear invalid data in the cluster to release disk space.
     • Temporarily reduce the number of index copies. After the disk or node capacity is expanded, change the number of index copies back to the original value.
       1. On the Dev Tools page of Kibana, run the following command to temporarily reduce the number of index copies:

          PUT index_name/_settings
          {
            "number_of_replicas": 1
          }

          The output is as follows.

          Figure 12 Index status of read-only-allow-delete
          

          The disk usage exceeds the maximum value allowed by the disk space. The system forcibly sets the read_only_allow_delete attribute for all indexes in the cluster. Run the following command to set the attribute value to null, and then run the command in 2.a to reduce the number of index copies:

          PUT /_settings
          {
            "index.blocks.read_only_allow_delete": null
          }

       2. Expand the number of nodes or node storage capacity of the cluster by referring to Scaling Out a Cluster.
       3. After the scale-out is complete, run the command in step 2.a to change the number of index copies back. After all index shards are allocated, the cluster status changes to Available.