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Whenever any ES nodes go down, Elasticsearch immediately begins regenerating metadata (reallocating those shards) when an ES nodes becomes unresponsive. In controlled situations such as a rolling restart , you can take steps steps can be taken to minimize the impact.

A rolling restart of the ES cluster (keeping it operational while taking nodes offline one at a time) may be needed to upgrade the Elasticsearch version or to perform maintenance on the server itself ( such as an OS update or hardware change). This process keep the cluster operational while taking nodes offline one at a time. Because Elasticsearch prefers to keep data fully replicated and evenly balanced, it must be made to pause rebalancing until the rolling restart completes.

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1. Start maintenance mode by changing the ES settings.

2. Complete the maintenance work (such as upgrading ES) and rolling restart. 

3. Stop maintenance mode by restoring the ES settings.

You should follow Follow the steps in the Elasticsearch documentation, with this important addition for versions before Elasticsearch 7:

1. When you first configure the Elasticsearch cluster, you set the Set the discovery.zen.minimum_master_nodes value in the /etc/elasticsearch/elasticsearch.yml config file to be: (number of master-eligible-nodes/2, rounded down) + 1 when the elasticsearch cluster is first configured.
   Verify the number is smaller than the number of currently available nodes. If not, run Run the following command now to set that number to be <current_min_master_nodes>: (current number of master-eligible-nodes/2, rounded down) + 1 if not.

   Code Block
   language bash
   curl -s -XPUT 'http://ES_NODE:9200/_cluster/settings' \ --data-binary '{"transient": {"discovery.zen.minimum_master_nodes" : "<current_min_master_nodes>"}}'

2. Follow the Elasticsearch rolling restart procedure for the version:

   • ES Rolling Upgrades (current)

   • ES 5.6 Rolling Upgrades

   • ES 2.3.3 Rolling Restarts

3. After completing the rolling restart, reset Reset discovery.zen.minimum_master_nodes to its original value, or adjust it based on the current number of expected available nodes after completing the rolling restart:

   Code Block
   language bash
   curl -s -XPUT 'http://ES_NODE:9200/_cluster/settings' \ --data-binary '{"transient": {"discovery.zen.minimum_master_nodes" : "<original_min_master_nodes>"}}'