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Elastic Docs ›Elasticsearch Guide [6.4] ›Modules ›Cluster

Miscellaneous cluster settings

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Miscellaneous cluster settings

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Metadata

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An entire cluster may be set to read-only with the following dynamic setting:

cluster.blocks.read_only
Make the whole cluster read only (indices do not accept write operations), metadata is not allowed to be modified (create or delete indices).
cluster.blocks.read_only_allow_delete
Identical to cluster.blocks.read_only but allows to delete indices to free up resources.

Don’t rely on this setting to prevent changes to your cluster. Any user with access to the cluster-update-settings API can make the cluster read-write again.

Index Tombstones

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The cluster state maintains index tombstones to explicitly denote indices that have been deleted. The number of tombstones maintained in the cluster state is controlled by the following property, which cannot be updated dynamically:

cluster.indices.tombstones.size
Index tombstones prevent nodes that are not part of the cluster when a delete occurs from joining the cluster and reimporting the index as though the delete was never issued. To keep the cluster state from growing huge we only keep the last cluster.indices.tombstones.size deletes, which defaults to 500. You can increase it if you expect nodes to be absent from the cluster and miss more than 500 deletes. We think that is rare, thus the default. Tombstones don’t take up much space, but we also think that a number like 50,000 is probably too big.

Logger

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The settings which control logging can be updated dynamically with the logger. prefix. For instance, to increase the logging level of the indices.recovery module to DEBUG, issue this request:

PUT /_cluster/settings
{
  "transient": {
    "logger.org.elasticsearch.indices.recovery": "DEBUG"
  }
}

Persistent Tasks Allocations

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Plugins can create a kind of tasks called persistent tasks. Those tasks are usually long-live tasks and are stored in the cluster state, allowing the tasks to be revived after a full cluster restart.

Every time a persistent task is created, the master nodes takes care of assigning the task to a node of the cluster, and the assigned node will then pick up the task and execute it locally. The process of assigning persistent tasks to nodes is controlled by the following property, which can be updated dynamically:

cluster.persistent_tasks.allocation.enable

Enable or disable allocation for persistent tasks:

  • all - (default) Allows persistent tasks to be assigned to nodes
  • none - No allocations are allowed for any type of persistent task

This setting does not affect the persistent tasks that are already being executed. Only newly created persistent tasks, or tasks that must be reassigned (after a node left the cluster, for example), are impacted by this setting.

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