[[misc-cluster-settings]]
==== Miscellaneous cluster settings

[[cluster-read-only]]
===== Metadata

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

`cluster.blocks.read_only`::
      (<<dynamic-cluster-setting,Dynamic>>)
      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`::
      (<<dynamic-cluster-setting,Dynamic>>)
      Identical to `cluster.blocks.read_only` but allows to delete indices
      to free up resources.

WARNING: Don't rely on this setting to prevent changes to your cluster. Any
user with access to the <<cluster-update-settings,cluster-update-settings>>
API can make the cluster read-write again.


[[cluster-shard-limit]]
===== Cluster shard limit

There is a soft limit on the number of shards in a cluster, based on the number
of nodes in the cluster. This is intended to prevent operations which may
unintentionally destabilize the cluster.

IMPORTANT: This limit is intended as a safety net, not a sizing recommendation. The
exact number of shards your cluster can safely support depends on your hardware
configuration and workload, but should remain well below this limit in almost
all cases, as the default limit is set quite high.

If an operation, such as creating a new index, restoring a snapshot of an index,
or opening a closed index would lead to the number of shards in the cluster
going over this limit, the operation will fail with an error indicating the
shard limit.

If the cluster is already over the limit, due to changes in node membership or
setting changes, all operations that create or open indices will fail until
either the limit is increased as described below, or some indices are
<<indices-open-close,closed>> or <<indices-delete-index,deleted>> to bring the
number of shards below the limit.

The cluster shard limit defaults to 1,000 shards per data node. 
Both primary and replica shards of all open indices count toward the limit, 
including unassigned shards. 
For example, an open index with 5 primary shards and 2 replicas counts as 15 shards. 
Closed indices do not contribute to the shard count. 

You can dynamically adjust the cluster shard limit with the following setting:

`cluster.max_shards_per_node`::
     (<<dynamic-cluster-setting,Dynamic>>)
     Controls the number of shards allowed in the cluster per data node.

With the default setting, a 3-node cluster allows 3,000 shards total, across all open indexes. 
If you reduce the limit to 500, the cluster would allow 1,500 shards total.

NOTE: If there are no data nodes in the cluster, the limit will not be enforced.
This allows the creation of indices during cluster creation if dedicated master
nodes are set up before data nodes.

[[user-defined-data]]
===== User-defined cluster metadata

User-defined metadata can be stored and retrieved using the Cluster Settings API.
This can be used to store arbitrary, infrequently-changing data about the cluster
without the need to create an index to store it. This data may be stored using
any key prefixed with `cluster.metadata.`.  For example, to store the email
address of the administrator of a cluster under the key `cluster.metadata.administrator`,
issue this request:

[source,console]
-------------------------------
PUT /_cluster/settings
{
  "persistent": {
    "cluster.metadata.administrator": "sysadmin@example.com"
  }
}
-------------------------------

IMPORTANT: User-defined cluster metadata is not intended to store sensitive or
confidential information. Any information stored in user-defined cluster
metadata will be viewable by anyone with access to the
<<cluster-get-settings,Cluster Get Settings>> API, and is recorded in the
{es} logs.

[[cluster-max-tombstones]]
===== Index tombstones

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 setting:

`cluster.indices.tombstones.size`::
(<<static-cluster-setting,Static>>)
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.

include::{es-repo-dir}/indices/dangling-indices-list.asciidoc[tag=dangling-index-description]
You can use the <<dangling-indices-api,Dangling indices API>> to manage
this situation.

[[cluster-logger]]
===== Logger

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

[source,console]
-------------------------------
PUT /_cluster/settings
{
  "transient": {
    "logger.org.elasticsearch.indices.recovery": "DEBUG"
  }
}
-------------------------------


[[persistent-tasks-allocation]]
===== Persistent tasks allocation

Plugins can create a kind of tasks called persistent tasks. Those tasks are
usually long-lived 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 node 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 settings:

`cluster.persistent_tasks.allocation.enable`::
+
--
(<<dynamic-cluster-setting,Dynamic>>)
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.
--

`cluster.persistent_tasks.allocation.recheck_interval`::
     (<<dynamic-cluster-setting,Dynamic>>)
     The master node will automatically check whether persistent tasks need to
     be assigned when the cluster state changes significantly. However, there
     may be other factors, such as memory usage, that affect whether persistent
     tasks can be assigned to nodes but do not cause the cluster state to change.
     This setting controls how often assignment checks are performed to react to
     these factors. The default is 30 seconds. The minimum permitted value is 10
     seconds.
