elasticsearch/docs/reference/how-to/fix-common-cluster-issues.asciidoc

[[fix-common-cluster-issues]]
== Fix common cluster issues

This guide describes how to fix common errors and problems with {es} clusters.

[discrete]
=== Error: disk usage exceeded flood-stage watermark, index has read-only-allow-delete block

This error indicates a data node is critically low on disk space and has reached
the <<cluster-routing-flood-stage,flood-stage disk usage watermark>>. To prevent
a full disk, when a node reaches this watermark, {es} blocks writes to any index
with a shard on the node. If the block affects related system indices, {kib} and
other {stack} features may become unavailable.

{es} will automatically remove the write block when the affected node's disk
usage goes below the <<cluster-routing-watermark-high,high disk watermark>>. To
achieve this, {es} automatically moves some of the affected node's shards to
other nodes in the same data tier.

To verify that shards are moving off the affected node, use the <<cat-shards,cat
shards API>>.

[source,console]
----
GET _cat/shards?v=true
----

If shards remain on the node, use the <<cluster-allocation-explain,cluster
allocation explanation API>> to get an explanation for their allocation status.

[source,console]
----
GET _cluster/allocation/explain
{
  "index": "my-index",
  "shard": 0,
  "primary": false,
  "current_node": "my-node"
}
----
// TEST[s/^/PUT my-index\n/]
// TEST[s/"primary": false,/"primary": false/]
// TEST[s/"current_node": "my-node"//]

To immediately restore write operations, you can temporarily increase the disk
watermarks and remove the write block.

[source,console]
----
PUT _cluster/settings
{
  "persistent": {
    "cluster.routing.allocation.disk.watermark.low": "90%",
    "cluster.routing.allocation.disk.watermark.high": "95%",
    "cluster.routing.allocation.disk.watermark.flood_stage": "97%"
  }
}

PUT */_settings?expand_wildcards=all
{
  "index.blocks.read_only_allow_delete": null
}
----
// TEST[s/^/PUT my-index\n/]

As a long-term solution, we recommend you add nodes to the affected data tiers
or upgrade existing nodes to increase disk space. To free up additional disk
space, you can delete unneeded indices using the <<indices-delete-index,delete
index API>>.

[source,console]
----
DELETE my-index
----
// TEST[s/^/PUT my-index\n/]

When a long-term solution is in place, reset or reconfigure the disk watermarks.

[source,console]
----
PUT _cluster/settings
{
  "persistent": {
    "cluster.routing.allocation.disk.watermark.low": null,
    "cluster.routing.allocation.disk.watermark.high": null,
    "cluster.routing.allocation.disk.watermark.flood_stage": null
  }
}
----

[discrete]
[[circuit-breaker-errors]]
=== Circuit breaker errors

{es} uses <<circuit-breaker,circuit breakers>> to prevent nodes from running out
of JVM heap memory. If Elasticsearch estimates an operation would exceed a
circuit breaker, it stops the operation and returns an error.

By default, the <<parent-circuit-breaker,parent circuit breaker>> triggers at
95% JVM memory usage. To prevent errors, we recommend taking steps to reduce
memory pressure if usage consistently exceeds 85%.

[discrete]
[[diagnose-circuit-breaker-errors]]
==== Diagnose circuit breaker errors

**Error messages**

If a request triggers a circuit breaker, {es} returns an error with a `429` HTTP
status code.

[source,js]
----
{
  'error': {
    'type': 'circuit_breaking_exception',
    'reason': '[parent] Data too large, data for [<http_request>] would be [123848638/118.1mb], which is larger than the limit of [123273216/117.5mb], real usage: [120182112/114.6mb], new bytes reserved: [3666526/3.4mb]',
    'bytes_wanted': 123848638,
    'bytes_limit': 123273216,
    'durability': 'TRANSIENT'
  },
  'status': 429
}
----
// NOTCONSOLE

{es} also writes circuit breaker errors to <<logging,`elasticsearch.log`>>. This
is helpful when automated processes, such as allocation, trigger a circuit
breaker.

[source,txt]
----
Caused by: org.elasticsearch.common.breaker.CircuitBreakingException: [parent] Data too large, data for [<transport_request>] would be [num/numGB], which is larger than the limit of [num/numGB], usages [request=0/0b, fielddata=num/numKB, in_flight_requests=num/numGB, accounting=num/numGB]
----

**Check JVM memory usage**

If you've enabled Stack Monitoring, you can view JVM memory usage in {kib}. In
the main menu, click **Stack Monitoring**. On the Stack Monitoring **Overview**
page, click **Nodes**. The **JVM Heap** column lists the current memory usage
for each node.

You can also use the <<cat-nodes,cat nodes API>> to get the current
`heap.percent` for each node.

[source,console]
----
GET _cat/nodes?v=true&h=name,node*,heap*
----

To get the JVM memory usage for each circuit breaker, use the
<<cluster-nodes-stats,node stats API>>.

[source,console]
----
GET _nodes/stats/breaker
----

[discrete]
[[prevent-circuit-breaker-errors]]
==== Prevent circuit breaker errors

**Reduce JVM memory pressure**

High JVM memory pressure often causes circuit breaker errors. See
<<high-jvm-memory-pressure>>.

**Avoid using fielddata on `text` fields**

For high-cardinality `text` fields, fielddata can use a large amount of JVM
memory. To avoid this, {es} disables fielddata on `text` fields by default. If
you've enabled fielddata and triggered the <<fielddata-circuit-breaker,fielddata
circuit breaker>>, consider disabling it and using a `keyword` field instead.
See <<fielddata>>.

**Clear the fieldata cache**

If you've triggered the fielddata circuit breaker and can't disable fielddata,
use the <<indices-clearcache,clear cache API>> to clear the fielddata cache.
This may disrupt any in-flight searches that use fielddata.

[source,console]
----
POST _cache/clear?fielddata=true
----
// TEST[s/^/PUT my-index\n/]

[discrete]
[[high-cpu-usage]]
=== High CPU usage

{es} uses <<modules-threadpool,thread pools>> to manage CPU resources for
concurrent operations. High CPU usage typically means one or more thread pools
are running low.

If a thread pool is depleted, {es} will <<rejected-requests,reject requests>>
related to the thread pool. For example, if the `search` thread pool is
depleted, {es} will reject search requests until more threads are available.

[discrete]
[[diagnose-high-cpu-usage]]
==== Diagnose high CPU usage

**Check CPU usage**

include::{es-repo-dir}/tab-widgets/cpu-usage-widget.asciidoc[]

**Check hot threads**

If a node has high CPU usage, use the <<cluster-nodes-hot-threads,nodes hot
threads API>> to check for resource-intensive threads running on the node.

[source,console]
----
GET _nodes/my-node,my-other-node/hot_threads
----
// TEST[s/\/my-node,my-other-node//]

This API returns a breakdown of any hot threads in plain text.

[discrete]
[[reduce-cpu-usage]]
==== Reduce CPU usage

The following tips outline the most common causes of high CPU usage and their
solutions.

**Scale your cluster**

Heavy indexing and search loads can deplete smaller thread pools. To better
handle heavy workloads, add more nodes to your cluster or upgrade your existing
nodes to increase capacity.

**Spread out bulk requests**

While more efficient than individual requests, large <<docs-bulk,bulk indexing>>
or <<search-multi-search,multi-search>> requests still require CPU resources. If
possible, submit smaller requests and allow more time between them.

**Cancel long-running searches**

Long-running searches can block threads in the `search` thread pool. To check
for these searches, use the <<tasks,task management API>>.

[source,console]
----
GET _tasks?actions=*search&detailed
----

The response's `description` contains the search request and its queries.
`running_time_in_nanos` shows how long the search has been running.

[source,console-result]
----
{
  "nodes" : {
    "oTUltX4IQMOUUVeiohTt8A" : {
      "name" : "my-node",
      "transport_address" : "127.0.0.1:9300",
      "host" : "127.0.0.1",
      "ip" : "127.0.0.1:9300",
      "tasks" : {
        "oTUltX4IQMOUUVeiohTt8A:464" : {
          "node" : "oTUltX4IQMOUUVeiohTt8A",
          "id" : 464,
          "type" : "transport",
          "action" : "indices:data/read/search",
          "description" : "indices[my-index], search_type[QUERY_THEN_FETCH], source[{\"query\":...}]",
          "start_time_in_millis" : 4081771730000,
          "running_time_in_nanos" : 13991383,
          "cancellable" : true
        }
      }
    }
  }
}
----
// TESTRESPONSE[skip: no way to get tasks]

To cancel a search and free up resources, use the API's `_cancel` endpoint.

[source,console]
----
POST _tasks/oTUltX4IQMOUUVeiohTt8A:464/_cancel
----

For additional tips on how to track and avoid resource-intensive searches, see
<<avoid-expensive-searches,Avoid expensive searches>>.

[discrete]
[[high-jvm-memory-pressure]]
=== High JVM memory pressure

High JVM memory usage can degrade cluster performance and trigger
<<circuit-breaker-errors,circuit breaker errors>>. To prevent this, we recommend
taking steps to reduce memory pressure if a node's JVM memory usage consistently
exceeds 85%.

[discrete]
[[diagnose-high-jvm-memory-pressure]]
==== Diagnose high JVM memory pressure

**Check JVM memory pressure**

include::{es-repo-dir}/tab-widgets/jvm-memory-pressure-widget.asciidoc[]

**Check garbage collection logs**

As memory usage increases, garbage collection becomes more frequent and takes
longer. You can track the frequency and length of garbage collection events in
<<logging,`elasticsearch.log`>>. For example, the following event states {es}
spent more than 50% (21 seconds) of the last 40 seconds performing garbage
collection.

[source,log]
----
[timestamp_short_interval_from_last][INFO ][o.e.m.j.JvmGcMonitorService] [node_id] [gc][number] overhead, spent [21s] collecting in the last [40s]
----

[discrete]
[[reduce-jvm-memory-pressure]]
==== Reduce JVM memory pressure

**Reduce your shard count**

Every shard uses memory. In most cases, a small set of large shards uses fewer
resources than many small shards. For tips on reducing your shard count, see
<<size-your-shards>>.

[[avoid-expensive-searches]]
**Avoid expensive searches**

Expensive searches can use large amounts of memory. To better track expensive
searches on your cluster, enable <<index-modules-slowlog,slow logs>>.

Expensive searches may have a large <<paginate-search-results,`size` argument>>,
use aggregations with a large number of buckets, or include
<<query-dsl-allow-expensive-queries,expensive queries>>. To prevent expensive
searches, consider the following setting changes:

* Lower the `size` limit using the
<<index-max-result-window,`index.max_result_window`>> index setting.

* Decrease the maximum number of allowed aggregation buckets using the
<<search-settings-max-buckets,search.max_buckets>> cluster setting.

* Disable expensive queries using the
<<query-dsl-allow-expensive-queries,`search.allow_expensive_queries`>> cluster
setting.

[source,console]
----
PUT _settings
{
  "index.max_result_window": 5000
}

PUT _cluster/settings
{
  "persistent": {
    "search.max_buckets": 20000,
    "search.allow_expensive_queries": false
  }
}
----
// TEST[s/^/PUT my-index\n/]

**Prevent mapping explosions**

Defining too many fields or nesting fields too deeply can lead to
<<mapping-limit-settings,mapping explosions>> that use large amounts of memory.
To prevent mapping explosions, use the <<mapping-settings-limit,mapping limit
settings>> to limit the number of field mappings.

**Spread out bulk requests**

While more efficient than individual requests, large <<docs-bulk,bulk indexing>>
or <<search-multi-search,multi-search>> requests can still create high JVM
memory pressure. If possible, submit smaller requests and allow more time
between them.

**Upgrade node memory**

Heavy indexing and search loads can cause high JVM memory pressure. To better
handle heavy workloads, upgrade your nodes to increase their memory capacity.

[discrete]
[[red-yellow-cluster-status]]
=== Red or yellow cluster status

A red or yellow cluster status indicates one or more shards are missing or
unallocated. These unassigned shards increase your risk of data loss and can
degrade cluster performance.

[discrete]
[[diagnose-cluster-status]]
==== Diagnose your cluster status

**Check your cluster status**

Use the <<cluster-health,cluster health API>>.

[source,console]
----
GET _cluster/health?filter_path=status,*_shards
----

A healthy cluster has a green `status` and zero `unassigned_shards`. A yellow
status means only replicas are unassigned. A red status means one or
more primary shards are unassigned.

**View unassigned shards**

To view unassigned shards, use the <<cat-shards,cat shards API>>.

[source,console]
----
GET _cat/shards?v=true&h=index,shard,prirep,state,node,unassigned.reason&s=state
----

Unassigned shards have a `state` of `UNASSIGNED`. The `prirep` value is `p` for
primary shards and `r` for replicas.

To understand why an unassigned shard is not being assigned and what action
you must take to allow {es} to assign it, use the
<<cluster-allocation-explain,cluster allocation explanation API>>.

[source,console]
----
GET _cluster/allocation/explain?filter_path=index,node_allocation_decisions.node_name,node_allocation_decisions.deciders.*
{
  "index": "my-index",
  "shard": 0,
  "primary": false,
  "current_node": "my-node"
}
----
// TEST[s/^/PUT my-index\n/]
// TEST[s/"primary": false,/"primary": false/]
// TEST[s/"current_node": "my-node"//]

[discrete]
[[fix-red-yellow-cluster-status]]
==== Fix a red or yellow cluster status

A shard can become unassigned for several reasons. The following tips outline the
most common causes and their solutions.

**Re-enable shard allocation**

You typically disable allocation during a <<restart-cluster,restart>> or other
cluster maintenance. If you forgot to re-enable allocation afterward, {es} will
be unable to assign shards. To re-enable allocation, reset the
`cluster.routing.allocation.enable` cluster setting.

[source,console]
----
PUT _cluster/settings
{
  "persistent" : {
    "cluster.routing.allocation.enable" : null
  }
}
----

**Recover lost nodes**

Shards often become unassigned when a data node leaves the cluster. This can
occur for several reasons, ranging from connectivity issues to hardware failure.
After you resolve the issue and recover the node, it will rejoin the cluster.
{es} will then automatically allocate any unassigned shards.

To avoid wasting resources on temporary issues, {es} <<delayed-allocation,delays
allocation>> by one minute by default. If you've recovered a node and don’t want
to wait for the delay period, you can call the <<cluster-reroute,cluster reroute
API>> with no arguments to start the allocation process. The process runs
asynchronously in the background.

[source,console]
----
POST _cluster/reroute
----

**Fix allocation settings**

Misconfigured allocation settings can result in an unassigned primary shard.
These settings include:

* <<shard-allocation-filtering,Shard allocation>> index settings
* <<cluster-shard-allocation-filtering,Allocation filtering>> cluster settings
* <<shard-allocation-awareness,Allocation awareness>> cluster settings

To review your allocation settings, use the <<indices-get-settings,get index
settings>> and <<cluster-get-settings,cluster get settings>> APIs.

[source,console]
----
GET my-index/_settings?flat_settings=true&include_defaults=true

GET _cluster/settings?flat_settings=true&include_defaults=true
----
// TEST[s/^/PUT my-index\n/]

You can change the settings using the <<indices-update-settings,update index
settings>> and <<cluster-update-settings,cluster update settings>> APIs.

**Allocate or reduce replicas**

To protect against hardware failure, {es} will not assign a replica to the same
node as its primary shard. If no other data nodes are available to host the
replica, it remains unassigned. To fix this, you can:

* Add a data node to the same tier to host the replica.

* Change the `index.number_of_replicas` index setting to reduce the number of
replicas for each primary shard. We recommend keeping at least one replica per
primary.

[source,console]
----
PUT _settings
{
  "index.number_of_replicas": 1
}
----
// TEST[s/^/PUT my-index\n/]

**Free up or increase disk space**

{es} uses a <<disk-based-shard-allocation,low disk watermark>> to ensure data
nodes have enough disk space for incoming shards. By default, {es} does not
allocate shards to nodes using more than 85% of disk space.

To check the current disk space of your nodes, use the <<cat-allocation,cat
allocation API>>.

[source,console]
----
GET _cat/allocation?v=true&h=node,shards,disk.*
----

If your nodes are running low on disk space, you have a few options:

* Upgrade your nodes to increase disk space.

* Delete unneeded indices to free up space. If you use {ilm-init}, you can
update your lifecycle policy to use <<ilm-searchable-snapshot,searchable
snapshots>> or add a delete phase. If you no longer need to search the data, you
can use a <<snapshot-restore,snapshot>> to store it off-cluster.

* If you no longer write to an index, use the <<indices-forcemerge,force merge
API>> or {ilm-init}'s <<ilm-forcemerge,force merge action>> to merge its
segments into larger ones.
+
[source,console]
----
POST my-index/_forcemerge
----
// TEST[s/^/PUT my-index\n/]

* If an index is read-only, use the <<indices-shrink-index,shrink index API>> or
{ilm-init}'s <<ilm-shrink,shrink action>> to reduce its primary shard count.
+
[source,console]
----
POST my-index/_shrink/my-shrunken-index
----
// TEST[s/^/PUT my-index\n{"settings":{"index.number_of_shards":2,"blocks.write":true}}\n/]

* If your node has a large disk capacity, you can increase the low disk
watermark or set it to an explicit byte value.
+
[source,console]
----
PUT _cluster/settings
{
  "persistent": {
    "cluster.routing.allocation.disk.watermark.low": "30gb"
  }
}
----
// TEST[s/"30gb"/null/]

**Reduce JVM memory pressure**

Shard allocation requires JVM heap memory. High JVM memory pressure can trigger
<<circuit-breaker,circuit breakers>> that stop allocation and leave shards
unassigned. See <<high-jvm-memory-pressure>>.

**Recover data for a lost primary shard**

If a node containing a primary shard is lost, {es} can typically replace it
using a replica on another node. If you can't recover the node and replicas
don't exist or are irrecoverable, you'll need to re-add the missing data from a
<<snapshot-restore,snapshot>> or the original data source.

WARNING: Only use this option if node recovery is no longer possible. This
process allocates an empty primary shard. If the node later rejoins the cluster,
{es} will overwrite its primary shard with data from this newer empty shard,
resulting in data loss.

Use the <<cluster-reroute,cluster reroute API>> to manually allocate the
unassigned primary shard to another data node in the same tier. Set
`accept_data_loss` to `true`.

[source,console]
----
POST _cluster/reroute
{
  "commands": [
    {
      "allocate_empty_primary": {
        "index": "my-index",
        "shard": 0,
        "node": "my-node",
        "accept_data_loss": "true"
      }
    }
  ]
}
----
// TEST[s/^/PUT my-index\n/]
// TEST[catch:bad_request]

If you backed up the missing index data to a snapshot, use the
<<restore-snapshot-api,restore snapshot API>> to restore the individual index.
Alternatively, you can index the missing data from the original data source.

[discrete]
[[rejected-requests]]
=== Rejected requests

When {es} rejects a request, it stops the operation and returns an error with a
`429` response code. Rejected requests are commonly caused by:

* A <<high-cpu-usage,depleted thread pool>>. A depleted `search` or `write`
thread pool returns a `TOO_MANY_REQUESTS` error message.

* A <<circuit-breaker-errors,circuit breaker error>>.

* High <<index-modules-indexing-pressure,indexing pressure>> that exceeds the
<<memory-limits,`indexing_pressure.memory.limit`>>.

[discrete]
[[check-rejected-tasks]]
==== Check rejected tasks

To check the number of rejected tasks for each thread pool, use the
<<cat-thread-pool,cat thread pool API>>. A high ratio of `rejected` to
`completed` tasks, particularly in the `search` and `write` thread pools, means
{es} regularly rejects requests.

[source,console]
----
GET /_cat/thread_pool?v=true&h=id,name,active,rejected,completed
----

[discrete]
[[prevent-rejected-requests]]
==== Prevent rejected requests

**Fix high CPU and memory usage**

If {es} regularly rejects requests and other tasks, your cluster likely has high
CPU usage or high JVM memory pressure. For tips, see <<high-cpu-usage>> and
<<high-jvm-memory-pressure>>.

**Prevent circuit breaker errors**

If you regularly trigger circuit breaker errors, see <<circuit-breaker-errors>>
for tips on diagnosing and preventing them.

[discrete]
[[task-queue-backlog]]
=== Task queue backlog

A backlogged task queue can prevent tasks from completing and 
put the cluster into an unhealthy state. 
Resource constraints, a large number of tasks being triggered at once,
and long running tasks can all contribute to a backlogged task queue.

[discrete]
[[diagnose-task-queue-backlog]]
==== Diagnose a task queue backlog

**Check the thread pool status**

A <<high-cpu-usage,depleted thread pool>> can result in <<rejected-requests,rejected requests>>. 

You can use the <<cat-thread-pool,cat thread pool API>> to 
see the number of active threads in each thread pool and
how many tasks are queued, how many have been rejected, and how many have completed. 

[source,console]
----
GET /_cat/thread_pool?v&s=t,n&h=type,name,node_name,active,queue,rejected,completed
----

**Inspect the hot threads on each node**

If a particular thread pool queue is backed up, 
you can periodically poll the <<cluster-nodes-hot-threads,Nodes hot threads>> API 
to determine if the thread has sufficient 
resources to progress and gauge how quickly it is progressing.

[source,console]
----
GET /_nodes/hot_threads
----

**Look for long running tasks**

Long-running tasks can also cause a backlog. 
You can use the <<tasks,task management>> API to get information about the tasks that are running. 
Check the `running_time_in_nanos` to identify tasks that are taking an excessive amount of time to complete. 

[source,console]
----
GET /_tasks?filter_path=nodes.*.tasks
----

[discrete]
[[resolve-task-queue-backlog]]
==== Resolve a task queue backlog

**Increase available resources** 

If tasks are progressing slowly and the queue is backing up, 
you might need to take steps to <<reduce-cpu-usage>>. 

In some cases, increasing the thread pool size might help.
For example, the `force_merge` thread pool defaults to a single thread.
Increasing the size to 2 might help reduce a backlog of force merge requests.

**Cancel stuck tasks**

If you find the active task's hot thread isn't progressing and there's a backlog, 
consider canceling the task.