---
title: Force merge
description: Phases allowed: hot, warm. Force merges the index into the specified maximum number of segments. This operation is performed on a best effort basis. For...
url: https://www.elastic.co/docs/reference/elasticsearch/index-lifecycle-actions/ilm-forcemerge
products:
  - Elasticsearch
---

# Force merge
Phases allowed: hot, warm.
[Force merges](https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-indices-forcemerge) the index into the specified maximum number of [segments](https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-indices-segments). This operation is performed on a best effort basis. For example, shards that are relocating during a `forcemerge` will not be merged.
To use the `forcemerge` action in the `hot` phase, the `rollover` action **must** be present. If no rollover action is configured, ILM will reject the policy.
<admonition title="Performance considerations">
  Force merge is a resource-intensive operation. If too many force merges are triggered at once, it can negatively impact your cluster. For example, this can happen when you
  - modify an existing ILM policy's phase `min_age`, causing indices to trigger the force merge at a faster rate.
  - apply an ILM policy that includes a force merge action to existing indices. If the indices meet the `min_age` criteria, they can immediately proceed through multiple actions. You can prevent this by increasing the `min_age` or setting `index.lifecycle.origination_date` to change how the index age is calculated.
  - run the [ILM Move Step API](https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-ilm-move-to-step) against multiple indices.
  If you experience a force merge task queue backlog, you might need to increase the size of the force merge threadpool so indices can be force merged in parallel. To do this, configure the `thread_pool.force_merge.size` [cluster setting](https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-cluster-get-settings).
  <important>
    Note that `thread_pool.force_merge.size` is an advanced setting. Adjusting it can cause cascading performance impacts. Monitor cluster performance and increment the size of the thread pool slowly to reduce the backlog.Lucene automatically merges segments only up to a `5GB` size limit. Segments that exceed this limit will no longer automatically clear deleted documents. Therefore, it's important not to perform a forced merge until you have completed all modifications to the index.
  </important>
  Force merging will be performed by the node hosting the shard. Usually, the [node's role](https://www.elastic.co/docs/deploy-manage/distributed-architecture/clusters-nodes-shards/node-roles#set-node-roles) matches the [data tier](https://www.elastic.co/docs/manage-data/lifecycle/data-tiers) of the ILM phase that the index is in. One of the exceptions is when you have manually disabled [ILM Migrate](https://www.elastic.co/docs/reference/elasticsearch/index-lifecycle-actions/ilm-migrate) and have specified custom allocations using [ILM allocate](https://www.elastic.co/docs/reference/elasticsearch/index-lifecycle-actions/ilm-allocate). The other exception is searchable snapshots; force merges for [ILM Searchable Snapshots](https://www.elastic.co/docs/reference/elasticsearch/index-lifecycle-actions/ilm-searchable-snapshot) using `force_merge_index` are performed in the phase that the index is in **prior** to the `searchable_snapshot` action. You may want to explicitly choose in which data tier the force merge should occur, for example:
  - A force merge in the `hot` phase will use hot nodes. Merges may be faster on this potentially higher performance hardware but may have the tradeoff of impacting ingestion.
  - A force merge in the `warm` phase will use warm nodes. Merges may take longer to perform on potentially lower performance hardware but will avoid impacting ingestion in the `hot` tier.
  - [ILM Searchable Snapshot](https://www.elastic.co/docs/reference/elasticsearch/index-lifecycle-actions/ilm-searchable-snapshot) performance is dependant upon the shard having been force merged, so by default this ILM action will enable `force_merge_index`. This will trigger force merges in the preceding node data tier for `cold` and `frozen` phases.
  We recommend that merges be targetted against SSD and not HDD disks.Merges are one of the more expensive background tasks a cluster must perform. Merge frequencies spike during ingest as new segments are created. You can bypass some of the background merge overhead by [optimizing ingestion settings](https://www.elastic.co/docs/deploy-manage/production-guidance/optimize-performance/indexing-speed) such as:
  - Increasing the [`index.refresh_interval`](/docs/reference/elasticsearch/index-settings/index-modules#dynamic-index-settings) setting.
  - [Using bulk requests](https://www.elastic.co/docs/deploy-manage/production-guidance/optimize-performance/indexing-speed#_use_bulk_requests). For examples, refer to Elastic's client-side product tuning guides:
    - The Elastic Agent [performance tuning settings](https://www.elastic.co/docs/reference/fleet/elasticsearch-output#output-elasticsearch-performance-tuning-settings) in the [Fleet Settings UI](https://www.elastic.co/docs/reference/fleet/fleet-settings#output-settings)
  - The Logstash [tuning settings](https://www.elastic.co/docs/reference/logstash/tuning-logstash#tuning-logstash-settings)
</admonition>


## Options

<definitions>
  <definition term="max_num_segments">
    (Required, integer) Number of segments to merge to. To fully merge the index, set to `1`.
  </definition>
  <definition term="index_codec">
    (Optional, string) Codec used to compress the document store. The only accepted value is `best_compression`, which uses [ZSTD](https://en.wikipedia.org/wiki/Zstd) for a higher compression ratio but slower stored fields performance. To use the default LZ4 codec, omit this argument.
    <warning>
      If using `best_compression`, ILM will [close](https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-indices-close) and then [re-open](https://www.elastic.co/docs/api/doc/elasticsearch/operation/operation-indices-open) the index prior to the force merge. While closed, the index will be unavailable for read or write operations.
    </warning>
  </definition>
</definitions>


## Example

```json

{
  "policy": {
    "phases": {
      "warm": {
        "actions": {
          "forcemerge" : {
            "max_num_segments": 1
          }
        }
      }
    }
  }
}
```