---
mapped_pages:
  - https://www.elastic.co/guide/en/elasticsearch/reference/current/ilm-forcemerge.html
---

# Force merge [ilm-forcemerge]

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).

::::{note}
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-init}} will reject the policy.

:::::{admonition} Performance considerations
:name: ilm-forcemerge-performance

Force merge is a resource-intensive operation. If too many force merges are triggered at once, it can negatively impact your cluster. This can happen when you apply an {{ilm-init}} policy that includes a force merge action to existing indices. If they meet the `min_age` criteria, they can immediately proceed through multiple phases. You can prevent this by increasing the `min_age` or setting `index.lifecycle.origination_date` to change how the index age is calculated.

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}
This can have cascading performance impacts. Monitor cluster performance and increment the size of the thread pool slowly to reduce the backlog.
::::


Force merging will be performed by the nodes within the current phase of the index. A forcemerge in the `hot` phase will use hot nodes with potentially faster nodes, while impacting ingestion more. A forcemerge in the `warm` phase will use warm nodes and potentially take longer to perform, but without impacting ingestion in the `hot` tier.

:::::


## Options [ilm-forcemerge-options]

`max_num_segments`
:   (Required, integer) Number of segments to merge to. To fully merge the index, set to `1`.

`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-init}} 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.
    ::::



## Example [ilm-forcemerge-action-ex]

```console
PUT _ilm/policy/my_policy
{
  "policy": {
    "phases": {
      "warm": {
        "actions": {
          "forcemerge" : {
            "max_num_segments": 1
          }
        }
      }
    }
  }
}
```