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@@ -6,16 +6,36 @@ Phases allowed: hot, warm.
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<<indices-forcemerge,Force merges>> the index into
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the specified maximum number of <<indices-segments,segments>>.
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-
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This action makes the index <<dynamic-index-settings,read-only>>.
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-To use the `forcemerge` action in the `hot` phase, the `rollover` action *must* be present.
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-If no rollover action is configured, {ilm-init} will reject the policy.
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-
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[NOTE]
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The `forcemerge` action is best effort. It might happen that some of the
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shards are relocating, in which case they will not be merged.
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+To use the `forcemerge` action in the `hot` phase, the `rollover` action *must* be present.
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+If no rollover action is configured, {ilm-init} will reject the policy.
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+
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+[[ilm-forcemerge-performance]]
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+.Performance considerations
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+****
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+Force merge is a resource-intensive operation.
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+If too many force merges are triggered at once, it can negatively impact your cluster.
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+This can happen when you apply an {ilm-init} policy that includes a force merge action
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+to existing indices.
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+If they meet the `min_age` criteria, they can immediately proceed through multiple phases.
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+You can prevent this by increasing the `min_age` or setting `index.lifecycle.origination_date`
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+to change how the index age is calculated.
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+
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+If you experience a force merge task queue backlog,
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+you might need to increase the size of the force merge threadpool so
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+indices can be force merged in parallel.
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+To do this, configure the `thread_pool.force_merge.size` <<cluster-get-settings,cluster setting>>.
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+
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+IMPORTANT: This can have cascading performance impacts.
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+Monitor cluster performance and increment the size of the thread pool slowly to reduce the backlog.
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+****
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+
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+
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[[ilm-forcemerge-options]]
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==== Options
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Stef Nestor