# CCR bootstrap of large clusters

**URL:** <https://forum.opensearch.org/t/ccr-bootstrap-of-large-clusters/28148>\
**Category:** Cross-Cluster Replication\
**Created:** [June 23, 2026, 3:10am UTC](https://forum.opensearch.org/t/ccr-bootstrap-of-large-clusters/28148 "2026-06-23T03:10:16Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![gchakkalakkal1](https://avatars.discourse-cdn.com/v4/letter/g/46a35a/32.png) [@gchakkalakkal1](https://forum.opensearch.org/u/gchakkalakkal1)\
**Post date:** [June 23, 2026, 3:10am UTC](https://forum.opensearch.org/t/ccr-bootstrap-of-large-clusters/28148/1 "2026-06-23T03:10:16Z")

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**Versions** (relevant - OpenSearch/Dashboard/Server OS/Browser): 3.2

**Describe the issue** :CCR bootstrap starts successfully and begins transferring shard segment files from the leader to the follower cluster.

1. For some large shards, file transfer fails with a `CorruptIndexException` (or related index corruption error).
2. The affected follower shard transitions to a failed state.
3. CCR replication for the shard does not automatically recover and requires manual intervention.
4. In large clusters with hundreds or thousands of shards, even a small number of shard failures can prevent successful completion of the bootstrap process.

**Configuration** :

**Relevant Logs or Screenshots** :

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**Author:** ![Anthony](https://sea1.discourse-cdn.com/flex019/user_avatar/forum.opensearch.org/anthony/32/9939_2.png) [@Anthony](https://forum.opensearch.org/u/Anthony)\
**Post date:** [June 24, 2026, 11:06am UTC](https://forum.opensearch.org/t/ccr-bootstrap-of-large-clusters/28148/2 "2026-06-24T11:06:39Z")

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@gchakkalakkal1 This looks like it could be a known CCR bug ([#1465](https://github.com/opensearch-project/cross-cluster-replication/issues/1465), [#1482](https://github.com/opensearch-project/cross-cluster-replication/issues/1482)): leader-side segment reads used a Lucene `IndexInput` opened with `IOContext.READONCE`, which on newer JDKs backs onto a thread-confined memory segment. When a large shard’s transfer spans multiple chunk requests handled by different threads, accessing it cross-thread throws `IllegalStateException: confined`, which can surface as a corrupt/incomplete file. Fixed upstream in [PR #1520](https://github.com/opensearch-project/cross-cluster-replication/pull/1520), merged April 2025 and included in the official `3.2.0.0` plugin release.

Two things that would help confirm:

1. Does your actual stack trace mention `IllegalStateException: confined` or `MemorySegmentIndexInput`?
2. Are you on the official OpenSearch distribution, or an AWS Managed Service? Managed offerings have shipped older CCR plugin builds under a given version label even after the upstream fix landed.

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**Author:** ![system](https://us1.discourse-cdn.com/flex019/uploads/mauve_hedgehog/original/2X/3/33547ea01a5b12dcca2958411d3edd97ae2ea8c1.png) [@system](https://forum.opensearch.org/u/system)\
**Post date:** [August 23, 2026, 11:07am UTC](https://forum.opensearch.org/t/ccr-bootstrap-of-large-clusters/28148/3 "2026-08-23T11:07:36Z")

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