tkykenmt · tkykenmt · Jan 11, 2026 · Jan 11, 2026
diff --git a/docs/features/opensearch/s3-repository.md b/docs/features/opensearch/s3-repository.md
@@ -83,6 +83,7 @@ Configure in `opensearch.yml`:
 | `s3.client.default.endpoint` | Custom S3 endpoint |
 | `s3.client.default.region` | AWS region |
 | `s3.client.default.protocol` | HTTP or HTTPS |
+| `s3.client.default.legacy_md5_checksum_calculation` | Enable legacy MD5 checksum for S3-compatible storage (v3.4.0+) |
 
 ### Usage Example
 
@@ -210,6 +211,7 @@ graph TB
 
 | Version | PR | Description |
 |---------|-----|-------------|
+| v3.4.0 | [#19788](https://github.com/opensearch-project/OpenSearch/pull/19788) | Add LEGACY_MD5_CHECKSUM_CALCULATION to repository-s3 settings list |
 | v3.3.0 | [#18800](https://github.com/opensearch-project/OpenSearch/pull/18800) | Switch default async HTTP client to AWS CRT for improved throughput |
 | v3.3.0 | [#19220](https://github.com/opensearch-project/OpenSearch/pull/19220) | Fix S3-compatible repository checksum trailing headers issue |
 | v3.1.0 | [#18312](https://github.com/opensearch-project/OpenSearch/pull/18312) | Add support for SSE-KMS and S3 bucket owner verification |
@@ -219,6 +221,7 @@ graph TB
 
 ## References
 
+- [PR #19788](https://github.com/opensearch-project/OpenSearch/pull/19788): Add LEGACY_MD5_CHECKSUM_CALCULATION to settings list
 - [PR #18800](https://github.com/opensearch-project/OpenSearch/pull/18800): AWS CRT async HTTP client implementation
 - [Issue #18535](https://github.com/opensearch-project/OpenSearch/issues/18535): Feature request for S3CrtClient support
 - [PR #19220](https://github.com/opensearch-project/OpenSearch/pull/19220): S3-compatible repository checksum fix
@@ -235,6 +238,7 @@ graph TB
 
 ## Change History
 
+- **v3.4.0** (2025-10): Added `legacy_md5_checksum_calculation` to opensearch.yml settings for S3-compatible storage configuration
 - **v3.3.0** (2025-09): Switched default async HTTP client to AWS CRT for ~5-7% throughput improvement; added `s3_async_client_type` setting
 - **v3.3.0** (2025-09): Fixed S3-compatible repository compatibility by making checksum trailing headers conditional
 - **v3.1.0** (2025-07): Added SSE-KMS support, bucket owner verification, removed legacy server_side_encryption setting

diff --git a/docs/releases/v3.4.0/features/opensearch/s3-repository.md b/docs/releases/v3.4.0/features/opensearch/s3-repository.md
@@ -0,0 +1,83 @@
+# S3 Repository
+
+## Summary
+
+This release adds the `LEGACY_MD5_CHECKSUM_CALCULATION` setting to the list of repository-s3 plugin settings, enabling it to be configured in `opensearch.yml` in addition to repository creation time. This completes the fix for S3-compatible storage compatibility that was introduced in v3.3.0.
+
+## Details
+
+### What's New in v3.4.0
+
+The `s3.client.<client_name>.legacy_md5_checksum_calculation` setting was added to the list of registered settings in `S3RepositoryPlugin`, allowing users to configure legacy MD5 checksum calculation behavior through `opensearch.yml`.
+
+### Background
+
+In OpenSearch 3.0.0, the AWS SDK was upgraded to version 2.30.31, which introduced mandatory checksum trailing headers. This broke compatibility with S3-compatible storage providers (like Outscale, MinIO, etc.) that don't support the new checksum format.
+
+The fix in v3.3.0 ([PR #19220](https://github.com/opensearch-project/OpenSearch/pull/19220)) introduced the `legacy_md5_checksum_calculation` setting to enable legacy MD5 checksum behavior for compatibility. However, this setting was only configurable at repository creation time, not in `opensearch.yml`.
+
+### Technical Changes
+
+#### Configuration Enhancement
+
+The setting `S3ClientSettings.LEGACY_MD5_CHECKSUM_CALCULATION` is now registered in `S3RepositoryPlugin.getSettings()`:
+
+```java
+// S3RepositoryPlugin.java
+public List<Setting<?>> getSettings() {
+    return Arrays.asList(
+        // ... other settings ...
+        S3ClientSettings.LEGACY_MD5_CHECKSUM_CALCULATION,
+        // ... other settings ...
+    );
+}
+```
+
+#### New Configuration Option
+
+| Setting | Description | Default |
+|---------|-------------|---------|
+| `s3.client.<client_name>.legacy_md5_checksum_calculation` | Enable legacy MD5 checksum calculation for S3-compatible storage | `false` |
+
+### Usage Example
+
+Configure in `opensearch.yml` for cluster-wide S3-compatible storage support:
+
+```yaml
+# opensearch.yml
+s3.client.default.legacy_md5_checksum_calculation: true
+s3.client.default.endpoint: "https://s3-compatible-endpoint.example.com"
+```
+
+Or configure per-client:
+
+```yaml
+# opensearch.yml
+s3.client.minio.legacy_md5_checksum_calculation: true
+s3.client.minio.endpoint: "http://minio.local:9000"
+```
+
+### Migration Notes
+
+If you're using S3-compatible storage and previously had to configure `legacy_md5_checksum_calculation` at repository creation time, you can now move this configuration to `opensearch.yml` for easier management across all repositories using the same client.
+
+## Limitations
+
+- The setting only affects S3-compatible storage providers that don't support AWS SDK v2's checksum trailing headers
+- When enabled, uses MD5 checksums instead of the newer CRC-based checksums
+
+## Related PRs
+
+| PR | Description |
+|----|-------------|
+| [#19788](https://github.com/opensearch-project/OpenSearch/pull/19788) | Add S3Repository.LEGACY_MD5_CHECKSUM_CALCULATION to list of repository-s3 settings |
+
+## References
+
+- [Issue #18240](https://github.com/opensearch-project/OpenSearch/issues/18240): Snapshot repositories don't work with S3 compatible storage in 3.0.0
+- [PR #19220](https://github.com/opensearch-project/OpenSearch/pull/19220): Fix issue with s3-compatible repositories due to missing checksum trailing headers (v3.3.0)
+- [Register Snapshot Repository](https://docs.opensearch.org/3.0/api-reference/snapshots/create-repository/): OpenSearch documentation
+
+## Related Feature Report
+
+- [Full feature documentation](../../../../features/opensearch/s3-repository.md)
diff --git a/docs/releases/v3.4.0/index.md b/docs/releases/v3.4.0/index.md
@@ -8,6 +8,7 @@
 - [Dependency Updates (OpenSearch Core)](features/opensearch/dependency-updates-opensearch-core.md) - 32 dependency updates including Netty 4.2.4 for HTTP/3 readiness
 - [Lucene Upgrade](features/opensearch/lucene-upgrade.md) - Bump Apache Lucene from 10.3.1 to 10.3.2 with MaxScoreBulkScorer bug fix
 - [Node Stats Bugfixes](features/opensearch/node-stats-bugfixes.md) - Fix negative CPU usage values in node stats on certain Linux distributions
+- [S3 Repository](features/opensearch/s3-repository.md) - Add LEGACY_MD5_CHECKSUM_CALCULATION to opensearch.yml settings for S3-compatible storage
 - [Security Kerberos Integration](features/opensearch/security-kerberos-integration.md) - Update Hadoop to 3.4.2 and enable Kerberos integration tests for JDK-24+
 - [Settings Bugfixes](features/opensearch/settings-bugfixes.md) - Fix duplicate registration of dynamic settings and patch version build issues
 - [Stats Builder Pattern Deprecations](features/opensearch/stats-builder-pattern-deprecations.md) - Deprecated constructors in 30+ Stats classes in favor of Builder pattern