Related to #26969, the AWS SDK has improved since Elasticsearch 1.4 and I think it's more than time to remove the S3OutputStream class and to replace it by other utilities provided by the AWS SDK. I'd like to replace it with com.amazonaws.services.s3.transfer.TransferManager (from AWS SDK v1.11.211) which takes care of uploading objects using one or multiple requests, potentially by executing them concurrently.

With the security manager enabled and all the legacy code in the S3 plugin the change is not that simple but I'd like to do it soon. I think we can expect less GCs, less allocations, less connections issues and upload should be faster too. That's also the opportunity to prepare the field for AWS SDK v2.