Index build is taking very long and failing on Documentdb. How to resolve that?

There could be 2 possible reasons.

• Amazon DocumentDB utilizes local storage on an instance as part of the index creation process. You can monitor this disk usage using the FreeLocalStorage CloudWatch metric (CloudWatch -> Metrics -> DocDB -> Instance Metrics). When an index build consumes all of the local disk and fails, you will receive an error. You can try to scale the instance and then re try index creation.
• Background index builds in Amazon DocumentDB do not start until all queries on the primary instance that started before the index build was initiated complete executing. If there is a long running query, background index builds will block until the query finishes and thus can take longer than expected to complete. This is true even if collections are empty

https://docs.aws.amazon.com/documentdb/latest/developerguide/troubleshooting.index-creation.html