Backfill

Backfill moves the documents that already exist on the source cluster into the Amazon OpenSearch Service domain or Amazon OpenSearch Serverless NextGen collection. Migration Assistant takes a point-in-time snapshot of the source in Amazon Simple Storage Service (Amazon S3) — the only time the source is touched — and then uses Reindex-from-Snapshot (RFS) to read Lucene segment files directly from the snapshot and bulk-index them into the target.

Run metadata migration before backfill so that index settings, mappings, templates, and aliases are already in place on the target. See Migrate metadata. The snapshot used for metadata migration is fully compatible with the RFS backfill phase, so the snapshot you create in this chapter is the same one RFS reads from.