Compressing the stored fields file (the .fdt file) directly allows to have better compression on the size of the index, specifically when indexing (and storing) small documents. The compression will be considerably more effective compared to compressing each doc on its own (when setting compress on the _source mapper). The downside is that more data needs to be uncompressed when loading documents.

The settings to control it is `index.store.compress.stored_fields` set to `true` (it defaults to `false`), and can be enabled dynamically using the update settings API. This allows to enabled compression at a later stage (i.e. old time based indices), and then optimize the index to make sure it gets compressed.

Compressing the stored fields file (the .fdt file) directly allows to have better compression on the size of the index, specifically when indexing (and storing) small documents. The compression will be considerably more effective compared to compressing each doc on its own (when setting compress on the _source mapper). The downside is that more data needs to be uncompressed when loading documents.

The settings to control it is `index.store.compress.stored_fields` set to `true` (it defaults to `false`), and can be enabled dynamically using the update settings API. This allows to enabled compression at a later stage (i.e. old time based indices), and then optimize the index to make sure it gets compressed.