In Search 5, all directory-related options could be configured on a per-index basis, or even per-shard when sharding was enabled.
This was both complicated, and potentially limiting for us, since resources (thread pools, ...) could not easily be shared between multiple indexes.
In Search 6, we currently only allow one set of directory-related options at the backend level.
While this is simpler and could allow more optimization, it is also very restricting for users: they won't be able to, for example, store different indexes or different shards of the same index on different disks.
They could put each index in a different backend, but that would prevent multi-index searches (as it's not possible to search across multiple backends).
Maybe we should find some middle ground?
The current structure of configuration properties is fine, but we could introduce an alternative syntax when multiple storage configurations are needed.
"store" is just a first attempt at naming this concept, but we should look for a better term.