Sharding is done appending the shard name to the index name. This typically creates the following directory structure
/var/data/index/User.fr /var/data/index/User.uk /var/data/index/User.us
We could have a template based directory setting to let people have the following type of structures
dir_template=/var/data/index/User/{shard} /var/data/index/User/fr /var/data/index/User/uk /var/data/index/User/us
or
dir_template=/var/data/index/{shard}/User /var/data/index/fr/User /var/data/index/uk/User /var/data/index/us/User