Today ReadOnlyEngine loads the terms dictionary for all fields off-heap only if the underlying directory is using mmap to access the file. This means that frozen indices can consume more heap if they are opened with niofs, which defeats one of the main benefit since they could use more memory than a non-frozen index opened with the default directory.
I am opening this issue to discuss whether we should always load terms dictionary off-heap for fields contained in a read-only index. Currently only frozen indices would be concerned but this decision would affect searchable snapshots too since they cannot use mmap to load data from a remote filesystem.