Useful summary. Made me want to read about the ghost cache. The cache layout is:

Probationary 10% + Traditional 90% (of memory).

The idea is that most items are not requested again prior to dropping out of probationary and so don't take up space and process in the Traditional Cache.

Some infrequently used items however would remain inside the Traditional cache but never make it through the probationary cache. So a small "ghost" cache is used to specifically detect these items rejected from the Probationary cache.

Next time we go to fetch a ghost item they go directly into the Traditional cache. The ghost cache is just storing metadata (pointers essentially).