The in-memory blobstore can be enabled from the GeoServer GUI.
When using the in-process one (i.e. GUAVA based) not all the eviction policies are supported.
Problem is that we get a nasty stacktrace if we try configure one which is not supported.
We should do much better than this as we know upfront which are supported and which ones are not.
I would also update the doc for the cache.