An in-memory database requires an in-memory cache that is used only for the im-memory database. Then you can create in-memory device(s) on the cache, to host the in-memory database.
Creating in-memory cache
Right-click the Caches directory and select New Cache.
Specify the name of the cache. Click Next.
Specify the size of the cache. Click Calculate overhead to see the amount of overhead associated with the size you have indicated. Click Next.
Select In-Memory Database as the Cache Type. A summary dialog appears: verify the catch type is inmemory_storage. Click Finish.