Couchbase Server never deletes entire items from disk unless a client explicitly deletes the item from the database or the expiration value for the item is reached.
The ejection mechanism removes an item from RAM, while keeping a copy of the key and metadata for that document in RAM and also keeping copy of that document on disk.
Important: If only memcached buckets are used with Couchbase Server, the server provides only a caching layer as storage and no data persistence on disk. If your server runs out of space in RAM, items are evicted from RAM on a least recently used basis (LRU). Eviction means the server removes the key, metadata and all other data for the item from RAM. After eviction, the item is irretrievable.