 Buffer cache eviction

Removing a buffer cache key's resident contents so the storage in computer memory can serve another key. An unreferenced clean buffer can be reused directly in the buffer cache state model; an unreferenced dirty buffer requires successful buffer cache writeback first. A pinned buffer cannot be chosen regardless of cleanliness. LRU replacement ranks eligible candidates; it does not override lifetime protection. If every candidate is retained, a cache miss cannot obtain space until something releases a reference or capacity changes. This is a resource shortage, not permission to discard live contents.