The Cache Forgets So the Vault Remembers
The Cache Forgets So the Vault Remembers
Seed: CWFS (Cache Worm File System), Plan 9 file server storage hierarchy.
Central fact: CWFS keeps two kinds of truth at once by giving each a different relationship to forgetting — a fast disk cache that is allowed to forget, and a WORM backing store that is architecturally forbidden to.
An active filesystem needs to answer requests quickly and needs to change constantly; a permanent archive needs the opposite of both — it needs to be slow to reach and utterly unwilling to change. CWFS does not choose between these; it stacks them. The cache layer is ordinary rewritable disk, holding whatever has been touched recently, free to evict, overwrite, and lose anything not currently needed, because losing it costs nothing — the truth was never only there. Underneath sits the WORM layer: Write Once, Read Many, a storage medium (originally literal write-once optical platters, later emulated on ordinary disk with the same discipline enforced in software) where a block, once written, is permanent by construction, not by policy. There is no delete command for the worm layer because there is nothing behind it to check whether deletion was authorized — the medium itself does not know how to unwrite.
The cache's entire purpose is to protect the vault from being consulted too often — every read that can be answered from the fast, forgetful layer is one that never has to disturb the slow, permanent one. And every write that matters enough to keep eventually flows downward, converting a cache entry that could vanish tomorrow into a vault entry that cannot vanish ever.
Speed and permanence never had to compromise with each other. They were simply given different floors, and a staircase between them that only goes one way.