• Stable storage. Information residing in stable storage is never lost (never
should be taken with a grain of salt, since theoretically never cannot be
guaranteed—for example, it is possible, although extremely unlikely, that
a black hole may envelop the earth and permanently destroy all data!). Although
stable storage is theoretically impossible to obtain, it can be closely
approximated by techniques that make data loss extremely unlikely. To implement
stable storage, we replicate the information in several nonvolatile
storage media (usually disk) with independent failure modes. Updates must
be done with care to ensure that a failure during an update to stable storage
does not cause a loss of information. Section 16.2.1 discusses stable-storage
implementation.