For a transaction to be durable, its changes need to be written to stable storage.
Similarly, for a transaction to be atomic, log records need to be written to stable
storage before any changes are made to the database on disk. Clearly, the degree
to which a system ensures durability and atomicity depends on how stable its
implementation of stable storage really is. In some cases, a single copy on disk is
considered sufficient, but applications whose data are highly valuable andwhose