Media failure logic and Structure of Recovery manager

Media failure logic:

In the event of a hard system error (one non-volatile storage integrity), there should be minimum of lost work. Redundant copies of the object should be maintained, for illustration on magnetic tape that is stored in a vault. It is significant that the archive mechanism have independent failure modes from the regular storage subsystem. Therefore using doubly redundant disk storage would protect against a disk head crash but wouldn't protect against a bug in the disk driver routine or a fire in the machine room.

The archive mechanism intermittently writes a checkpoint of the data base contents to magnetic tape and writes a redo log of all update actions to magnetic tape, Then recovering from a hard breakdown is accomplished by locating the most recent surviving version on tape, loading it back into the system and then redoing every updates from that point forward using the surviving log tapes.

While performing a system checkpoint reasons relatively few disk writes as well as takes only a few seconds copying the whole database to tape is potentially a lengthy operation. Opportunely there is a (little used) trick one can take a fuzzy dump or an object by writing it to archive with an idle task. Following the dump is taken the log generated during the fuzzy dump is merged with the fuzzy dump to produce a sharp dump.

The particulars of this algorithm are left as an exercise for the reader.

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