Failure with Loss of Nonvolatile Storage

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Failure with Loss of Nonvolatile Storage

Until now, we have considered only the case where a failure results in the loss of information residing in volatile storage while the content of the nonvolatile storage remains intact. Although failures in which the content of nonvolatile storage is lost are rare, we nevertheless need to be prepared to deal with this type of failure. In this section, we discuss only disk storage. Our discussions apply as well to other nonvolatile storage types.

The basic scheme is to dump the entire content of the database to stable storage periodically—say, once per day. For example, we may dump the database to one or more magnetic tapes. If a failure occurs that results in the loss of physical database blocks, the system uses the most recent dump in restoring the database to a previous consistent state. Once this restoration has been accomplished, the system uses the log to bring the database system to the most recent consistent state.

More precisely, no transaction may be active during the dump procedure, and a procedure similar to checkpointing must take place:

1. Output all log records currently residing in main memory onto stable storage.

2. Output all buffer blocks onto the disk.

3. Copy the contents of the database to stable storage.

4. Output a log record <dump> onto the stable storage.

Steps 1, 2, and 4 correspond to the three steps used for checkpoints in Section 17.4.3.

To recover from the loss of nonvolatile storage, the system restores the database to disk by using the most recent dump. Then, it consults the log and redoes all the transactions that have committed since the most recent dump occurred. Notice that no undo operations need to be executed.

A dump of the database contents is also referred to as an archival dump, since we can archive the dumps and use them later to examine old states of the database.

Dumps of a database and checkpointing of buffers are similar.

The simple dump procedure described here is costly for the following two reasons.

First, the entire database must be be copied to stable storage, resulting in considerable data transfer. Second, since transaction processing is halted during the dump procedure, CPU cycles are wasted. Fuzzy dump schemes have been developed, which allow transactions to be active while the dump is in progress. They are similar to fuzzy checkpointing schemes; see the bibliographical notes for more details.

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