High speed on-line backup when using logical log operations

Abstract

Media recovery protects a database from failures of the stable medium by maintaining an extra copy of the database, called the backup, and a media recovery log. When a failure occurs, the database is “restored” from the backup, and the media recovery log is used to roll forward the database to the desired time, usually the current time. Backup must be both fast and “on-line”, i.e. concurrent with on-going update activity. Conventional online backup sequentially copies from the stable database, almost independent of the database cache manager, but requires page-oriented log operations. But results of logical operations must be flushed to a stable database (a backup is a stable database) in a constrained order to guarantee recovery. This order is not naturally achieved for the backup by a cache manager concerned only with crash recovery. We describe a “full speed” backup, only loosely coupled to the cache manager, and hence similar to current online backups, but effective for general logical log operations. This requires additional logging of cached objects to guarantee media recoverability. We then show how logging can be greatly reduced when log operations have a constrained form which nonetheless provides very useful additional logging efficiency for database systems.