Manajemen Basis Data Pertemuan 6 Matakuliah: M0264/Manajemen Basis Data Tahun: 2008

Bina Nusantara The Log Database Recovery (Pemulihan Kerusakan) Media Recovery (Media Pemulihan) Objectives

Bina Nusantara The following actions are recorded in the log: – Ti writes an object: the old value and the new value. Log record must go to disk before the changed page! – Ti commits/aborts: a log record indicating this action. Log records are chained together by Xact id, so it’s easy to undo a specific Xact. Log is often duplexed and archived on stable storage. All log related activities (and in fact, all CC related activities such as lock/unlock, dealing with deadlocks etc.) are handled transparently by the DBMS. The Log

Bina Nusantara The Log A log record is written for each of the following actions : –Updating a page –Commit –Abort –End –Undoing an update

Bina Nusantara Database Recovery Recovery Control –Some causes of database failure. –Purpose of transaction log file. –Purpose of checkpointing. –How to recover following database failure.

Bina Nusantara Database Recovery Process of restoring database to a correct state in the event of a failure. Need for Recovery Control –Two types of storage: volatile (main memory) and nonvolatile. –Volatile storage does not survive system crashes. –Stable storage represents information that has been replicated in several nonvolatile storage media with independent failure modes.

Bina Nusantara Database Recovery Types of Failures –System crashes, resulting in loss of main memory. –Media failures, resulting in loss of parts of secondary storage. –Application software errors. –Natural physical disasters. –Carelessness or unintentional destruction of data or facilities. –Sabotage.

Bina Nusantara Database Recovery DBMS should provide following facilities to assist with recovery: –Backup mechanism, which makes periodic backup copies of database. –Logging facilities, which keep track of current state of transactions and database changes. –Checkpoint facility, which enables updates to database in progress to be made permanent. –Recovery manager, which allows DBMS to restore database to consistent state following a failure.

Bina Nusantara Media Recovery Media recovery is based on periodically making copy of the database. Because copying a large database object such as a file can take a long time and the DBMS must be allowed to continue with its operation in the meantime, creating a copy is handled in manner similar to taking a fuzzy checkpoint.