Sample/practice exam, questions and answers Database Recovery PDF

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Sample answer of the benefits of Database Recovery and common methodologies....

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Database Recovery Backup and recovery is one of the most important aspects of database administration. If a database crashed and there was no way to recover it, the devastating results to a business could include lost data, lost revenue and customer dissatisfaction. A backup is a representative copy of data. This copy can include important parts of a database such as the control file, redo logs, and datafiles. A backup protects data from application error and acts as a safeguard against unexpected data loss, by providing a way to restore original data. Backups are divided into physical backups and logical backups. Physical backups are copies of physical database files. The phrase "backup and recovery" usually refers to the transfer of copied files from one location to another, along with the various operations performed on these files. Oracle performs crash recovery and instance recovery automatically after an instance failure. In the case of media failure, a database administrator (DBA) must initiate a recovery operation. Recovering a backup involves two distinct operations: rolling the backup forward to a more recent time by applying redo data, and rolling back all changes made in uncommitted transactions to their original state. In general, recovery refers to the various operations involved in restoring, rolling forward, and rolling back a backup. Backup and recovery refers to the various strategies and operations involved in protecting the database against data loss and reconstructing the database should a loss occur.` Redo log buffer is part of the system global area memory structure which records all changes made to the database. For any transaction to be considered complete, it first has to be saved in the redo log file. This is initialised by the redo log buffer and the process responsible for writing data from the redo log buffer to the redo log file is LGWR (log writer). Given that the space allocated for redo log files is limited, they are overwritten when full. When doing so, log writer signals creating a checkpoint and also calls the DBWR to commit all the transactions to the data files before the checkpoint can be called. CKPT process is responsible for it. In order to ensure the ability of a complete and up to date recovery in case of any time of system crash or failure, it is necessary to archive the redo log files at this stage. Therefore when called, the checkpoint process starts archiving the redo log file and puts a time stamp on the file. The actual process responsible for writing the file from the redo log file to the archive file is the archive process or archiver. Oracle ensures that in case of any type of failure or crash, there is a recovery strategy that allows for a backup or roll forward of all the completed transactions. Therefore disks with both the redo log files and archive log files, as well as datafiles are mirrored. In case of any of the failures of the mirrored disks, oracle simply uses the mirrored disk. Moreover, another mirror is created. In case of unmirrored disk failure or the entire system crash, SMON (system monitor) process is responsible for getting the database back to the current version. To do so, it recovers the last saved backup copy of the database and in order to roll forward all the transactions that have been committed after the last backup (which can be checked by their time stamps), it redoes them from the redo log file. If a crash of the entire system occurs in the distributed environment, the more advanced RECO process is responsible for recovery. The full recovery procedure of oracle is thanks to the logging of transactions....