Backup, as it is called, is the process of copying a database to a dump device. In this context, a dump device is a tape or disk that is used to hold a copy of the database. It is also common to refer to the copy of the database stored in the dump device as a backup or dump of the original database. As shown in the following figure: ORACLE database backups are divided into two types: physical backups and logical backups. Physical backup is the process of copying the operating system files that actually make up the database from one place to another, usually from disk to tape. A physical backup of a database can be performed using Oracle's Recovery Manager (RMAN) or operating system commands. Logical backup is the process of extracting data from the database and storing it in a binary file using the SQL language.The logical backup tool provided by Oracle is EXP. Logical backup of the database is a complement to physical backup. According to the state of the database at the time of the physical backup, the backup can be divided into consistent backup (consistent backup) and inconsistent backup (inconsistent backup) two kinds: Consistent backup: Consistent backup is when the database of all read-write database files and control files have the same system change number (SCN), and the data files do not contain any data other than the current SCN. do not contain any changes beyond the current SCN. When doing a database checkpoint, Oracle makes all control files and data files consistent. Oracle also considers read-only tablespaces and offline tablespaces to be consistent. The only way to put the database in a consistent state is to shut down the database normally (with the shutdown normal or shutdown immediate command). Therefore, a backup is only consistent if the database is shut down normally (with the shutdown normal or shutdown immediate command). Inconsistent backup: An inconsistent backup is a backup when the system change number (SCN) of the database's readable and writable database files and control files are in an inconsistent condition. For a 7*24 working database, since it is impossible to shut down the computer and the database data is constantly changing, only inconsistent backup can be performed. Under the condition of inconsistent SCN number, the database can only be started when the SCN is consistent by applying the redo log. Therefore, if an inconsistent backup is performed, the database must be set to the archived state and the redo logs must be archived for it to be meaningful. A backup is inconsistent if the database is open. The database is in the closed state, but closed by unusual means. For example, the database was shut down by shutdown abort or power failure. What is recovery Recovery is the process of transforming a database from a faulty state to a fault-free state. Depending on the cause of the failure, recovery is divided into two types: Instance recovery. This recovery is an Oracle instance failure, Oracle automatic recovery. Media Recovery. This type of recovery is the recovery done when the media that holds the database fails. All references to recovery later in this book refer to media recovery. Load (restore) physical backups and recovery (Recover) Physical backups are the means of media recovery. Load is the backup back to disk, recovery is the use of redo logs (part of the physical backup) to modify the data files back to disk (another part of the physical backup), so as to restore the database process. As shown in the following figure: Recovery methods are categorized into two types based on the extent of database recovery: Full recovery: restores the database to the state of the database at the time of database failure. This is done by loading a database backup and applying the full redo log. Incomplete recovery: restores the database to the state of the database at some point before the database failed. This recovery is done by loading a database backup and applying partial redo logs. The online redo log must be reset with the resetlogs option when you start the database after performing an incomplete recovery. For example, at 10:00 a.m., the database is suspended due to disk corruption. Now two methods are used to recover the database, the first method allows the database to be used normally and the data in the database after recovery is the same as that at the time of the corruption (10:00 a.m.), then the first recovery method is a full recovery; the second method allows the database to be used normally but only allows the data in the database after recovery to be the same as that before the corruption (e.g., at 9:00 a.m.) and fails to recover the database to the time of the failure (10:00 a.m.). The second recovery method belongs to the incomplete recovery type if the database backup is consistent. In fact, if the database backup is a consistent backup, the loaded database can be used and thus can also be restored without redo logging to the point at which the database was backed up. This is also a type of incomplete recovery. The Relationship Between Backup and Recovery Backing up an ORACLE database is analogous to buying health insurance - you don't realize the importance of it until you encounter an illness, and the amount of money you get depends on the type of policy. Similarly, the feasibility, difficulty and time spent on recovery of a database after a failure varies with the type and frequency of backups made. A database failure is a special event in the operation of a database that affects the normal use of the database. There are many types of database failures, the most serious is media failure (
Free software Free software download Free software download network Free software download site Free software download center Software download center Software download center Free software download site
www.chenganzhiyxrj.com