Oracle Database uses several database system files that reside on the database server. These are different from data files, which are physical files that belong to database containers.
The following files are required for database startup:
- Control files store metadata about the data files and online redo log files (for example, their names and statuses). The database instance requires this information to open the database. Control files also contain metadata that must be accessible when the database is not open. Each multitenant container database (CDB) has one unique control file, although multiple identical copies are permitted. Pluggable databases (PDBs) do not have separate control files.
- The parameter file defines how the database instance is configured when it starts up. You can use an initialization parameter file (pfile) or a server parameter file (spfile).
- Online redo log files store changes to the database as they occur and are used for data recovery. These files are part of the CDB.
Note: Oracle recommends that you maintain multiple copies of the control files and online redo log files in separate locations to avoid a single point of failure. This is called multiplexing. For details, see Multiple Control Files and Multiple Copies of Online Redo Log Files
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The database also uses the following system files, which reside outside the CDB:
- Automatic Diagnostic Repository (ADR) is a file-based repository for database diagnostic data, such as traces, dumps, the alert log, health monitor reports, and more. It has a unified directory structure across multiple instances and multiple products. The database, Oracle Automatic Storage Management (Oracle ASM), the listener, Oracle Clusterware, and other Oracle products or components store all diagnostic data in the ADR. Each instance of each product stores diagnostic data in its own home directory within the ADR.
- (Optional) Backup files are used for database recovery. You typically restore a backup file when a media failure or user error has damaged or deleted the original file.
- (Optional) Archived redo log files contain an ongoing history of the data changes that the database instance generates. You can use these files and a backup of the database to recover lost data files.
- (Optional) The password file enables users with the SYSDBA, SYSOPER, SYSBACKUP, SYSDG, SYSKM, SYSRAC, and SYSASM roles to connect remotely to the database instance and perform administrative tasks.
- (Optional) Keystores (previously called wallets) are secure software containers that store authentication and signing credentials. Oracle Database supports software keystores, Oracle Key Vault, and other PKCS#11 compatible key management devices.
- (Optional) Block change tracking file contains changed blocks to improve the performance of incremental backups. During an incremental backup, instead of scanning all data blocks to identify which blocks have changed, Oracle Recovery Manager (RMAN) uses this file to identify the changed blocks that need to be backed up.
- (Optional) Flashback Database uses flashback logs to access past versions of data blocks and some information from archived redo logs. This enables you to return a database to its state at a time in the recent past. Flashback Database requires that you configure a fast recovery area for a database because the flashback logs can only be stored there. You can use Flashback Database only if flashback logs are available, so you must set up your database in advance to create flashback logs.