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Standby Database Directory Structure

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Learn Oracle - Standby Database Directory Structure Considerations

The directory structure of the various standby databases is important because it determines the path names for the standby datafiles, archived redo log files, and standby redo log files. If possible, the datafiles, log files, and control files on the primary and standby systems should have the same names and path names and use Optimal Flexible Architecture (OFA) naming conventions. The archival directories on the standby database should also be identical between sites, including size and structure. This strategy allows other operations such as backups, switchovers, and failovers to execute the same set of steps, reducing the maintenance complexity.

Otherwise, oracle DBA must set the filename conversion parameters (as shown in Table ) or rename the data file. Nevertheless, if oracle DBA need to use a system with a different directory structure or place the standby and primary databases on the same system, you can do so with a minimum of extra administration.

The three basic configuration options are illustrated in Figure . These include:

  • A standby database on the same system as the primary database that uses a different directory structure than the primary system. This is illustrated in Figure as Standby1.

    If oracle DBA have a standby database on the same system as the primary database, you must use a different directory structure. Otherwise, the standby database attempts to overwrite the primary database files.

  • A standby database on a separate system that uses the same directory structure as the primary system. This is illustrated in Figure as Standby2. This is the recommended method.

  • A standby database on a separate system that uses a different directory structure than the primary system. This is illustrated in Figure as Standby3.

Standby Configuration

Table describes possible configurations of primary and standby databases and the consequences of each. In the table, note that the service name defaults to the concatenation of the DB_UNIQUE_NAME and DB_DOMAIN initialization parameters. You must specify a unique value for the DB_UNIQUE_NAME initialization parameter when more than one member of a Data Guard configuration resides on the same system. Oracle recommends that the value of the DB_UNIQUE_NAME initialization parameter always be unique, even if each database is located on a separate system.

Standby System Directory Structure Consequences
Same as primary system Different than primary system (required)
  • oracle DBA must set the DB_UNIQUE_NAME initialization parameter.
  • You can either manually rename files or set up the DB_FILE_NAME_CONVERT and LOG_FILE_NAME_CONVERT initialization parameters on the standby database to automatically update the path names for primary database datafiles and archived redo log files and standby redo log files in the standby database control file
  • The standby database does not protect against disasters that destroy the system on which the primary and standby databases reside, but it does provide switchover capabilities for planned maintenance..
Separate system Same as primary system
  • You do not need to rename primary database files, archived redo log files, and standby redo log files in the standby database control file, although oracle DBA can still do so if you want a new naming scheme (for example, to spread the files among different disks).
  • By locating the standby database on separate physical media, you safeguard the data on the primary database against disasters that destroy the primary system.
Separate system Different than primary system
  • oracle DBA can either manually rename files or set up the DB_FILE_NAME_CONVERT and LOG_FILE_NAME_CONVERT initialization parameters on the standby database to automatically rename the datafiles .
  • By locating the standby database on separate physical media, you safeguard the data on the primary database against disasters that destroy the primary system.

Learn Oracle -Online Redo Logs, Archived Redo Logs, and Standby Redo Logs

The most crucial structures for Data Guard recovery operations are online redo logs, archived redo logs, and standby redo logs. Redo data transmitted from the primary database is received by the remote file server (RFS) process on the standby system where the RFS process writes the redo data to archived log files or standby redo log files. Redo data can be applied either after the redo is written to the archived redo log file or standby redo log file, or, if real-time apply is enabled, directly from the standby redo log file as it is being filled.

This article assumes that you already understand the concepts behind online redo logs and archived redo logs.

Learn Oracle - Online Redo Logs and Archived Redo Logs
The transmission of redo is integral to maintaining the transactional consistency of the primary and standby databases. Both online redo logs and archived redo logs are required in a Data Guard environment:

Online redo logs

Every instance of an Oracle primary database and logical standby database has an online redo log to protect the database in case of an instance failure. Physical standby databases do not use an online redo log, because physical standby databases are not opened for read/write I/O. Changes are not made to the physical standby database and new redo data is not generated.

Archived redo logs An archived redo log is required because archiving is the method used to keep standby databases transactionally consistent with the primary database. Primary databases, and both physical and logical standby databases all use an archived redo log. Oracle databases are set up, by default, to run in ARCHIVELOG mode so that the archiver (ARCn) process automatically copies each filled online redo log file to one or more archived redo log files.

Unlike physical standby databases, logical standby databases are open databases that generate redo data and have multiple log files, including online redo log files, archived redo log files, and standby redo log files (if configured).

Both the size of the online redo log files and the frequency with which a log switch occurs can affect the generation of the archived redo log files at the primary site. The Oracle Database High Availability Overview provides recommendations for log group sizing.

An Oracle database will attempt a checkpoint at each log switch. Therefore, if the size of the online redo log file is too small, frequent log switches lead to frequent checkpointing and negatively affect system performance on the standby database.

Standby Redo Logs

A standby redo log is similar to an online redo log, except that a standby redo log is used to store redo data received from another database.

A standby redo log is required if oracle DBA want to implement:

  • The maximum protection and maximum availability levels of data protection

  • Real-time apply

  • Cascaded destinations

A standby redo log provides a number of advantages:

  • Standby redo log files can reside on raw devices, which may be important if either or both the primary and standby databases reside in a Real Application Clusters environment.

  • Standby redo log files can be multiplexed using multiple members, improving reliability over archived log files.

  • During a failover, Data Guard can recover and apply more redo data from standby redo log files than from the archived log files alone.

  • The archiver (ARCn) process or the log writer (LGWR) process on the primary database can transmit redo data directly to remote standby redo log files, potentially eliminating the need to register a partial archived log file (for example, to recover after a standby database crashes). See Chapter 5 for more information.



Related Topics:

Learn Oracle - Oracle Data Guard
Learn Oracle - Standby Database Types

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We learned Standby database types today. Next we will move to User Interfaces for Administering Data Guard Configurations.

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