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Getting Started with Data Guard

A Data Guard configuration contains a primary database and up to nine associated standby databases. This chapter describes the following considerations for getting started with Data Guard:

• Standby Database Types
• User Interfaces for Administering Data Guard Configurations
• Data Guard Operational Prerequisites
• Standby Database Directory Structure Considerations
• Online Redo Logs, Archived Redo Logs, and Standby Redo Logs

2.1 Standby Database Types

A standby database is a transactionally consistent copy of an Oracle production database that is initially created from a backup copy of the primary database. Once the standby database is created and configured, Data Guard automatically maintains the standby database by transmitting primary database redo data to the standby system, where the redo data is applied to the standby database.

A standby database can be one of two types: a physical standby database or a logical standby database. If needed, either type of standby database can assume the role of the primary database and take over production processing. A Data Guard configuration can include physical standby databases, logical standby databases, or a combination of both types.

The following sections describe standby databases in more detail. See Oracle High Availability Architecture and Best Practices for information that can help you determine which type is most appropriate for your business.

2.1.1 Physical Standby Databases

A physical standby database is physically identical to the primary database, with on disk database structures that are identical to the primary database on a block-for-block basis. The database schema, including indexes, are identical.

Data Guard maintains a physical standby database by performing Redo Apply. When it is not performing recovery, a physical standby database can be open in read-only mode.

• Redo Apply

  The physical standby database is maintained by applying redo data from the archived redo log files or directly from standby redo log files on the standby system using the Oracle recovery mechanism. The recovery operation applies changes block for block using the data block address. The database cannot be opened while redo is being applied.

• Open read-only

  The physical standby database can be open in read-only mode so that you can execute queries on the database. While opened in read-only mode, the standby database can continue to receive redo data, but application of the redo data from the log files is deferred until the database resumes Redo Apply.

Although the physical standby database cannot perform both Redo Apply and be opened in read-only mode at the same time, you can switch between them. For example, you can run a physical standby database to perform Redo Apply, then open it in read-only mode for applications to run reports, and then change it back to perform Redo Apply to apply any outstanding archived redo log files. You can repeat this cycle, alternating between Redo Apply and read-only, as necessary.

In either case, the physical standby database is available to perform backups. Furthermore, the physical standby database will continue to receive redo data even if archived redo log files or standby redo log files are not being applied at that moment.

Benefits of a Physical Standby Database

A physical standby database provides the following benefits:

• Disaster recovery and high availability

  A physical standby database enables a robust and efficient disaster recovery and high availability solution. Easy-to-manage switchover and failover capabilities allow easy role reversals between primary and physical standby databases, minimizing the downtime of the primary database for planned and unplanned outages.

• Data protection

  Using a physical standby database, Data Guard can ensure no data loss, even in the face of unforeseen disasters. A physical standby database supports all datatypes, and DDL and DML operations that the primary database can support. It also provides a safeguard against data corruptions and user errors. Storage level physical corruptions on the primary database do not propagate to the standby database. Similarly, logical corruptions or user errors that cause the primary database to be permanently damaged can be resolved. Finally, the redo data is validated when it is applied to the standby database.

• Reduction in primary database workload

  Oracle Recovery Manager (RMAN) can use physical standby databases to off-load backups from the primary database saving valuable CPU and I/O cycles. The physical standby database can also be opened in read-only mode for reporting and queries.

• Performance

  The Redo Apply technology used by the physical standby database applies changes using low-level recovery mechanisms, which bypass all SQL level code layers; therefore, it is the most efficient mechanism for applying changes. This makes the Redo Apply technology an efficient mechanism to propagate changes among databases.

2.1.2 Logical Standby Databases

A logical standby database is initially created as an identical copy of the primary database, but it later can be altered to have a different structure. The logical standby database is updated by executing SQL statements. This allows users to access the standby database for queries and reporting at any time. Thus, the logical standby database can be used concurrently for data protection and reporting operations.

Data Guard automatically applies information from the archived redo log file or standby redo log file to the logical standby database by transforming the data in the log files into SQL statements and then executing the SQL statements on the logical standby database. Because the logical standby database is updated using SQL statements, it must remain open. Although the logical standby database is opened in read/write mode, its target tables for the regenerated SQL are available only for read-only operations. While those tables are being updated, they can be used simultaneously for other tasks such as reporting, summations, and queries. Moreover, these tasks can be optimized by creating additional indexes and materialized views on the maintained tables.

A logical standby database has some restrictions on datatypes, types of tables, and types of DDL and DML operations. Section 4.1.1 describes the unsupported datatypes and storage attributes for tables.

Benefits of a Logical Standby Database

A logical standby database provides similar disaster recovery, high availability, and data protection benefits as a physical standby database. It also provides the following specialized benefits:

• Efficient use of standby hardware resources

  A logical standby database can be used for other business purposes in addition to disaster recovery requirements. It can host additional database schemas beyond the ones that are protected in a Data Guard configuration, and users can perform normal DDL or DML operations on those schemas any time. Because the logical standby tables that are protected by Data Guard can be stored in a different physical layout than on the primary database, additional indexes and materialized views can be created to improve query performance and suit specific business requirements.

• Reduction in primary database workload

  A logical standby database can remain open at the same time its tables are updated from the primary database, and those tables are simultaneously available for read access. This makes a logical standby database an excellent choice to do queries, summations, and reporting activities, thereby off-loading the primary database from those tasks and saving valuable CPU and I/O cycles.

2.2 User Interfaces for Administering Data Guard Configurations

You can use the following interfaces to configure, implement, and manage a Data Guard configuration:

• Oracle Enterprise Manager

  Enterprise Manager provides a GUI interface for the Data Guard broker that automates many of the tasks involved in creating, configuring, and monitoring a Data Guard environment. See Oracle Data Guard Broker and the Oracle Enterprise Manager online help for information about the GUI and its wizards.

• Command-line interface:
  • SQL*Plus

  Several SQL*Plus statements use the STANDBY keyword to specify operations on a standby database. Other SQL statements do not include standby-specific syntax, but they are useful for performing operations on a standby database. See Chapter 13 for a list of the relevant statements.

• Data Guard broker command-line interface

2.3 Data Guard Operational Prerequisites

The following sections describe operational requirements for using Data Guard:

• Hardware and Operating System Requirements
• Oracle Software Requirements

2.3.1 Hardware and Operating System Requirements

The following list describes hardware and operating system requirements for using Data Guard:

• The operating system and platform architecture on the primary and standby locations must be the same.

  For example, this means a Data Guard configuration with a primary database on a 32-bit Solaris system must have a standby database that is configured on a 32-bit Solaris system. Similarly, a primary database on a 64-bit HP-UX system must be configured with a standby database on a 64-bit HP-UX system, and a primary database on a 32-bit Linux on Intel system must be configured with a standby database on a 32-bit Linux on Intel system, and so forth.

• The hardware (for example, the number of CPUs, memory size, storage configuration) can be different between the primary and standby systems.

  If the standby system is smaller than the primary system, you may have to restrict the work that can be done on the standby system after a switchover or failover. The standby system must have enough resources available to receive and apply all redo data from the primary database. The logical standby database requires additional resources to translate the redo data into SQL statements and then execute the SQL on the logical standby database.

• The operating system running on the primary and standby locations must be the same, but the operating system release does not need to be the same. In addition, the standby database can use a different directory structure from the primary database.

2.3.2 Oracle Software Requirements

The following list describes Oracle software requirements for using Data Guard:

• Oracle Data Guard is available only as a feature of Oracle Database Enterprise Edition. It is not available with Oracle Database Standard Edition. This means the same release of Oracle Database Enterprise Edition must be installed on the primary database and all standby databases in a Data Guard configuration.

  Note: It is possible to simulate a standby database environment with databases running Oracle Database Standard Edition. You can do this by manually transferring archived redo log files using an operating system copy utility or using custom scripts that periodically send archived redo log files from one database to the other. The consequence is that this configuration does not provide the ease-of-use, manageability, performance, and disaster-recovery capabilities available with Data Guard.

  Using Data Guard SQL Apply, you will be able to perform a rolling upgrade of the Oracle database software from patchset release 10.1.0.n to the next database 10.1.0.(n+1) patchset release. During a rolling upgrade, you can run different releases of the Oracle database (10.1.0.1 and higher) on the primary and logical standby databases while you upgrade them, one at a time. For complete information, see Section 9.2 and the ReadMe file for the applicable Oracle Database 10g patchset release.

• If you are currently running Oracle Data Guard on Oracle8i database software, see Oracle Database Upgrade Guide for complete information about upgrading to Oracle Data Guard.

• The primary database must run in ARCHIVELOG mode.
• The primary database can be a single instance database or a multi-instance Real Application Clusters database. The standby databases can be single instance databases or multi-instance Real Application Clusters (RAC) databases, and these standby databases can be a mix of both physical and logical types. See Oracle High Availability Architecture and Best Practices for more information about configuring and using Oracle Data Guard with RAC.
• Each primary database and standby database must have its own control file.
• If a standby database is located on the same system as the primary database, the archival directories for the standby database must use a different directory structure than the primary database. Otherwise, the standby database may overwrite the primary database files.
• To protect against unlogged direct writes in the primary database that cannot be propagated to the standby database, turn on FORCE LOGGING at the primary database before performing datafile backups for standby creation. Keep the database in FORCE LOGGING mode as long as the standby database is required.
• The user accounts you use to manage the primary and standby database instances must have SYSDBA system privileges.
• Oracle recommends that when you set up Oracle Automatic Storage Management (ASM) and Oracle Managed Files (OMF) in a Data Guard configuration, set it up symmetrically on the primary and standby database. That is, if any database in the Data Guard configuration uses ASM, OMF, or both, then every database in the configuration should use ASM, OMF, or both, respectively. See the scenario in Section 10.9 for more information.

  Note: Because some applications that perform updates involving time-based data cannot handle data entered from multiple time zones, consider setting the time zone for the primary and remote standby systems to be the same to ensure the chronological ordering of records is maintained after a role transition.

2.4 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, you must set the filename conversion parameters (as shown in Table 2-1) or rename the datafile. Nevertheless, if you 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 2-1. 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 2-1 as Standby1.

  If you 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 2-1 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 2-1 as Standby3.

  Note: if any database in the Data Guard configuration uses ASM, OMF, or both, then every database in the configuration should use ASM, OMF, or both, respectively. See Chapter 10 for a scenario describing how to set up OMF in a Data Guard configuration.

Figure 2-1 Possible Standby Configurations

Text description of the illustration sbr81097.gif

Table 2-1 describes possible configurations of primary and standby databases and the consequences of each. In the table, note that the service name defaults to the global database name, which is a concatenation of the database name (DB_NAME) and domain name (DB_DOMAIN) parameters. If you do not explicitly specify unique service names when the primary and standby databases reside on the same system, the same default global database name will be in effect for both the primary and standby databases.

Table 2-1  Standby Database Location and Directory Options

Standby System	Directory Structure	Consequences
Same as primary system	Different than primary system (required)	You 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. (See Section 3.1.3.) You must explicitly set up unique service names for the primary and standby databases with the SERVICE_NAMES initialization parameter. 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 you 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	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 rename the datafiles (see Section 3.1.3). By locating the standby database on separate physical media, you safeguard the data on the primary database against disasters that destroy the primary system.

2.5 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 documentation assumes that you already understand the concepts behind online redo logs and archived redo logs. Section 2.5.1 supplements the basic concepts by providing information that is specific to Data Guard configurations. Section 2.5.2 provides detailed information about using standby redo log files.

See Oracle Database Administrator's Guide for more information about redo logs and archive logs, and Section 6.2.1 for information about real-time apply.

2.5.1 Online Redo Logs and Archived Redo Logs

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 associated online redo log to protect the database in case of an instance failure. Physical standby databases do not have an associated online redo log, because physical standby databases are never opened for read/write I/O; changes are not made to the database and 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 each have 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.

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 High Availability Architecture and Best Practices 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.

2.5.2 Standby Redo Logs

A standby redo log is similar in all ways to an online redo log, except that a standby redo log is used only when the database is running in the standby role to store redo data received from the primary database.

A standby redo log is required to implement:

• The maximum protection and maximum availability levels of data protection (described in Section 1.4 and in more detail in Section 5.6)
• Real-time apply (described in Section 6.2)
• Cascaded redo log destinations (described in Appendix C)

Configuring standby redo log files is highly recommended on all standby databases in a Data Guard configuration, because they provide a number of advantages:

• Because a standby redo log consists of preallocated files, a standby redo log avoids the operating system overhead of file system metadata updates common with sequential files (such as with an archive log).
• 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.

Section 5.6.2 describes how to configure standby redo log files.