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Oracle® Database VLDB and Partitioning Guide
11g Release 1 (11.1)

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8 Backing Up and Recovering VLDBs

Backup and recovery is one of the most crucial and important jobs for a DBA to protect business data. As the data store grows larger each year, DBAs are continually challenged to ensure that critical data is backed up and that it can be recovered quickly and easily to meet business needs. Very large databases are unique in that they are large and data may come from a myriad of resources. OLTP and data warehouse systems have some distinct characteristics. Generally the availability considerations for a very large OLTP system are no different from the considerations for a small OLTP system. Assuming a fixed allowed downtime, a large OLTP system requires more hardware resources than a small OLTP system.

This chapter proposes an efficient backup and recovery strategy for very large databases to reduce the overall resources necessary to support backup and recovery by leveraging some of the special characteristics that differentiate data warehouses from OLTP systems. This chapter contains the following topics:

Data Warehousing

A data warehouse is a system which is designed to support analysis and decision-making. In a typical enterprise, hundreds or thousands of users may rely on the data warehouse to provide the information to help them understand their business and make better decisions. Therefore, availability is a key requirement for data warehousing. This chapter will address one key aspect of data warehousing availability: the recovery of data after a data loss.

Before looking at the backup and recovery techniques in detail, it is important to discuss specific techniques for backup and recovery of a data warehouse. In particular, one legitimate question might be: why shouldn't a data warehouse's backup and recovery strategy be just like that of every other database system?

A DBA should initially approach the task of data warehouse backup and recovery by applying the same techniques that are used in OLTP systems: the DBA must decide what information to protect and quickly recover when media recovery is required, prioritizing data according to its importance and the degree to which it changes. However, the issue that commonly arises for data warehouses is that an approach that is efficient and cost-effective for a 100 GB OLTP system may not be viable for a 10 TB data warehouse. The backup and recovery may take 100 times longer or require 100 times more storage.

Data Warehouse Characteristics

There are four key differences between data warehouses and OLTP systems that have significant impacts on backup and recovery:

  1. A data warehouse is typically much larger than an OLTP system. Data warehouses over 10's of terabytes are not uncommon and the largest data warehouses grow to orders of magnitude larger. Thus, scalability is a particularly important consideration for data warehouse backup and recovery.

  2. A data warehouse often has lower availability requirements than an OLTP system. While data warehouses are mission critical, there is also a significant cost associated with the ability to recover multiple terabytes in a few hours compared to recovering in a day. Some organizations may determine that in the unlikely event of a failure requiring the recovery of a significant portion of the data warehouse, they may tolerate an outage of a day or more if they can save significant expenditures in backup hardware and storage.

  3. A data warehouse is typically updated via a controlled process called the ETL (Extract, Transform, Load) process, unlike in OLTP systems where end-users are modifying data themselves. Because the data modifications are done in a controlled process, the updates to a data warehouse are often known and reproducible from sources other than redo logs.

  4. A data warehouse contains historical information, and often, significant portions of the older data in a data warehouse are static. For example, a data warehouse may track five years of historical sales data. While the most recent year of data may still be subject to modifications (due to returns, restatements, and so on), the last four years of data may be entirely static. The advantage of static data is that it does not need to be backed up frequently.

These four characteristics are key considerations when devising a backup and recovery strategy that is optimized for data warehouses.

Oracle Backup and Recovery

In general, backup and recovery refers to the various strategies and procedures involved in protecting your database against data loss and reconstructing the database after any kind of data loss. A backup is a representative copy of data. This copy can include important parts of a database such as the control file, archived redo logs, and data files. A backup protects data from application error and acts as a safeguard against unexpected data loss, by providing a way to restore original data.

Physical Database Structures Used in Recovering Data

Before you begin to think seriously about a backup and recovery strategy, the physical data structures relevant for backup and recovery operations must be identified. The files and other structures that make up an Oracle database store data and safeguard it against possible failures. Three basic components are required for the recovery of an Oracle database:

  • Datafiles

  • Redo logs

  • Control file

Datafiles

An Oracle database consists of one or more logical storage units called tablespaces. Each tablespace in an Oracle database consists of one or more files called datafiles, which are physical files located on or attached to the host operating system in which Oracle is running.

The data in a database is collectively stored in the datafiles that constitute each tablespace of the database. The simplest Oracle database would have one tablespace, stored in one datafile. Copies of the datafiles of a database are a critical part of any backup strategy. The sheer size of the datafiles is the main challenge from a VLDB backup and recovery perspective.

Redo Logs

Redo logs record all changes made to a database's datafiles. With a complete set of redo logs and an older copy of a datafile, Oracle can reapply the changes recorded in the redo logs to re-create the database at any point between the backup time and the end of the last redo log. Each time data is changed in an Oracle database, that change is recorded in the online redo log first, before it is applied to the datafiles.

An Oracle database requires at least two online redo log groups. In each group there is at least one online redo log member, an individual redo log file where the changes are recorded. At intervals, Oracle rotates through the online redo log groups, storing changes in the current online redo log while the groups not in use can be copied to an archive location, where they are called archived redo logs (or, collectively, the archived redo log). For high availability reasons, production systems should always use multiple online redo members per group, preferably on different storage systems. Preserving the archived redo log is a major part of your backup strategy, as it contains a record of all updates to datafiles. Backup strategies often involve copying the archived redo logs to disk or tape for longer-term storage.

Control Files

The control file contains a crucial record of the physical structures of the database and their status. Several types of information stored in the control file are related to backup and recovery:

  • Database information required to recover from crashes or to perform media recovery

  • Database structure information, such as datafile details

  • Redo log details

  • Archived log records

  • A record of past RMAN backups

Oracle's datafile recovery process is in part guided by status information in the control file, such as the database checkpoints, current online redo log file, and the datafile header checkpoints for the datafiles. Loss of the control file makes recovery from a data loss much more difficult. The control file should be backed up regularly, to preserve the latest database structural changes, and to simplify recovery.

Backup Type

Backups are divided into physical backups and logical backups:

  • Physical backups are backups of the physical files used in storing and recovering your database, such as datafiles, control files, and archived redo logs. Ultimately, every physical backup is a copy of files storing database information to some other location, whether on disk or offline storage, such as tape.

  • Logical backups contain logical data (for example, tables or stored procedures) extracted from a database with the Oracle Data Pump (export/import) utilities. The data is stored in a binary file that can be imported into an Oracle database.

Physical backups are the foundation of any backup and recovery strategy. Logical backups are a useful supplement to physical backups in many circumstances but are not sufficient protection against data loss without physical backups.

Reconstructing the contents of all or part of a database from a backup typically involves two phases: retrieving a copy of the datafile from a backup, and reapplying changes to the file since the backup, from the archived and online redo logs, to bring the database to the desired recovery point in time. To restore a datafile or control file from backup is to retrieve the file from the backup location on tape, disk, or other media, and make it available to the Oracle Database. To recover a datafile, is to take a restored copy of the datafile and apply to it the changes recorded in the database's redo logs. To recover a whole database is to perform recovery on each of its datafiles.

Backup Tools

Oracle provides the following tools to manage backup and recovery of Oracle databases. Each tool gives you a choice of several basic methods for making backups. The methods include:

  • Recovery Manager (RMAN)

    RMAN reduces the administration work associated with your backup strategy by maintaining an extensive record of metadata about all backups and needed recovery-related files. In restore and recovery operations, RMAN uses this information to eliminate the need for the user to identify needed files. RMAN is performant, supporting file multiplexing and parallel streaming, and verifies blocks for physical and (optionally) logical corruptions, on backup and restore.

    Backup activity reports can be generated using V$BACKUP views and also through Enterprise Manager.

  • Oracle Enterprise Manager

    Enterprise Manager is Oracle's management console that utilizes Recovery Manager for its backup and recovery features. Backup and restore jobs can be intuitively set up and run, with notification of any problems to the user.

  • Oracle Data Pump

    Data Pump provides high speed, parallel, bulk data and metadata movement of Oracle database contents. This utility makes logical backups by writing data from an Oracle database to operating system files. This data can later be imported into an Oracle database.

  • User-Managed Backups

    The database is backed up manually by executing commands specific to your operating system.

Recovery Manager (RMAN)

Oracle Recovery Manager (RMAN), a command-line and Enterprise Manager-based tool, is the Oracle-preferred method for efficiently backing up and recovering your Oracle database. RMAN is designed to work intimately with the server, providing block-level corruption detection during backup and restore. RMAN optimizes performance and space consumption during backup with file multiplexing and backup set compression, and integrates with leading tape and storage media products via the supplied Media Management Library (MML) API.

RMAN takes care of all underlying database procedures before and after backup or restore, freeing dependency on operating system and SQL*Plus scripts. It provides a common interface for backup tasks across different host operating systems, and offers features not available through user-managed methods, such as data file and tablespace-level backup and recovery, parallelization of backup/recovery data streams, incremental backups, autobackup of control file upon database structural changes, backup retention policy, and detailed history of all backups.

See Also:

Oracle Database Backup and Recovery User's Guide for more information on RMAN

Oracle Enterprise Manager

Although Recovery Manager is commonly used as a command line utility, Oracle Enterprise Manager enables backup and recovery using a GUI. Oracle Enterprise Manager (EM) supports commonly used Backup and Recovery features:

  • Backup Configurations to customize and save commonly used configurations for repeated use

  • Backup and Recovery wizards to walk the user through the steps of creating a backup script and submitting it as a scheduled job

  • Backup Job Library to save commonly used Backup jobs that can be retrieved and applied to multiple targets

  • Backup Job Task to submit any RMAN job using a user-defined RMAN script

Backup Management

Enterprise Manager provides the ability to view and perform maintenance against RMAN backups. You can view the RMAN backups, archive logs, control file backups, and image copies. If you select the link on the RMAN backup, then it will display all files that are located in that backup. Extensive statistics about backup jobs, including average throughput, compression ratio, start/end time, and files composing the backup piece can also be viewed from the console.

Oracle Data Pump

Physical backups can be supplemented by using the Data Pump (export/import) utilities to make logical backups of data. Logical backups store information about the schema objects created for a database. Data Pump loads data and metadata into a set of operating system files that can be imported on the same system or moved to another system and imported there.

The dump file set is made up of one or more disk files that contain table data, database object metadata, and control information. The files are written in a binary format. During an import operation, the Data Pump Import utility uses these files to locate each database object in the dump file set.

User-Managed Backups

If you do not want to use Recovery Manager, operating system commands can be used, such as the UNIX dd or tar commands to make backups. In order to create a user-managed online backup, the database must manually be placed into hot backup mode. Hot backup mode causes additional writes to the online log files, increasing their size.

Backup operations can also be automated by writing scripts. You can make a backup of the entire database at once or back up individual tablespaces, datafiles, control files, or archived logs. An entire database backup can be supplemented with backups of individual tablespaces, datafiles, control files, and archived logs.

Operating system commands or third party backup software can be used to perform database backups. Conversely, the third party software must be used to restore the backups of the database.

Data Warehouse Backup and Recovery

Data warehouse recovery is not any different from an OLTP system. However, a data warehouse may not require all of the data to be recovered from a backup, or in the event of a complete failure, restoration of the entire database before user access can commence. An efficient and fast recovery of a data warehouse begins with a well-planned backup.

The next several sections will help you to identify what data should be backed up and guide you to the method and tools that will allow you to recover critical data in the shortest amount of time.

Recovery Time Objective (RTO)

A Recovery Time Objective, or RTO, is the time duration in which you want to be able to recover your data. Your backup and recovery plan should be designed to meet RTOs your company chooses for its data warehouse. For example, you may determine that 5% of the data must be available within 12 hours, 50% of the data must be available after a complete loss of the Oracle Database within 2 days, and the remainder of the data be available within 5 days. In this case you have two RTOs. Your total RTO is 7.5 days.

To determine what your RTO should be, you must first identify the impact of the data not being available. To establish an RTO, follow these four steps:

  1. Analyze and Identify: Understand your recovery readiness, risk areas, and the business costs of unavailable data. In a data warehouse, you should identify critical data that must be recovered in the n days after an outage.

  2. Design: Transform the recovery requirements into backup and recovery strategies. This can be accomplished by organizing the data into logical relationships and criticality.

  3. Build and Integrate: Deploy and integrate the solution into your environment to backup and recover your data. Document the backup and recovery plan.

  4. Manage and Evolve: Test your recovery plans at regular intervals. Implement change management processes to refine and update the solution as your data, IT infrastructure, and business processes change.

Recovery Point Objective (RPO)

A Recovery Point Objective, or RPO, is the maximum amount of data that can be lost before causing detrimental harm to the organization. RPO indicates the data loss tolerance of a business process or an organization in general. This data loss is often measured in terms of time, for example, 5 hours or 2 days worth of data loss. A zero RPO means that no committed data should be lost when media loss occurs, while a 24 hour RPO can tolerate a day's worth of data loss.

More Data Means a Longer Backup Window

The most obvious characteristic of the data warehouse is the size of the database. This can be upwards of 100's of terabytes. Hardware is the limiting factor to a fast backup and recovery. However, today's tape storage continues to evolve to accommodate the amount of data that may need to be offloaded to tape (for example, advent of Virtual Tape Libraries which utilize disk internally with the standard tape access interface). RMAN can fully utilize, in parallel, all available tape devices to maximize backup and recovery performance.

Essentially, the time required to back up a large database can be derived from the minimum throughput among: production disk, HBA/network to tape devices, and tape drive streaming specifications * the number of tape drives. The host CPU can also be a limiting factor to overall backup performance, if RMAN backup encryption or compression is used. Backup and recovery windows can be adjusted to fit any business requirements, given adequate hardware resources.

Divide and Conquer

In a data warehouse, there may be times when the database is not being fully utilized. While this window of time may be several contiguous hours, it is not enough to backup the entire database. Therefore, you may want to consider breaking up the database backup over a number of days. RMAN allows you to specify how long a given backup job is allowed to run. When using BACKUP ... DURATION, you can choose between running the backup to completion as quickly as possible and running it more slowly to minimize the load the backup may impose on your database.

In the following example, RMAN will backup all database files that have not been backed up in the last 7 days first, run for 4 hours, and read the blocks as fast as possible.

BACKUP DATABASE NOT BACKED UP SINCE 'sysdate - 7' PARTIAL DURATION 4:00 MINIMIZE TIME;

Each time this RMAN command is run, it will backup the datafiles that have not been backed up in the last 7 days first. You do not need to manually specify the tablespaces or datafiles to be backed up each night. Over the course of several days, all of your database files will be backed up.

While this is a simplistic approach to database backup, it is easy to implement and provides more flexibility in backing up large amounts of data. Do note that in case of a recovery, RMAN may point you to multiple different storage devices in order to perform the restore. As a result, your recovery time may be longer.

The Data Warehouse Recovery Methodology

Devising a backup and recovery strategy can be a daunting task. When you have 100's of terabytes of data that must be protected and recovered in the case of a failure, the strategy can be very complex. The remainder of this chapter contains several best practices that can be implemented to ease the administration of backup and recovery.

Best Practice 1: Use ARCHIVELOG Mode

Archived redo logs are crucial for recovery when no data can be lost, since they constitute a record of changes to the database. Oracle can be run in either of two modes:

Running the database in ARCHIVELOG mode has the following benefits:

Running the database in NOARCHIVELOG mode has the following consequences:

Is Downtime Acceptable?

Oracle Database backups can be made while the database is open or closed. Planned downtime of the database can be disruptive to operations, especially in global enterprises that support users in multiple time zones, up to 24-hours per day. In these cases, it is important to design a backup plan to minimize database interruptions.

Depending on the business, some enterprises can afford downtime. If the overall business strategy requires little or no downtime, then the backup strategy should implement an online backup. The database needs never to be taken down for a backup. An online backup requires the database to be in ARCHIVELOG mode.

Given the size of a data warehouse (and consequently the amount of time to back up a data warehouse), it is generally not viable to make an offline backup of a data warehouse, which would be necessitated if one were using NOARCHIVELOG mode.

Best Practice 2: Use RMAN

Many data warehouses, which were developed on earlier releases of the Oracle Database, may not have integrated RMAN for backup and recovery. However, just as there is a preponderance of reasons to leverage ARCHIVELOG mode, there is a similarly compelling list of reasons to adopt RMAN. Consider the following:

  1. Trouble-free backup and recovery

  2. Corrupt block detection

  3. Archive log validation and management

  4. Block Media Recovery (BMR)

  5. Easily integrates with Media Managers

  6. Backup and restore optimization

  7. Backup and restore validation

  8. Downtime free backups

  9. Incremental backups

  10. Extensive reporting

Best Practice 3: Use Block Change Tracking

Enabling block change tracking allows incremental backups to be completed faster, by only reading and writing the changed blocks since the last full or incremental backup. For data warehouses, this can be extremely helpful if the database typically undergoes a low to medium percentage of changes.

See Also:

Oracle Database Backup and Recovery User's Guide for more information on block change tracking

Best Practice 4: Use RMAN Multi-Section Backups

With the advent of bigfile tablespaces, data warehouses have the opportunity to consolidate a large number of datafiles into fewer, better managed data files. For backing up very large datafiles, RMAN provides multi-section backups as a way to parallelize the backup operation within the file itself, such that sections of a file are backed up in parallel, rather than backing up on a per-file basis.

For example, a 1 TB data file can be sectioned into 10 100 GB backup pieces, with each section backed up in parallel, rather than the entire 1 TB file backed up at once. Thus, the overall backup time for large datafiles can be dramatically reduced.

See Also:

Oracle Database Backup and Recovery User's Guide for more information on configuring multi-section backups

Best Practice 5: Leverage Read-Only Tablespaces

One of the biggest issues facing a data warehouse is the sheer size of a typical data warehouse. Even with powerful backup hardware, backups may still take several hours. Thus, one important consideration in improving backup performance is minimizing the amount of data to be backed up. Read-only tablespaces are the simplest mechanism to reduce the amount of data to be backed up in a data warehouse. Even with incremental backups, both backup and recovery will be faster if tablespaces are set to read-only.

The advantage of a read-only tablespace is that data only needs to be backed up once. If a data warehouse contains five years of historical data and the first four years of data can be made read-only, then theoretically the regular backup of the database would only back up 20% of the data. This can dramatically reduce the amount of time required to back up the data warehouse.

Most data warehouses store their data in tables that have been range-partitioned by time. In a typical data warehouse, data is generally active for a period ranging anywhere from 30 days to one year. During this period, the historical data can still be updated and changed (for example, a retailer may accept returns up to 30 days beyond the date of purchase, so that sales data records could change during this period). However, once data reaches a certain age, it is often known to be static.

By leveraging partitioning, users can make the static portions of their data read-only. Currently, Oracle supports read-only tablespaces rather than read-only partitions or tables. To take advantage of the read-only tablespaces and reduce the backup window, a strategy of storing constant data partitions in a read-only tablespace should be devised. Here are two strategies for implementing a rolling window.

  1. Implement a regularly scheduled process to move partitions from a read-write tablespace to a read-only tablespace when the data matures to the point where it is entirely static.

  2. Create a series of tablespaces, each containing a small number of partitions and regularly modify one tablespace from read-write to read-only as the data in that tablespace ages.

One consideration is that backing up data is only half of the recovery process. If you configure a tape system so that it can backup the read-write portions of a data warehouse in 4 hours, the corollary is that a tape system might take 20 hours to recover the database if a complete recovery is necessary when 80% of the database is read-only.

Best Practice 6: Plan for NOLOGGING Operations in Your Backup/Recovery Strategy

In general, one of the highest priorities for a data warehouse is performance. Not only must the data warehouse provide good query performance for online users, but the data warehouse must also be efficient during the ETL process so that large amounts of data can be loaded in the shortest amount of time.

One common optimization leveraged by data warehouses is to execute bulk-data operations using the NOLOGGING mode. The database operations which support NOLOGGING modes are direct-path loads and inserts, index creation, and table creation. When an operation runs in NOLOGGING mode, data is not written to the redo log (or more precisely, only a small set of metadata is written to the redo log). This mode is widely used within data warehouses and can improve the performance of bulk data operations by up to 50%.

However, the tradeoff is that a NOLOGGING operation cannot be recovered using conventional recovery mechanisms, since the necessary data to support the recovery was never written to the log file. Moreover, subsequent operations to the data upon which a NOLOGGING operation has occurred also cannot be recovered even if those operations were not using NOLOGGING mode. Because of the performance gains provided by NOLOGGING operations, it is generally recommended that data warehouses utilize NOLOGGING mode in their ETL process.

The presence of NOLOGGING operations must be taken into account when devising the backup and recovery strategy. When a database is relying on NOLOGGING operations, the conventional recovery strategy (of recovering from the latest tape backup and applying the archived logfiles) is no longer applicable because the log files will not be able to recover the NOLOGGING operation.

The first principle to remember is, don't make a backup when a NOLOGGING operation is occurring. Oracle does not currently enforce this rule, so DBAs must schedule the backup jobs and the ETL jobs such that the NOLOGGING operations do not overlap with backup operations.

There are two approaches to backup and recovery in the presence of NOLOGGING operations; ETL or incremental backups. If you are not using NOLOGGING operations in your data warehouse, then you do not have to choose either of the following options: you can recover your data warehouse using archived logs. However, the following options may offer some performance benefits over an archive log-based approach in the event of recovery. You can also use flashback logs and guaranteed restore points to flashback your database to a previous point in time.

Extract, Transform, and Load

The ETL process uses several Oracle features and a combination of methods to load (re-load) data into a data warehouse. These features consist of:

  • Transportable Tablespaces

    Transportable Tablespaces allow users to quickly move a tablespace across Oracle databases. It is the most efficient way to move bulk data between databases. Oracle Database provides the ability to transport tablespaces across platforms. If the source platform and the target platform are of different endianness, then RMAN will convert the tablespace being transported to the target format.

  • SQL*Loader

    SQL*Loader loads data from external flat files into tables of an Oracle Database. It has a powerful data parsing engine that puts little limitation on the format of the data in the datafile.

  • Data Pump (export/import)

    Oracle Data Pump enables high speed movement of data and metadata from one Oracle database to another. This technology is the basis for Oracle's Data Pump Export and Data Pump Import utilities.

  • External Tables

    External Tables is a complement to existing SQL*Loader functionality. It enables you to access data in external sources as if it were in a table in the database. External tables can also be used with the Data Pump driver in order to export data from an Oracle database, using CREATE TABLE ... AS SELECT * FROM, and then import data into an Oracle database.

The ETL Strategy

One approach is to take regular database backups and also store the necessary data files to re-create the ETL process for that entire week. In the event where a recovery is necessary, the data warehouse could be recovered from the most recent backup. Then, instead of rolling forward by applying the archived redo logs (as would be done in a conventional recovery scenario), the data warehouse could be rolled forward by re-running the ETL processes. This paradigm assumes that the ETL processes can be easily replayed, which would typically involve storing a set of extract files for each ETL process.

A sample implementation of this approach is to make a backup of the data warehouse every weekend, and then store the necessary files to support the ETL process each night. Thus, at most, 7 days of ETL processing would need to be re-applied in order to recover a database. The data warehouse administrator can easily project the length of time to recover the data warehouse, based upon the recovery speeds from tape and performance data from previous ETL runs.

Essentially, the data warehouse administrator is gaining better performance in the ETL process via NOLOGGING operations, at a price of slightly more complex and a less automated recovery process. Many data warehouse administrators have found that this is a desirable trade-off.

One downside to this approach is that the burden is on the data warehouse administrator to track all of the relevant changes that have occurred in the data warehouse. This approach will not capture changes that fall outside of the ETL process. For example, in some data warehouses, end-users may create their own tables and data structures. Those changes will be lost in the event of a recovery.

This restriction needs to be conveyed to the end-users. Alternatively, one could also mandate that end-users create all private database objects in a separate tablespace, and during recovery, the DBA could recover this tablespace using conventional recovery while recovering the rest of the database using the approach of replaying the ETL process.

Incremental Backup

A more automated backup and recovery strategy in the presence of NOLOGGING operations leverages RMAN's incremental backup capability. Incremental backups provide the capability to backup only the changed blocks since the previous backup. Incremental backups of datafiles capture data changes on a block-by-block basis, rather than requiring the backup of all used blocks in a datafile. The resulting backup sets are generally smaller and more efficient than full datafile backups, unless every block in the datafile is change.

When you enable block change tracking, Oracle tracks the physical location of all database changes. RMAN automatically uses the change tracking file to determine which blocks need to be read during an incremental backup. The block change tracking file is approximately 1/30000 of the total size of the database.

See Also:

Oracle Database Backup and Recovery User's Guide for more information on block change tracking and how to enable it

The Incremental Approach

A typical backup and recovery strategy using this approach is to backup the data warehouse every weekend, and then take incremental backups of the data warehouse every night following the completion of the ETL process. Note that incremental backups, like conventional backups, must not be run concurrently with NOLOGGING operations. In order to recover the data warehouse, the database backup would be restored, and then each night's incremental backups would be re-applied.

Although the NOLOGGING operations were not captured in the archivelogs, the data from the NOLOGGING operations is present in the incremental backups. Moreover, unlike the previous approach, this backup and recovery strategy can be completely managed using RMAN.

Flashback Database and Guaranteed Restore Points

Flashback Database is a fast, continuous point-in-time recovery method to repair widespread logical errors. Flashback Database relies on additional logging, called flashback logs, which are created in the Flash Recovery Area and retained for a user-defined period of time according to the recovery needs. These logs track the original block images when they are updated.

When a Flashback Database operation is executed, just the block images corresponding to the changed data are restored and recovered, versus traditional data file restore where all blocks from the backup need to be restored before recovery can start. Flashback logs are created proportionally to redo logs.

For very large and active databases, it may be infeasible to keep all needed flashback logs for continuous point-in-time recovery. However, there may be a need to create a specific point-in-time snapshot (for example, right before a nightly batch job) in the event of logical errors during the batch run. For this scenario, guaranteed restore points (GRP) can be created without enabling flashback logging.

When the GRP is created, flashback logs are maintained just to satisfy flashback database to the GRP and no other point in time, thus saving space. For example, a GRP can be created followed by a nologging batch job. As long as there are no prior nologging operations within the last hour of the creation time of the GRP, flashback database to the GRP will undo the nologging batch job. To flash back to a time after the nologging batch job finishes, then create the GRP at least one hour away from the end of the batch job.

Estimating flashback log space for GRP in this scenario depends on how much of the database will change over the number of days you intend to keep GRP. For example, to keep a GRP for 2 days and you expect 100 GB of the database to change, then plan for 100 GB for the flashback logs. Note that the 100 GB refers to the subset of the database changed after the GRP is created and not the frequency of changes.

Best Practice 7: Not All Tablespaces Are Created Equal

Not all of the tablespaces in a data warehouse are equally significant from a backup and recovery perspective. DBA's can leverage this information to devise more efficient backup and recovery strategies when necessary. The basic granularity of backup and recovery is a tablespace, so different tablespaces can potentially have different backup and recovery strategies.

On the most basic level, temporary tablespaces never need to be backed up (a rule which RMAN enforces). Moreover, in some data warehouses, there may be tablespaces dedicated to scratch space for end-users to store temporary tables and incremental results. These tablespaces are not explicit temporary tablespaces but are essentially functioning as temporary tablespaces. Depending upon the business requirements, these tablespaces may not need to be backed up and restored; instead, in the case of a loss of these tablespaces, the end-users would re-create their own data objects.

In many data warehouses, some data is more important than other data. For example, the sales data in a data warehouse may be crucial and in a recovery situation this data must be online as soon as possible. But, in the same data warehouse, a table storing clickstream data from the corporate website may be much less mission-critical. The business may tolerate this data being offline for a few days or may even be able to accommodate the loss of several days of clickstream data in the event of a loss of database files. In this scenario, the tablespaces containing sales data must be backed up often, while the tablespaces containing clickstream data need only to be backed up once every week or two.

While the simplest backup and recovery scenario is to treat every tablespace in the database the same, Oracle provides the flexibility for a DBA to devise a backup and recovery scenario for each tablespace as needed.