If any control file fails, then your database becomes unavailable. If you have a control file copy, however, you can shut down your database and re-create the failed control file from the copy, then restart your database. Another option is to delete the failed control file from the CONTROL_FILES initialization parameter and restart your database using the remaining control files.

The online redo log stores a copy of the changes made to data. If a failure requires a data file to be restored from backup, then the recent data changes that are missing from the restored data file can be obtained from the online redo log files, so work is never lost. The online redo log files are used to recover a database after hardware, software, or media failure. To protect against a failure involving the online redo log file itself, Oracle Database can multiplex the online redo log file so that two or more identical copies of the online redo log file can be maintained on different disks.

The online redo log for a database consists of groups of online redo log files. A group consists of an online redo log file and its multiplexed copies. Each identical copy is considered to be a member of that group. Each group is defined by a number, such as Group 1.

Figure 6-2 shows the configuration of a database that has three online redo log groups and two members in each group. For each group, the members are stored on separate disks for maximum availability. For example, the members of Group 1 are the redo log files A_LOG1 and B_LOG1.

Figure 6-2 Online Redo Log Groups and Their Members

Description of "Figure 6-2 Online Redo Log Groups and Their Members"

The database log writer process (LGWR) writes redo records from the memory buffer to a redo log group until the log files in that group reach their storage size limit, or until you request a log switch operation. The LGWR process then writes to the next log group. The LGWR process performs this action in a circular fashion so that the oldest group is overwritten by the most recent redo records.

These archived redo log files extend the amount of redo data that can be saved and are used for recovery. Archived redo log files are required to recover a backup of the database from the time of the backup to the current time. Archiving can be either enabled or disabled for the database, but Oracle strongly recommends that you enable archiving. Oracle also recommends that you configure the database to write archived redo log files to the fast recovery area.

Data files can be broken down into the following components:

• Segment

  A segment contains a specific type of database object. For example, a table is stored in a table segment, and an index is stored in an index segment. A data file can contain many segments.

• Extent

  An extent is a contiguous set of data blocks within a segment. Oracle Database allocates space for segments in units of one extent. When the existing extents of a segment are full, the database allocates another extent for that segment.

• Data block

  A data block, also called a database block, is the smallest unit of I/O to database storage. An extent consists of several contiguous data blocks. The database uses a default block size at database creation.

  After the database has been created, it is not possible to change the default block size without re-creating the database. It is possible, however, to create a tablespace with a block size different than the default block size.

Segments, extents, and data blocks are all logical structures. Only Oracle Database can determine how many data blocks are in a file. The operating system recognizes only files and operating system blocks, not the number of data blocks in an Oracle Database file. Each data block maps to one or more operating system blocks.

A database is divided into logical storage units called tablespaces, which group related logical structures (such as tables, views, and other database objects). For example, all application objects can be grouped into a single tablespace to simplify maintenance operations.

A tablespace consists of one or more physical data files. Database objects assigned to a tablespace are stored in the physical data files of that tablespace.

When you create an Oracle database, some tablespaces already exist, such as SYSTEM and SYSAUX.

Tablespaces provide a means to physically locate data on storage. When you define the data files that comprise a tablespace, you specify a storage location for these files. For example, you might specify a data file location for a certain tablespace as a designated host directory (implying a certain disk volume) or designated Oracle Automatic Storage Management disk group. Any schema objects assigned to that tablespace then get located in the specified storage location. Tablespaces also provide a unit of backup and recovery. The backup and recovery features of Oracle Database enable you to back up or recover at the tablespace level.

Table 6-1 describes some tablespaces included in the database.

You can create new tablespaces to support your user and application data requirements. During tablespace creation, you set the following parameters:

• Locally Managed Tablespaces

• Tablespace Types

• Tablespace Status

• Autoextend Tablespace

• Encrypted Tablespaces

Other storage structures that can exist in an Oracle database include the initialization parameter file, the password file, and backup files.

You can use Oracle Enterprise Manager Database Express (EM Express) to view location and status information about control files.

1. In EM Express, from the Storage menu, select Control Files.

   The Control Files page appears.

   
   Description of the illustration exp_control_files_page.gif

   This page shows whether your database has a multiplexed control file. The List of Control Files and Control Files sections give you more detailed information about your control files.

2. To back up the current control file to a trace file, click Backup to Trace.

   See Oracle Database Backup and Recovery User’s Guide for more information about backing up the control file to a trace file.

You can use Oracle Enterprise Manager Database Express (EM Express) to view status and multiplexing information about online redo log files.

1. In EM Express, from the Storage menu, select Redo Log Groups.

   The Redo Log Groups page appears. This page shows the attributes of the online redo log groups for your database.

   When an online redo log group contains only one member, it is not multiplexed. Note the Status attribute for the online redo log groups. The online redo log group with status Current is the one currently being written to disk by the log writer.

2. (Optional) Expand a redo log group number to view information about online redo log group members.

You can use Oracle Enterprise Manager Database Express (EM Express) to view status information about archived redo log files.

1. In EM Express, from the Storage menu, select Archive Logs.

   The Archive Logs page appears.

2. View information on the Archive Logs page about the redo logs that have been archived.

You can use Oracle Enterprise Manager Database Express (EM Express) to view configuration, size, and status information about tablespaces and data files.

1. In EM Express, from the Storage menu, select Tablespaces.

   The Tablespaces page appears.

2. To view information about the data files for a particular tablespace, expand that tablespace. Information about the data files appears underneath the tablespace.

1. In Oracle Enterprise Manager Database Express (EM Express), from the Storage menu, select Tablespaces.

   The Tablespaces page appears.

   
   Description of the illustration exp_tablespaces_page.gif
2. To create a new tablespace, click the Create button.

   The Create Tablespace wizard appears, showing the General page.

3. In the Name field, enter a name for the tablespace.
4. In the Bigfile section, select Smallfile.

   Note:

   If you select Use bigfile tablespace, then the tablespace can have only one data file. Bigfile tablespaces are used with very large databases that use Oracle Automatic Storage Management or other logical volume managers that support striping, RAID, and dynamically extensible logical volumes.

   EM Express does not support Oracle Automatic Storage Management database instances.

5. In the Status section, select Online.
6. To go to the next page in the wizard, click the right arrow button.

   The Add Datafiles page appears.

7. For the Datafiles field, enter the name for the datafile. If the datafile name includes a number in the suffix (such as df_1), you can click the + button or press the Enter key to create multiple data files with the options you select on the Add Datafiles page.
8. For the File Size field, enter appropriate values for your data file location and initial size.
9. Select Auto Extend.
10. For the Increment field, select the additional space to be added to the file each time it extends.
11. For the Maximum File Size field, enter the maximum size for this data file.
12. After adding the data files for the new tablespace, click the right arrow button to go to the next page in the wizard.

    The Space page appears.

    Note:

    On the Add Datafiles page and subsequent pages in the wizard, you can click the right arrow button to go to the next page, or you can click OK to create the tablespace without continuing through the wizard. The default values on on the remaining wizard pages will be used to create the tablespace.

13. For Block Size, select the block size to use for the tablespace.
14. For Extent Allocation, select Automatic.
15. Click the right arrow button to go to the next page in the wizard.

    The Logging page appears.

16. In the Logging section, select Logging.
17. Click the right arrow button to go to the final page in the wizard.

    The Segments page appears.

18. In the Segment Space Management section, select Automatic.
19. In the Compression section, select None.
20. Click OK to add the tablespace.

This section contains the following topics:

• Setting a Tablespace to Automatically Extend

• Setting the Datafile for a Smallfile Tablespace to Automatically Extend

• Taking a Tablespace Offline

1. In EM Express, from the Storage menu, select Tablespaces.

   The Tablespaces page appears.

2. Select the tablespace to drop, and then click Drop.

   The Drop Tablespace page appears.

   EM Express asks for confirmation to delete the tablespace and gives you the option to also delete the associated data files from the disk.

3. Choose from the following options and then click OK:

   • Drop Contents

   • Drop Datafiles

   • Drop Constraints

4. A confirmation is displayed and the deleted tablespace no longer appears on the Tablespaces page.

Oracle recommends that you multiplex the online redo log. Multiplexing provides better protection for data if an instance or media failure occurs. You can multiplex the online redo log using Oracle Enterprise Manager Database Express (EM Express).

1. In EM Express, from the Storage menu, select Redo Log Groups.

   The Redo Log Groups page appears.

2. Select a group and click Add Member.

   The Add Member page appears.

3. In the File Directory field, enter the directory where you want the data file to be stored on disk.

   You can create this file in the same directory as the other member of the redo log file group, but it is recommended that you store members on separate disk drives. That way, if there is a drive failure, then you still have access to one member.

4. In the File Name field, enter a file name for the new redo log member.

   For example, if your existing member file name is REDO01.log, then you might name this member REDO01a.log.

5. Click OK.

   A confirmation message appears.

6. Repeat Step 2 through Step 5 for every existing log group.

1. In EM Express, from the Storage menu, select Redo Log Groups.

   The Redo Log Groups page appears.

2. From the Actions list, select Switch Logfile, and then click OK.

   A confirmation message appears.

3. A confirmation message appears. Click Yes.

   The status of the group that had been Current changes to Active, and the status of the next group in the list changes from Active to Current.

• To undo any uncommitted changes made to the database if a rollback is necessary. A rollback can be needed because a user wants to undo the changes of a misguided or unintentional transaction, or it can be part of a recovery operation.

• To provide read consistency, which means that each user can get a consistent view of data, even while other changes may be occurring against the data. With read consistency, a user session does not see uncommitted changes made in other user sessions (sometimes referred to as dirty reads). For example, if a user issues a query at 10:00 a.m. and the query lasts for 15 minutes, then the query results reflect the entire state of the data at 10:00 a.m., regardless of update or insert operations performed by other users after the query started.

• To enable certain Oracle Flashback features, such as Oracle Flashback Query and Oracle Flashback Table, which enable you to view or recover data to a previous point in time.

Although by default Oracle Database manages undo data and the undo tablespace automatically, if your installation uses Oracle Flashback features, then you may have to perform some undo management tasks to ensure the success of these operations.

Oracle Flashback operations resulting in snapshot too old errors indicate that you must intervene to ensure that sufficient undo data is retained to support these operations.

The following methods better support Oracle Flashback operations:

• Set the minimum undo retention period for the autoextending tablespace to be as long as the longest expected Oracle Flashback operation.

  You achieve this goal by setting the UNDO_RETENTION initialization parameter. See Oracle Database Administrator’s Guide for details.

• Change the undo tablespace to a fixed size.

  For an autoextending undo tablespace, Oracle Database always automatically tunes the undo retention period to be slightly longer than the longest-running active query. However, this autotuned retention period may be insufficient to accommodate Oracle Flashback operations. If the undo tablespace has autoextending disabled, or has a fixed size, then Oracle Database uses a different method for tuning the undo retention period to better accommodate Oracle Flashback operations.

  To change the undo tablespace to a fixed size, you must choose a tablespace size that is sufficiently large. If you choose an undo tablespace size that is too small, then the following errors could occur:

  • DML could fail because there is not enough space to accommodate undo data for new transactions.

  • Long-running queries could fail with a snapshot too old error, which means that there was insufficient undo data for read consistency.

  Oracle Enterprise Manager Database Express (EM Express) includes an Undo Advisor to help you determine the minimum size for the fixed size of the undo tablespace. See "Computing the Minimum Undo Tablespace Size Using the Undo Advisor".

1. In EM Express, from the Storage menu, select Undo Management.

   The Undo Management Details page appears.

2. View the undo management information on the Undo Management Details page.

Description of the illustration exp_undo_mgmt_details_new.gif

See Also:

"About Undo Data"

If you must change the undo tablespace to a fixed size, then use the Undo Advisor in Oracle Enterprise Manager Database Express (EM Express) to help determine the minimum required size. You can also use the Undo Advisor to set the minimum undo retention period.

1. In EM Express, go to the Undo Management Details page.

   See "Viewing Undo Information".

2. In the Configuration section, view the Undo Advisor graph.
   
   Description of the illustration exp_undo_advisor.gif

   The Undo Advisor displays an orange line. There are several tick-marks, or points, that you can click on the orange line. The blue point on the orange line marks the target undo retention time that is currently set for the database.

3. To determine the recommended minimum size of the undo tablespace for a particular undo retention time, select the point on the orange line closest to the desired undo retention time.

   In the following figure, for example, the point on the orange line for the undo retention time of 3884 seconds has been clicked:

   
   Description of the illustration exp_undo_advsr_ex.gif

   The Undo Advisor shows its recommendation that the undo tablespace size should be at least 20 MB for this undo retention time of 3884 seconds.

   Note:

   Using the Undo Advisor to gets its recommendation for the required minimum tablespace size for a particular undo retention time does not change the size of the undo tablespace. You must follow the instructions in "Changing the Undo Tablespace to a Fixed Size" to change the size of the undo tablespace.

ou change the undo tablespace to a fixed size to prevent the tablespace from growing too large or to better support Oracle Flashback operations. You can use Oracle Enterprise Manager Database Express (EM Express) to change the undo tablespace to a fixed size.

1. In EM Express, go to the Undo Management Details page, as described in "Viewing Undo Information".
2. After determining the minimum required undo tablespace size, click the link after the Name field in the Tablespace section.

   The Tablespace page appears, with the undo tablespace displayed.

3. Select the undo tablespace. Then from the Actions menu, select Resize.

   The Resize Tablespace page appears.

4. In the File Size field, enter the computed minimum size for the undo tablespace.

   See "Computing the Minimum Undo Tablespace Size Using the Undo Advisor".

5. Click OK.

   A confirmation message appears.

You change the datafiles for an undo tablespace to a fixed size to prevent the tablespace from growing too large or to better support Oracle Flashback operations. You can use Oracle Enterprise Manager Database Express (EM Express) to change the datafiles for an undo tablespace to a fixed size.

1. In EM Express, go to the Undo Management Details page, as described in "Viewing Undo Information".
2. After determining the minimum required undo tablespace size, click the link after the Name field in the Tablespace section.

   The Tablespace page appears, with the undo tablespace displayed.

3. Select and expand the undo tablespace. Select one of the datafiles for the undo tablespace. Then from the Actions menu, select Resize.

   The Resize Datafile page appears.

4. In the Size field, enter the computed minimum size for the undo tablespace.

   See "Computing the Minimum Undo Tablespace Size Using the Undo Advisor".

5. Click OK.

   A confirmation message appears.

6. You can perform these steps for all the datafiles for the undo tablespace to change them all to a fixed size.

You can change the current analysis period and the desired undo retention period using Oracle Enterprise Manager Database Express (EM Express).

1. In EM Express, go to the Undo Management Details page, as described in "Viewing Undo Information".
2. Click Change Analysis Parameters.

   The Change Analysis Parameters page appears.

3. To change the analysis period, in the Analysis Period field, select the desired analysis period.
4. To change the undo retention period, select either:

   • Use Required Undo Retention

   • Specify Undo Retention in Seconds

     Enter the number of seconds to use for the retention period in the text box.

You can switch from one undo tablespace to another using Oracle Enterprise Manager Database Express (EM Express).

1. In EM Express, go to the Undo Management Details page, as described in "Viewing Undo Information".
2. Click Switch Undo Tablespace.

   The Switch Undo Tablespace page appears.

3. In the Switch to Undo Tablespace field, select the name of the undo tablespace you want to switch to. This field includes the names of available undo tablespaces for the database.

   For example, if the current undo tablespace is named UD1 and you want to switch to the undo tablespace named UD2, select UD2 in the Switch to Undo Tablespace field.

4. Click OK.

   A confirmation message appears.