7 Managing Archived Redo Logs

Running a Database in NOARCHIVELOG Mode

When you run your database in NOARCHIVELOG mode, you disable the archiving of the redo log. The database control file indicates that filled groups are not required to be archived. Therefore, when a filled group becomes inactive after a log switch, the group is available for reuse by LGWR.

NOARCHIVELOG mode protects a database from instance failure but not from media failure. Only the most recent changes made to the database, which are stored in the online redo log groups, are available for instance recovery. If a media failure occurs while the database is in NOARCHIVELOG mode, you can only restore the database to the point of the most recent full database backup. You cannot recover transactions subsequent to that backup.

In NOARCHIVELOG mode you cannot perform online tablespace backups, nor can you use online tablespace backups taken earlier while the database was in ARCHIVELOG mode. To restore a database operating in NOARCHIVELOG mode, you can use only whole database backups taken while the database is closed. Therefore, if you decide to operate a database in NOARCHIVELOG mode, take whole database backups at regular, frequent intervals.

Running a Database in ARCHIVELOG Mode

When you run a database in ARCHIVELOG mode, you enable the archiving of the redo log. The database control file indicates that a group of filled redo log files cannot be reused by LGWR until the group is archived. A filled group becomes available for archiving immediately after a redo log switch occurs.

The archiving of filled groups has these advantages:

• A database backup, together with online and archived redo log files, guarantees that you can recover all committed transactions in the event of an operating system or disk failure.

• If you keep an archived log, you can use a backup taken while the database is open and in normal system use.

• You can keep a standby database current with its original database by continuously applying the original archived redo logs to the standby.

You can configure an instance to archive filled redo log files automatically, or you can archive manually. For convenience and efficiency, automatic archiving is usually best. Figure 7-1 illustrates how the archiver process (ARC0 in this illustration) writes filled redo log files to the database archived redo log.

If all databases in a distributed database operate in ARCHIVELOG mode, you can perform coordinated distributed database recovery. However, if any database in a distributed database is in NOARCHIVELOG mode, recovery of a global distributed database (to make all databases consistent) is limited by the last full backup of any database operating in NOARCHIVELOG mode.

Figure 7-1 Redo Log File Use in ARCHIVELOG Mode

Description of the illustration admin056.gif

Setting the Initial Database Archiving Mode

You set the initial archiving mode as part of database creation in the CREATE DATABASE statement. Usually, you can use the default of NOARCHIVELOG mode at database creation because there is no need to archive the redo information generated by that process. After creating the database, decide whether to change the initial archiving mode.

If you specify ARCHIVELOG mode, you must have initialization parameters set that specify the destinations for the archive log files (see "Specifying Archive Destinations").

Changing the Database Archiving Mode

To change the archiving mode of the database, use the ALTER DATABASE statement with the ARCHIVELOG or NOARCHIVELOG clause. To change the archiving mode, you must be connected to the database with administrator privileges (AS SYSDBA).

The following steps switch the database archiving mode from NOARCHIVELOG to ARCHIVELOG:

1. Shut down the database instance.

   SHUTDOWN
   
   

   An open database must first be closed and any associated instances shut down before you can switch the database archiving mode. You cannot change the mode from ARCHIVELOG to NOARCHIVELOG if any datafiles need media recovery.

2. Back up the database.

   Before making any major change to a database, always back up the database to protect against any problems. This will be your final backup of the database in NOARCHIVELOG mode and can be used if something goes wrong during the change to ARCHIVELOG mode. See Oracle Database Backup and Recovery Basics for information about taking database backups.

3. Edit the initialization parameter file to include the initialization parameters that specify the destinations for the archive log files (see "Specifying Archive Destinations").

4. Start a new instance and mount, but do not open, the database.

   STARTUP MOUNT
   
   

   To enable or disable archiving, the database must be mounted but not open.

5. Change the database archiving mode. Then open the database for normal operations.

   ALTER DATABASE ARCHIVELOG;
   ALTER DATABASE OPEN;
   
   

6. Shut down the database.

   SHUTDOWN IMMEDIATE
   
   

7. Back up the database.

   Changing the database archiving mode updates the control file. After changing the database archiving mode, you must back up all of your database files and control file. Any previous backup is no longer usable because it was taken in NOARCHIVELOG mode.

   
   

   See Also: Oracle Real Application Clusters Administrator's Guide for more information about switching the archiving mode when using Real Application Clusters

   
   

Performing Manual Archiving

To operate your database in manual archiving mode, follow the procedure shown in "Changing the Database Archiving Mode". However, when you specify the new mode in step 5, use the following statement:

ALTER DATABASE ARCHIVELOG MANUAL;

When you operate your database in manual ARCHIVELOG mode, you must archive inactive groups of filled redo log files or your database operation can be temporarily suspended. To archive a filled redo log group manually, connect with administrator privileges. Ensure that the database is mounted but not open. Use the ALTER SYSTEM statement with the ARCHIVE LOG clause to manually archive filled redo log files. The following statement archives all unarchived log files:

ALTER SYSTEM ARCHIVE LOG ALL;

When you use manual archiving mode, you cannot specify any standby databases in the archiving destinations.

Even when automatic archiving is enabled, you can use manual archiving for such actions as rearchiving an inactive group of filled redo log members to another location. In this case, it is possible for the instance to reuse the redo log group before you have finished manually archiving, and thereby overwrite the files. If this happens, the database writes an error message to the alert log.

Adjusting the Number of Archiver Processes

The LOG_ARCHIVE_MAX_PROCESSES initialization parameter specifies the number of ARCn processes that the database initially invokes. The default is two processes. There is usually no need specify this initialization parameter or to change its default value, because the database starts additional archiver processes (ARCn) as needed to ensure that the automatic processing of filled redo log files does not fall behind.

However, to avoid any runtime overhead of invoking additional ARCn processes, you can set the LOG_ARCHIVE_MAX_PROCESSES initialization parameter to specify up to ten ARCn processes to be started at instance startup. The LOG_ARCHIVE_MAX_PROCESSES parameter is dynamic, and can be changed using the ALTER SYSTEM statement. The database must be mounted but not open. The following statement increases (or decreases) the number of ARCn processes currently running:

ALTER SYSTEM SET LOG_ARCHIVE_MAX_PROCESSES=3;

Specifying Archive Destinations

You can choose whether to archive redo logs to a single destination or multiplex them. If you want to archive only to a single destination, you specify that destination in the LOG_ARCHIVE_DEST initialization parameter. If you want to multiplex the archived logs, you can choose whether to archive to up to ten locations (using the LOG_ARCHIVE_DEST_n parameters) or to archive only to a primary and secondary destination (using LOG_ARCHIVE_DEST and LOG_ARCHIVE_DUPLEX_DEST). The following table summarizes the multiplexing alternatives, which are further described in the sections that follow.

Understanding Archive Destination Status

Each archive destination has the following variable characteristics that determine its status:

• Valid/Invalid: indicates whether the disk location or service name information is specified and valid

• Enabled/Disabled: indicates the availability state of the location and whether the database can use the destination

• Active/Inactive: indicates whether there was a problem accessing the destination

Several combinations of these characteristics are possible. To obtain the current status and other information about each destination for an instance, query the V$ARCHIVE_DEST view.

The characteristics determining a locations status that appear in the view are shown in Table 7-1. Note that for a destination to be used, its characteristics must be valid, enabled, and active.

The LOG_ARCHIVE_DEST_STATE_n (where n is an integer from 1 to 10) initialization parameter lets you control the availability state of the specified destination (n).

• ENABLE indicates that the database can use the destination.

• DEFER indicates that the location is temporarily disabled.

• ALTERNATE indicates that the destination is an alternate.

The availability state of the destination is DEFER, unless there is a failure of its parent destination, in which case its state becomes ENABLE.

Normal Transmission Mode

In normal transmission mode, the archiving destination is another disk drive of the database server. In this configuration archiving does not contend with other files required by the instance and can complete more quickly. Specify the destination with either the LOG_ARCHIVE_DEST_n or LOG_ARCHIVE_DEST parameters.

It is good practice to move archived redo log files and corresponding database backups from the local disk to permanent inexpensive offline storage media such as tape. A primary value of archived logs is database recovery, so you want to ensure that these logs are safe should disaster strike your primary database.

Standby Transmission Mode

In standby transmission mode, the archiving destination is either a local or remote standby database.

If you are operating your standby database in managed recovery mode, you can keep your standby database synchronized with your source database by automatically applying transmitted archive logs.

To transmit files successfully to a standby database, either ARCn or a server process must do the following:

• Recognize a remote location

• Transmit the archived logs in conjunction with a remote file server (RFS) process that resides on the remote server

Each ARCn process has a corresponding RFS for each standby destination. For example, if three ARCn processes are archiving to two standby databases, then Oracle Database establishes six RFS connections.

You transmit archived logs through a network to a remote location by using Oracle Net Services. Indicate a remote archival by specifying a Oracle Net service name as an attribute of the destination. Oracle Database then translates the service name, through the tnsnames.ora file, to a connect descriptor. The descriptor contains the information necessary for connecting to the remote database. The service name must have an associated database SID, so that the database correctly updates the log history of the control file for the standby database.

The RFS process, which runs on the destination node, acts as a network server to the ARCn client. Essentially, ARCn pushes information to RFS, which transmits it to the standby database.

The RFS process, which is required when archiving to a remote destination, is responsible for the following tasks:

• Consuming network I/O from the ARCn process

• Creating file names on the standby database by using the STANDBY_ARCHIVE_DEST parameter

• Populating the log files at the remote site

• Updating the standby database control file (which Recovery Manager can then use for recovery)

Archived redo logs are integral to maintaining a standby database, which is an exact replica of a database. You can operate your database in standby archiving mode, which automatically updates a standby database with archived redo logs from the original database.

Specifying the Minimum Number of Successful Destinations

The optional initialization parameter LOG_ARCHIVE_MIN_SUCCEED_DEST=n determines the minimum number of destinations to which the database must successfully archive a redo log group before it can reuse online log files. The default value is 1. Valid values for n are 1 to 2 if you are using duplexing, or 1 to 10 if you are multiplexing.

Rearchiving to a Failed Destination

Use the REOPEN attribute of the LOG_ARCHIVE_DEST_n parameter to specify whether and when ARCn should attempt to rearchive to a failed destination following an error. REOPEN applies to all errors, not just OPEN errors.

REOPEN=n sets the minimum number of seconds before ARCn should try to reopen a failed destination. The default value for n is 300 seconds. A value of 0 is the same as turning off the REOPEN attribute; ARCn will not attempt to archive after a failure. If you do not specify the REOPEN keyword, ARCn will never reopen a destination following an error.

You cannot use REOPEN to specify the number of attempts ARCn should make to reconnect and transfer archived logs. The REOPEN attempt either succeeds or fails.

When you specify REOPEN for an OPTIONAL destination, the database can overwrite online logs if there is an error. If you specify REOPEN for a MANDATORY destination, the database stalls the production database when it cannot successfully archive. In this situation, consider the following options:

• Archive manually to the failed destination.

• Change the destination by deferring the destination, specifying the destination as optional, or changing the service.

• Drop the destination.

When using the REOPEN keyword, note the following:

• ARCn reopens a destination only when starting an archive operation from the beginning of the log file, never during a current operation. ARCn always retries the log copy from the beginning.

• If you specified REOPEN, either with a specified time the default, ARCn checks to see whether the time of the recorded error plus the REOPEN interval is less than the current time. If it is, ARCn retries the log copy.

• The REOPEN clause successfully affects the ACTIVE=TRUE destination state. The VALID and ENABLED states are not changed.

Dynamic Performance Views

Several dynamic performance views contain useful information about archived redo logs, as summarized in the following table.

For example, the following query displays which redo log group requires archiving:

SELECT GROUP#, ARCHIVED
   FROM SYS.V$LOG;

GROUP#     ARC
--------   ---
       1   YES
       2   NO

To see the current archiving mode, query the V$DATABASE view:

SELECT LOG_MODE FROM SYS.V$DATABASE;

LOG_MODE
------------
NOARCHIVELOG

The ARCHIVE LOG LIST Command

The SQL*Plus command ARCHIVE LOG LIST displays archiving information for the connected instance. For example:

SQL> ARCHIVE LOG LIST

Database log mode              Archive Mode
Automatic archival             Enabled
Archive destination            D:\oracle\oradata\IDDB2\archive
Oldest online log sequence     11160
Next log sequence to archive   11163
Current log sequence           11163

This display tells you all the necessary information regarding the archived redo log settings for the current instance:

• The database is currently operating in ARCHIVELOG mode.

• Automatic archiving is enabled.

• The archived redo log destination is D:\oracle\oradata\IDDB2\archive.

• The oldest filled redo log group has a sequence number of 11160.

• The next filled redo log group to archive has a sequence number of 11163.

• The current redo log file has a sequence number of 11163.