|Oracle® Database Administrator's Guide
10g Release 1 (10.1)
Part Number B10739-01
This chapter describes how to archive redo data. It contains the following topics:
Oracle Database lets you save filled groups of redo log files to one or more offline destinations, known collectively as the archived redo log, or more simply the archive log. The process of turning redo log files into archived redo log files is called archiving. This process is only possible if the database is running in
ARCHIVELOG mode. You can choose automatic or manual archiving.
An archived redo log file is a copy of one of the filled members of a redo log group. It includes the redo entries and the unique log sequence number of the identical member of the redo log group. For example, if you are multiplexing your redo log, and if group 1 contains identical member files
b_log1, then the archiver process (ARCn) will archive one of these member files. Should
a_log1 become corrupted, then ARCn can still archive the identical
b_log1. The archived redo log contains a copy of every group created since you enabled archiving.
When the database is running in
ARCHIVELOG mode, the log writer process (LGWR) cannot reuse and hence overwrite a redo log group until it has been archived. The background process ARCn automates archiving operations when automatic archiving is enabled. The database starts multiple archiver processes as needed to ensure that the archiving of filled redo logs does not fall behind.
You can use archived redo logs to:
Recover a database
Update a standby database
Get information about the history of a database using the LogMiner utility
See Also:The following sources document the uses for archived redo logs:
The choice of whether to enable the archiving of filled groups of redo log files depends on the availability and reliability requirements of the application running on the database. If you cannot afford to lose any data in your database in the event of a disk failure, use
ARCHIVELOG mode. The archiving of filled redo log files can require you to perform extra administrative operations.
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.
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.
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
Figure 7-1 Redo Log File Use in ARCHIVELOG Mode
This section describes how to set the archiving mode of the database and how to control the archiving process. The following topics are discussed:
See Also:your Oracle operating system specific documentation for additional information on controlling archiving modes
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").
To change the archiving mode of the database, use the
ALTER DATABASE statement with the
NOARCHIVELOG clause. To change the archiving mode, you must be connected to the database with administrator privileges (
The following steps switch the database archiving mode from
Shut down the database instance.
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
NOARCHIVELOG if any datafiles need media recovery.
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.
Edit the initialization parameter file to include the initialization parameters that specify the destinations for the archive log files (see "Specifying Archive Destinations").
Start a new instance and mount, but do not open, the database.
To enable or disable archiving, the database must be mounted but not open.
Change the database archiving mode. Then open the database for normal operations.
ALTER DATABASE ARCHIVELOG; ALTER DATABASE OPEN;
Shut down the database.
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
See Also:Oracle Real Application Clusters Administrator's Guide for more information about switching the archiving mode when using Real Application Clusters
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 file.
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;
Before you can archive redo logs, you must determine the destination to which you will archive and familiarize yourself with the various destination states. The dynamic performance (V$) views, listed in "Viewing Information About the Archived Redo Log", provide all needed archive information.
The following topics are contained in this section:
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_DUPLEX_DEST). The following table summarizes the multiplexing alternatives, which are further described in the sections that follow.
n is an integer from 1 to 10
|Local or remote|
n parameter (where n is an integer from 1 to 10) to specify from one to ten different destinations for archival. Each numerically suffixed parameter uniquely identifies an individual destination.
You specify the location for
n using the keywords explained in the following table:
||A local file system location.|
||Remote archival through Oracle Net service name.|
If you use the
LOCATION keyword, specify a valid path name for your operating system. If you specify
SERVICE, the database translates the net 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.
Perform the following steps to set the destination for archived redo logs using the
n initialization parameter:
Use SQL*Plus to shut down the database.
n initialization parameter to specify from one to ten archiving locations. The
LOCATION keyword specifies an operating system specific path name. For example, enter:
LOG_ARCHIVE_DEST_1 = 'LOCATION = /disk1/archive' LOG_ARCHIVE_DEST_2 = 'LOCATION = /disk2/archive' LOG_ARCHIVE_DEST_3 = 'LOCATION = /disk3/archive'
If you are archiving to a standby database, use the
SERVICE keyword to specify a valid net service name from the
tnsnames.ora file. For example, enter:
LOG_ARCHIVE_DEST_4 = 'SERVICE = standby1'
Optionally, set the
LOG_ARCHIVE_FORMAT initialization parameter, using
%t to include the thread number as part of the file name,
%s to include the log sequence number, and
%r to include the resetlogs ID (a timestamp value represented in ub4). Use capital letters (
%R) to pad the file name to the left with zeroes.
The incarnation of a database changes when you open it with the
The following example shows a setting of
LOG_ARCHIVE_FORMAT = arch_%t_%s_%r.arc
This setting will generate archived logs as follows for thread 1; log sequence numbers 100, 101, and 102; resetlogs ID 509210197. The identical resetlogs ID indicates that the files are all from the same database incarnation:
/disk1/archive/arch_1_100_509210197.arc, /disk1/archive/arch_1_101_509210197.arc, /disk1/archive/arch_1_102_509210197.arc /disk2/archive/arch_1_100_509210197.arc, /disk2/archive/arch_1_101_509210197.arc, /disk2/archive/arch_1_102_509210197.arc /disk3/archive/arch_1_100_509210197.arc, /disk3/archive/arch_1_101_509210197.arc, /disk3/archive/arch_1_102_509210197.arc
To specify a maximum of two locations, use the
LOG_ARCHIVE_DEST parameter to specify a primary archive destination and the
LOG_ARCHIVE_DUPLEX_DEST to specify an optional secondary archive destination. All locations must be local. Whenever the database archives a redo log, it archives it to every destination specified by either set of parameters.
Perform the following steps the use method 2:
Use SQL*Plus to shut down the database.
Specify destinations for the
LOG_ARCHIVE_DUPLEX_DEST parameter (you can also specify
LOG_ARCHIVE_DUPLEX_DEST dynamically using the
ALTER SYSTEM statement). For example, enter:
LOG_ARCHIVE_DEST = '/disk1/archive' LOG_ARCHIVE_DUPLEX_DEST = '/disk2/archive'
LOG_ARCHIVE_FORMAT initialization parameter as described in step 3 for method 1.
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
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.
Table 7-1 Destination Status
||The user has properly initialized the destination, which is available for archiving.|
||n/a||n/a||The user has not provided or has deleted the destination information.|
||An error occurred creating or writing to the destination file; refer to error data.|
||Destination is full (no disk space).|
||The user manually and temporarily disabled the destination.|
||The user manually and temporarily disabled the destination following an error; refer to error data.|
||n/a||n/a||n/a||A parameter error occurred; refer to error data.|
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
The two modes of transmitting archived logs to their destination are normal archiving transmission and standby transmission mode. Normal transmission involves transmitting files to a local disk. Standby transmission involves transmitting files through a network to either a local or remote standby database.
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
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.
Caution:You can maintain a standby database on a local disk, but Oracle strongly encourages you to maximize disaster protection by maintaining your standby database at a remote site.
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
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.
Sometimes archive destinations can fail, causing problems when you operate in automatic archiving mode. Oracle Database provides procedures to help you minimize the problems associated with destination failure. These procedures are discussed in the sections that follow:
The optional initialization parameter
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.
n parameter lets you specify whether a destination is
OPTIONAL (the default) or
n parameter uses all
MANDATORY destinations plus some number of nonstandby
OPTIONAL destinations to determine whether LGWR can overwrite the online log. The following rules apply:
MANDATORY attribute for a destination is the same as specifying
You must have at least one local destination, which you can declare
When you specify a value for
n, Oracle Database will treat at least one local destination as
MANDATORY, because the minimum value for
LOG_ARCHIVE_MIN_SUCCEED_DEST is 1.
MANDATORY destination fails, including a
MANDATORY standby destination, Oracle Database ignores the
LOG_ARCHIVE_MIN_SUCCEED_DEST value cannot be greater than the number of destinations, nor can it be greater than the number of
MANDATORY destinations plus the number of
OPTIONAL local destinations.
MANDATORY destination, and the database overwrites the online log without transferring the archived log to the standby site, then you must transfer the log to the standby manually.
If you are duplexing the archived logs, you can establish which destinations are mandatory or optional by using the
LOG_ARCHIVE_DUPLEX_DEST parameters. The following rules apply:
Any destination declared by
LOG_ARCHIVE_DEST is mandatory.
Any destination declared by
LOG_ARCHIVE_DUPLEX_DEST is optional if
LOG_ARCHIVE_MIN_SUCCEED_DEST = 1 and mandatory if
LOG_ARCHIVE_MIN_SUCCEED_DEST = 2.
In this scenario, you archive to three local destinations, each of which you declare as
OPTIONAL. Table 7-2 illustrates the possible values for
n in this case.
Table 7-2 LOG_ARCHIVE_MIN_SUCCEED_DEST Values for Scenario 1
|1||The database can reuse log files only if at least one of the |
|2||The database can reuse log files only if at least two of the |
|3||The database can reuse log files only if all of the |
|4 or greater|
This scenario shows that even though you do not explicitly set any of your destinations to
MANDATORY using the
n parameter, the database must successfully archive to one or more of these locations when
LOG_ARCHIVE_MIN_SUCCEED_DEST is set to 1, 2, or 3.
Consider a case in which:
You specify two
You specify two
No destination is a standby database.
Table 7-3 shows the possible values for
|1||The database ignores the value and uses the number of |
|2||The database can reuse log files even if no |
|3||The database can reuse logs only if at least one |
|4||The database can reuse logs only if both |
|5 or greater|
This case shows that the database must archive to the destinations you specify as
MANDATORY, regardless of whether you set
LOG_ARCHIVE_MIN_SUCCEED_DEST to archive to a smaller number of destinations.
REOPEN attribute of the
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
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.
REOPEN clause successfully affects the
ACTIVE=TRUE destination state. The
ENABLED states are not changed.
Background processes always write to a trace file when appropriate. (See the discussion of this topic in "Monitoring the Database Using Trace Files and the Alert File".) In the case of the archivelog process, you can control the output that is generated to the trace file. You do this by setting the
LOG_ARCHIVE_TRACE initialization parameter to specify a trace level. The following values can be specified:
|0||Disable archivelog tracing. This is the default.|
|1||Track archival of redo log file.|
|2||Track archival status for each archivelog destination.|
|4||Track archival operational phase.|
|8||Track archivelog destination activity.|
|16||Track detailed archivelog destination activity.|
|32||Track archivelog destination parameter modifications.|
|64||Track ARCn process state activity.|
|128||Track FAL (fetch archived log) server related activities.|
|256||Supported in a future release.|
|512||Tracks asynchronous LGWR activity.|
|1024||RFS physical client tracking.|
|2048||ARCn/RFS heartbeat tracking.|
|4096||Track real-time apply|
You can combine tracing levels by specifying a value equal to the sum of the individual levels that you would like to trace. For example, setting
LOG_ARCHIVE_TRACE=12, will generate trace level 8 and 4 output. You can set different values for the primary and any standby database.
The default value for the
LOG_ARCHIVE_TRACE parameter is 0. At this level, the archivelog process generates appropriate alert and trace entries for error conditions.
You can change the value of this parameter dynamically using the
ALTER SYSTEM statement. The database must be mounted but not open. For example:
ALTER SYSTEM SET LOG_ARCHIVE_TRACE=12;
Changes initiated in this manner will take effect at the start of the next archiving operation.
See Also:Oracle Data Guard Concepts and Administration for information about using this parameter with a standby database
Several dynamic performance views contain useful information about archived redo logs, as summarized in the following table.
|Dynamic Performance View||Description|
||Identifies whether the database is in |
||Displays historical archived log information from the control file. If you use a recovery catalog, the |
||Describes the current instance, all archive destinations, and the current value, mode, and status of these destinations.|
||Displays information about the state of the various archive processes for an instance.|
||Contains information about any backups of archived logs. If you use a recovery catalog, the |
||Displays all redo log groups for the database and indicates which need to be archived.|
||Contains log history information such as which logs have been archived and the SCN range for each archived log.|
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
SELECT LOG_MODE FROM SYS.V$DATABASE; LOG_MODE ------------ NOARCHIVELOG
See Also:Oracle Database Reference for detailed descriptions of dynamic performance views
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
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.
See Also:SQL*Plus User's Guide and Reference for more information on the