7 Configuring Recovery Manager and Archiving

Overview of Configuring RMAN for Real Application Clusters

Recovery Manager (RMAN) enables you to back up, restore, and recover datafiles, control files, server parameter files (SPFILEs) and archived redo logs. It is included with the Oracle server and does not require separate installation. You can run RMAN from the command line or use RMAN in the Backup Manager in Oracle Enterprise Manager.

Configuring the RMAN Snapshot Control File Location

The snapshot control file is a temporary snapshot control file that RMAN creates to re-synchronize from a read-consistent version of the control file. RMAN only needs a snapshot control file when re-synchronizing with the recovery catalog or when making a backup of the current control file. In RAC, the snapshot control file is only needed on the nodes on which RMAN performs backups; the snapshot control file does not need to be globally available to all instances in a RAC environment.

You can specify a cluster file system file or a raw device destination for the location of your snapshot control file. Run the following RMAN command to determine the configured location of the snapshot control file:

SHOW SNAPSHOT CONTROLFILE NAME;

You can change the configured location of the snapshot control file. For example, on UNIX-based systems you can specify the snapshot control file location as $ORACLE_HOME/dbs/scf/snap_prod.cf by entering the following at the RMAN prompt:

CONFIGURE SNAPSHOT CONTROLFILE NAME TO '$ORACLE_HOME/dbs/scf/snap_prod.cf';

This command globally sets the configuration for the location of the snapshot control file throughout your cluster database. Therefore, ensure that the directory $ORACLE_HOME/dbs/scf exists on all nodes that perform backups.

The CONFIGURE command creates persistent settings across RMAN sessions. Therefore, you do not need to run this command again unless you want to change the location of the snapshot control file. You can also specify a cluster file system file or a raw device destination for the location of your snapshot control file. This file is shared across all nodes in the cluster just like other datafiles in RAC. Refer to Oracle Database Backup and Recovery Reference for more information about configuring the snapshot control file.

Configuring the RMAN Control File and SPFILE Autobackup Feature

If you set CONFIGURE CONTROLFILE AUTOBACKUP to ON, then RMAN automatically creates a control file and an SPFILE backup after you run the BACKUP or COPY commands. RMAN can also automatically restore an SPFILE if this is required to start an instance to perform recovery. This means that the default location for the SPFILE must be available to all nodes in your RAC database.

These features are important in disaster recovery because RMAN can restore the control file even without a recovery catalog. RMAN can restore an autobackup of the control file even after the loss of both the recovery catalog and the current control file. You can change the default name that RMAN gives to this file with the CONFIGURE CONTROLFILE AUTOBACKUP FORMAT command. Note that if you specify an absolute path name in this command, then this path must exist identically on all nodes that participate in backups.

RMAN performs the control file autobackup on the first allocated channel. Therefore, when you allocate multiple channels with different parameters, especially when you allocate a channel with the CONNECT command, determine which channel will perform the control file autobackup. Always allocate the channel for this node first. Refer to the Oracle Database Backup and Recovery Advanced User's Guide for more information about using the control file autobackup feature.

Besides using the RMAN control file, you can also use Enterprise Manager to use the RMAN features.

Configuring Channels for RMAN in Real Application Clusters

This section describes how to configure channels for RMAN. You can configure channels to use automatic workload balancing or you can specify specific channels for specific instances as described in the following topics:

• Configuring Channels to use Automatic Workload Balancing

• Configuring Channels to Use a Specific Channel

CONFIGURE DEVICE TYPE [disk | sbt] PARALLELISM number of channels;
...

CONFIGURE CHANNEL DEVICE TYPE sbt CONNECT
'SYS/change_on_install@node1'
CONFIGURE CHANNEL DEVICE TYPE sbt CONNECT
'SYS/change_on_install@node2'
...

Managing Archived Redo Logs Using RMAN in Real Application Clusters

When a node generates an archived redo log, Oracle always records the filename of the log in the control file of the target database. If you are using a recovery catalog, then RMAN also records the archived redo log filenames in the recovery catalog when a re-synchronization occurs.

The archived redo log naming scheme that you use is important because when a node writes to a log with a specific filename on its file system, the file must be readable by any node that needs to access this archived redo log. For example, if node 1 archives a log to /oracle/arc_dest/log_1_100_23452345.arc, then node 2 can only back up this archived redo log only if it can read/oracle/arc_dest/log_1_100_23452345.arc on its own file system.

The backup and recovery strategy that you choose depends on how you configure the archiving destinations for each node. Whether only one node or all nodes perform archived redo log backups, you need to ensure that all archived redo logs are backed up. If you use RMAN parallelism during recovery, then the node that performs recovery must have read access to all archived redo logs in your cluster.

Multiple nodes can restore archived logs in parallel. However, during recovery, one one node applies the archived logs. Therefore, the node that is performing the recovery must be able to access all of the archived logs that are needed for the recovery operation. By default, the database determines the optimum number of parallel threads to use during the recovery operation. You can use the PARALLEL clause in the RECOVER command to change this number.

Guidelines and Considerations for Archived Redo Logs

The primary consideration is to ensure that all archived redo logs can be read from every node during recovery, and if possible during backups. This section illustrates the archived redo log naming issues for configuring archiving in your cluster database. The scenario described here is for a non-cluster file system archiving scheme. Assume that the following conditions are met:

• Configure each node to write to a local archiving directory that is named the same on each node.

• Do not set up a cluster file system (in other words, each node can only read from and write to its own local file system). Refer to information about cluster file systems later in this chapter.

• Do not use NFS (network file system) or mapped drives to enable the nodes in the cluster to gain read/write access to one another.

sid1.log_archive_dest_1 = (location=/arc_dest_1)
sid2.log_archive_dest_1 = (location=/arc_dest_2)
sid3.log_archive_dest_1 = (location=/arc_dest_3)

/arc_dest_1/log_1_62_23452345.arc 
/arc_dest_2/log_2_100_23452345.arc
/arc_dest_2/log_2_101_23452345.arc
/arc_dest_3/log_3_70_23452345.arc
/arc_dest_1/log_1_63_23452345.arc

Archived Redo Log File Conventions in RAC

For any archived redo log configuration, uniquely identify the archived redo logs with the LOG_ARCHIVE_FORMAT parameter. The format of this parameter is operating system-specific and it can include text strings, one or more variables, and a filename extension.

All of the thread parameters, in either upper or lower case, are mandatory for RAC. This enables Oracle to create unique names for archive logs across the incarnation. This requirement is in effect when the COMPATIBLE parameter is set to 10.0 or greater.

Use the %R or %r parameters to include the resetlogs identifier to avoid overwriting the logs from a previous incarnation. If you do not specify a log format, then the default is operating system-specific and includes %t, %s, and %r.

As an example, if the instance associated with redo thread number 1 sets LOG_ARCHIVE_FORMAT to log_%t_%s_%r.arc, then its archived redo log files are named:

log_1_1000_23435343.arc
log_1_1001_23452345.arc
log_1_1002_23452345.arc
...

RMAN Archiving Configuration Scenarios

This section describes the archiving scenarios for a RAC database. The two configuration scenarios in this chapter describe a three-node UNIX cluster for a RAC database. For both scenarios, the LOG_ARCHIVE_FORMAT that you specify for the instance performing recovery must be the same as the format that you specified for the instances that archived the files.

Automatic Storage Management and Cluster File System Archiving Scheme

The preferred configuration for RAC is to use Automatic Storage Management (ASM) for a recovery area with a different disk group for your recovery set than for your datafiles. Alternatively, you can use a cluster file system archiving scheme.

In this case, each node writes to a single cluster file system archived redo log destination and can read the archived redo log files of the other nodes. Read access is achieved for all nodes with a cluster file system. For example, if node 1 archives a log to /arc_dest/log_1_100_23452345.arc on the cluster file system, then any other node in the cluster can also read this file.

Note: The archive log naming format in this example is only for a CFS example. ASM uses an Oracle Managed Files (OMF)-based naming format.

If you do not use a cluster file system, then the archived redo log files cannot be on raw devices. This is because raw devices do not enable sequential writing of consecutive archive log files.

Figure 7-1 Cluster File System Archiving Scheme

Description of "Figure 7-1 Cluster File System Archiving Scheme"

Advantages of the Cluster File System Archiving Scheme

The advantage of this scheme is that none of the nodes uses the network to archive logs. Because the filename written by a node can be read by any node in the cluster, RMAN can back up all logs from any node in the cluster. Backup and restore scripts are simplified because each node has access to all archived redo logs.

Initialization Parameter Settings for the Cluster File System Archiving Scheme

In the cluster file system scheme, each node archives to a directory that is identified with the same name on all instances within the cluster database. To configure this, set values for the LOG_ARCH_DEST_n parameter for each instance using the sid designator as in the following example:

sid1.LOG_ARCHIVE_DEST_1="LOCATION=/arc_dest"
sid2.LOG_ARCHIVE_DEST_1="LOCATION=/arc_dest"
sid3.LOG_ARCHIVE_DEST_1="LOCATION=/arc_dest"

The following list shows archived redo log entry examples that would appear in the RMAN catalog or the in the control file based on the previous example. Note that any node can archive logs using any of the threads:

/arc_dest/log_1_999_23452345.arc
/arc_dest/log_1_1000_23435343.arc
/arc_dest/log_1_1001_23452345.arc <- thread 1 archived in node 3
/arc_dest/log_3_1563_23452345.arc <- thread 3 archived in node 2
/arc_dest/log_2_753_23452345.arc <- thread 2 archived in node 1
/arc_dest/log_2_754_23452345.arc
/arc_dest/log_3_1564_23452345.arc

Location of Archived Logs for the Cluster File System Archiving Scheme

Because the file system is shared and because each node is writing its archived redo logs to directory /arc_dest in the cluster file system, each node can read the logs written by itself as well as any other node.

sid1.LOG_ARCHIVE_DEST_1="LOCATION=/arc_dest_1"
sid2.LOG_ARCHIVE_DEST_1="LOCATION=/arc_dest_2"
sid3.LOG_ARCHIVE_DEST_1="LOCATION=/arc_dest_3"

/arc_dest_1/log_1_1000_23435343.arc
/arc_dest_2/log_1_1001_23452345.arc <- thread 1 archived in node 2
/arc_dest_2/log_3_1563_23452345.arc <- thread 3 archived in node 2
/arc_dest_1/log_2_753_23452345.arc <- thread 2 archived in node 1
/arc_dest_2/log_2_754_23452345.arc
/arc_dest_3/log_3_1564_23452345.arc

Changing the Archiving Mode in Real Application Clusters

You can run the ALTER DATABASE SQL statement to change the archiving mode in RAC as long as the database is mounted by the local instance but not open in any instances. You do not need to modify parameter settings to run this statement.