Instance Recovery in Oracle RAC

Learn about instance recovery in Oracle RAC.

Instance failure occurs when software or hardware problems disable an instance. After instance failure, Oracle AI Database automatically uses the online redo logs to perform recovery as described in this section.

Single Node Failure in Oracle RAC

Learn about single node failures in Oracle RAC.

Instance recovery in Oracle RAC does not include the recovery of applications that were running on the failed instance. Oracle Clusterware restarts the instance automatically.

Applications that were running on a node before it failed continue running by using failure recognition and recovery. This provides consistent and uninterrupted service if hardware or software fails. When one instance performs recovery for another instance, the remaining instance reads online redo logs generated by the failed instance and uses that information to ensure that committed transactions are recorded in the database. Thus, data from committed transactions is not lost. The instance performing recovery rolls back transactions that were active at the time of the failure and releases resources used by those transactions.

Note:

All online redo logs must be accessible for instance recovery. Therefore, Oracle recommends that you mirror your online redo logs.

Multiple-Node Failures in Oracle RAC

Learn how to manage multi-node failures in Oracle RAC.

When failures occur, if one instance remains active, then Oracle RAC performs instance recovery for any other instances that fail. If all instances of an Oracle RAC database fail, then Oracle AI Database automatically recovers the instances the next time one instance opens the database. The instance performing recovery can mount the database in either cluster database or exclusive mode from any node of an Oracle RAC database. This recovery procedure is the same for Oracle AI Database running in shared mode as it is for Oracle AI Database running in exclusive mode, except that one instance performs instance recovery for all of the failed instances.

Using RMAN to Create Backups in Oracle RAC

Oracle AI Database provides RMAN for backing up and restoring the database.

RMAN enables you to back up, restore, and recover data files, control files, SPFILEs, and archived redo logs. RMAN is included with the Oracle AI Database server and it is installed by default. You can run RMAN from the command line or you can use it from the Backup Manager in Oracle Enterprise Manager. In addition, RMAN is the recommended backup and recovery tool if you are using Oracle Automatic Storage Management (Oracle ASM). The procedures for using RMAN in Oracle RAC environments do not differ substantially from those for Oracle noncluster environments.

Channel Connections to Cluster Instances with RMAN

Learn about using RMAN for channel connections to cluster instances.

Channel connections to the instances are determined using the connect string defined by channel configurations. For example, in the following configuration, three channels are allocated using dbauser/pwd@service_name. If you configure the SQL Net service name with load balancing turned on, then the channels are allocated at a node as decided by the load balancing algorithm.

CONFIGURE DEVICE TYPE sbt PARALLELISM 3;
CONFIGURE DEFAULT DEVICE TYPE TO sbt;
CONFIGURE CHANNEL DEVICE TYPE SBT CONNECT 'dbauser/pwd@service_name'

However, if the service name used in the connect string is not for load balancing, then you can control at which instance the channels are allocated using separate connect strings for each channel configuration, as follows:

CONFIGURE DEVICE TYPE sbt PARALLELISM 3;
CONFIGURE CHANNEL 1.. CONNECT 'dbauser/pwd@mydb_1';
CONFIGURE CHANNEL 2.. CONNECT 'dbauser/pwd@mydb_2';
CONFIGURE CHANNEL 3.. CONNECT 'dbauser/pwd@mydb_3';

In the previous example, it is assumed that mydb_1, mydb_2 and mydb_3 are SQL*Net service names that connect to pre-defined nodes in your Oracle RAC environment. Alternatively, you can also use manually allocated channels to backup your database files. For example, the following command backs up the SPFILE, control file, data files and archived redo logs:

RUN
{
    ALLOCATE CHANNEL CH1 CONNECT 'dbauser/pwd@mydb_1';
    ALLOCATE CHANNEL CH2 CONNECT 'dbauser/pwd@mydb_2';
    ALLOCATE CHANNEL CH3 CONNECT 'dbauser/pwd@mydb_3';
    BACKUP DATABASE PLUS ARCHIVED LOG;
}

During a backup operation, if at least one channel allocated has access to the archived log, then RMAN automatically schedules the backup of the specific log on that channel. Because the control file, SPFILE, and data files are accessible by any channel, the backup operation of these files is distributed across the allocated channels.

For a local archiving scheme, there must be at least one channel allocated to all of the nodes that write to their local archived logs. For a cluster file system archiving scheme, if every node writes the archived logs in the same cluster file system, then the backup operation of the archived logs is distributed across the allocated channels.

During a backup, the instances to which the channels connect must be either all mounted or all open. For example, if the instance on node1 has the database mounted while the instances on node2 and node3 have the database open, then the backup fails.

Node Affinity Awareness of Fast Connections

Learn about fast connection node affinity awareness.

In some cluster database configurations, some nodes of the cluster have faster access to certain data files than to other data files. RMAN automatically detects this situation, which is known as node affinity awareness. When deciding which channel to use to back up a particular data file, RMAN gives preference to the nodes with faster access to the data files that you want to back up. For example, if you have a three-node cluster, and if node1 has faster read/write access to data files 7, 8, and 9 than the other nodes, then node1 has greater node affinity to those files than node2 and node3.

Deleting Archived Redo Logs after a Successful Backup

Learn how to delete archived redo logs after backups.

If you have configured the automatic channels as defined in section "Channel Connections to Cluster Instances with RMAN", then you can use the following example to delete the archived logs that you backed up n times. The device type can be DISK or SBT:

DELETE ARCHIVELOG ALL BACKED UP n TIMES TO DEVICE TYPE device_type;

During a delete operation, if at least one channel allocated has access to the archived log, then RMAN automatically schedules the deletion of the specific log on that channel. For a local archiving scheme, there must be at least one channel allocated that can delete an archived log. For a cluster file system archiving scheme, if every node writes to the archived logs on the same cluster file system, then the archived log can be deleted by any allocated channel.

If you have not configured automatic channels, then you can manually allocate the maintenance channels as follows and delete the archived logs.

ALLOCATE CHANNEL FOR MAINTENANCE DEVICE TYPE DISK CONNECT 'SYS/oracle@node1';
ALLOCATE CHANNEL FOR MAINTENANCE DEVICE TYPE DISK CONNECT 'SYS/oracle@node2';
ALLOCATE CHANNEL FOR MAINTENANCE DEVICE TYPE DISK CONNECT 'SYS/oracle@node3';
DELETE ARCHIVELOG ALL BACKED UP n TIMES TO DEVICE TYPE device_type;

Autolocation for Backup and Restore Commands

Learn about autolocation for the backup and restore commands.

RMAN automatically performs autolocation of all files that it must back up or restore. If you use the noncluster file system local archiving scheme, then a node can only read the archived redo logs that were generated by an instance on that node. RMAN never attempts to back up archived redo logs on a channel it cannot read.

During a restore operation, RMAN automatically performs the autolocation of backups. A channel connected to a specific node only attempts to restore files that were backed up to the node. For example, assume that log sequence 1001 is backed up to the drive attached to node1, while log 1002 is backed up to the drive attached to node2. If you then allocate channels that connect to each node, then the channel connected to node1 can restore log 1001 (but not 1002), and the channel connected to node2 can restore log 1002 (but not 1001).