Device Groups
In the Oracle Solaris Cluster software, all multihost devices must be under control of the Oracle Solaris Cluster software. The Oracle Solaris Cluster software automatically creates a raw device group for each disk and tape device in the cluster. However, these cluster device groups remain in an offline state until you access them as global devices.
Device groups provide a mechanism for Oracle Solaris Cluster to manage the mounting and unmounting of file systems which may reside in either ZFS storage pools or in volume manager disk groups. When a ZFS pool or disk group is managed by a device group, its file systems can be made globally accessible within the cluster or can failover from one node to another. If more than one node has direct physical connection to a device group, access to that data becomes highly available and can survive node failures.
Note - Device groups are independent of resource groups. One node can master a resource group (representing a group of data service processes). Another node can master the device groups that are being accessed by the data services. However, the best practice is to keep on the same node the device group that stores a particular application's data and the resource group that contains the application's resources (the application daemon). Refer to Relationship Between Resource Groups and Device Groups in Planning and Administering Data Services for Oracle Solaris Cluster 4.4 for more information about the association between device groups and resource groups.
Each cluster node that is physically attached to the multihost disks provides a path to the device group.
Device Group Failover
Because a disk enclosure is connected to more than one cluster node, all device groups in that enclosure are accessible through an alternate path if the node currently mastering the device group fails. The failure of the node that is mastering the device group does not affect access to the device group except for the time it takes to perform the recovery and consistency checks. During this time, all requests are blocked (transparently to the application) until the system makes the device group available.
Figure 15 Device Group Before and After Failover
Device Group Ownership
This section describes device group properties that enable you to balance performance and availability in a multiported disk configuration. Oracle Solaris Cluster software provides two properties that configure a multiported disk configuration: preferenced and numsecondaries. You can control the order in which nodes attempt to assume control if a failover occurs by using the preferenced property. Use the numsecondaries property to set the number of secondary nodes for a device group that you want.
A highly available service is considered down when the primary node fails and when no eligible secondary nodes can be promoted to primary nodes. If service failover occurs and the preferenced property is true, then the nodes follow the order in the node list to select a secondary node. The node list defines the order in which nodes attempt to assume primary control or transition from spare to secondary. You can dynamically change the preference of a device service by using the clsetup command. The preference that is associated with dependent service providers, for example a global file system, is identical to the preference of the device service.
Secondary nodes are check-pointed by the primary node during normal operation. In a multiported disk configuration, checkpointing each secondary node causes cluster performance degradation and memory overhead. Spare node support was implemented to minimize the performance degradation and memory overhead that checkpointing caused. By default, your device group has one primary and one secondary. The remaining available provider nodes become spares. If failover occurs, the secondary becomes primary and the node highest in priority on the node list becomes secondary.
You can set the number of secondary nodes that you want to any integer between one and the number of operational nonprimary provider nodes in the device group.
Note - If you are using Solaris Volume Manager, you must create the device group first, using the metaset command before you use the cldevicegroup command to set the property.
The default number of secondaries is 1. The actual number of secondary providers that is maintained by the framework is the number that you want, unless the number of operational nonprimary providers is less than the number that you want. You must alter the numsecondaries property and double-check the node list if you are adding or removing nodes from your configuration. Maintaining the node list and number of secondaries prevents conflict between the configured number of secondaries and the actual number that is allowed by the framework.
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Use the cldevicegroup command for Solaris Volume Manager device groups, in conjunction with the preferenced and numsecondaries property settings, to manage the addition of nodes to and the removal of nodes from your configuration.
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Refer to Overview of Administering Cluster File Systems in Administering an Oracle Solaris Cluster 4.4 Configuration for procedural information about changing device group properties.