Shared Devices

Sun Cluster systems use shared devices to provide cluster-wide, highly available access to any device in a cluster, from any node.

How Sun Cluster Uses Shared Devices

Generally, if a node fails while providing access to a shared device, the Sun Cluster software switches over to another path to the device and redirects the access to that path. This redirection is easy with shared devices because the same name is used for the device regardless of the path. Access to a remote device is performed in the same way as on a local device that uses the same name. Also, the API to access a shared device on a cluster is the same as the API that is used to access a device locally.

Sun Cluster shared devices include disks, CD-ROMs, and tapes. However, disks are the only multiported shared devices that are supported. This limited support means that CD-ROM and tape devices are not currently highly available devices. The local disks on each server are also not multiported, and thus are not highly available devices.

The DID framework assigns a common (normalized) logical name to each disk, CD-ROM, and tape device in the cluster. This assignment enables consistent access to each device from any node in the cluster.

Device ID

The Sun Cluster software manages shared devices through a construct that is known as the device ID (DID) driver. This driver is used to automatically assign unique IDs to every device in the cluster, including multihost disks, tape drives, and CD-ROMs.

The DID driver is an integral part of the shared device access feature of the cluster. The DID driver probes all nodes of the cluster and builds a list of unique disk devices. The DID driver also assigns each device a unique major and minor number that is consistent on all nodes of the cluster. Access to the shared devices is performed by using the normalized ID logical name, instead of the traditional Solaris logical name.

This approach ensures that any application accessing disks, such as Solaris Volume Manager, uses a consistent path across the cluster. This consistency is especially important for multihost disks, because the local major and minor numbers for each device can vary from node to node. These numbers can change the Solaris device naming conventions as well.