Multihost Devices

Disks that can be connected to more than one node at a time are multihost devices. In the Sun Cluster environment, multihost storage makes disks highly available. Sun Cluster requires multihost storage for two-node clusters to establish quorum. Greater than three-node clusters do not need multihost storage.

Multihost devices have the following characteristics.

• They can tolerate single node failures.

• They store application data and can also store application binaries and configuration files.

• They protect against node failures. If client requests are accessing the data through one node and it fails, the requests are switched over to use another node that has a direct connection to the same disks.

• They are either accessed globally through a primary node that “masters” the disks, or by direct concurrent access through local paths. The only application that uses direct concurrent access currently is Oracle Real Application Clusters.

A volume manager provides for mirrored or RAID-5 configurations for data redundancy of the multihost devices. Currently, Sun Cluster supports Solaris Volume Manager^TM and VERITAS Volume Manager, which is available for use in only SPARC based clusters, as volume managers, and the RDAC RAID-5 hardware controller on several hardware RAID platforms.

Combining multihost devices with disk mirroring and striping protects against both node failure and individual disk failure.

See Chapter 4, Frequently Asked Questions for questions and answers about multihost storage.

Multi-Initiator SCSI

This section applies only to SCSI storage devices and not to Fibre Channel storage used for the multihost devices.

In a standalone server, the server node controls the SCSI bus activities by way of the SCSI host adapter circuit connecting this server to a particular SCSI bus. This SCSI host adapter circuit is referred to as the SCSI initiator. This circuit initiates all bus activities for this SCSI bus. The default SCSI address of SCSI host adapters in Sun systems is 7.

Cluster configurations share storage between multiple server nodes, using multihost devices. When the cluster storage consists of singled-ended or differential SCSI devices, the configuration is referred to as multi-initiator SCSI. As this terminology implies, more than one SCSI initiator exists on the SCSI bus.

The SCSI specification requires that each device on a SCSI bus has a unique SCSI address. (The host adapter is also a device on the SCSI bus.) The default hardware configuration in a multi-initiator environment results in a conflict because all SCSI host adapters default to 7.

To resolve this conflict, on each SCSI bus, leave one of the SCSI host adapters with the SCSI address of 7, and set the other host adapters to unused SCSI addresses. Proper planning dictates that these “unused” SCSI addresses include both currently and eventually unused addresses. An example of addresses unused in the future is the addition of storage by installing new drives into empty drive slots.

In most configurations, the available SCSI address for a second host adapter is 6.

You can change the selected SCSI addresses for these host adapters by using one of the following tools to set the scsi-initiator-id property:

• eeprom(1M)

• The OpenBoot PROM on a SPARC based system

• The SCSI utility that you optionally run after the BIOS boots on an x86 based system

You can set this property globally for a node or on a per-host-adapter basis. Instructions for setting a unique scsi-initiator-id for each SCSI host adapter are included in the chapter for each disk enclosure in the Sun Cluster Hardware Collection.