Redundant Disk Storage

Supported Disk Interfaces

The following interfaces are supported by MC/LockManager for disks that are connected to two or more nodes (shared data disks):

Not all SCSI disks are supported. See the HP 9000 Servers Configuration Guide (available through your HP representative) for a list of currently supported disks.

External shared Fast/Wide SCSI buses must be equipped with in-line terminators for disks on a shared bus. Refer to the "Troubleshooting" chapter for additional information.

When planning and assigning SCSI bus priority, remember that one node can dominate a bus shared by multiple nodes, depending on what SCSI addresses are assigned to the controller for each node on the shared bus. All SCSI addresses, including the addresses of all interface cards, must be unique for all devices on a shared bus. See the manual Configuring HP-UX for Peripherals for information on SCSI bus addressing and priority.

Data Protection using Disk Arrays

It is required that you provide data protection for your highly available system. MC/LockManager supports the use of disk arrays configured in RAID Level 1 or RAID Level 5. The array provides data redundancy for the disks. This protection needs to be combined with the use of redundant SCSI interfaces between each node and the array. The use of redundant interfaces, configured with LVM's PV Links feature protects against single points of failure in the I/O channel, and RAID 1 or 5 configuration provides redundancy for the storage media. (PV links are also known as alternate links in LVM.)

Figure 2-6 “Cluster with High Availability Disk Array ” shows a similar cluster with a disk array connected to each node on two I/O channels. In this configuration, Logical Volume Manager's PV links are used to define the separate pathways to the data from one node.

Figure 2-6 Cluster with High Availability Disk Array

Monitoring of Disks with Event Monitoring Service

You can configure disk monitoring to detect a failed mechanism by using the disk monitor capabilities of the EMS HA Monitors, available as a separate product (B5735AA). Monitoring can be set up to trigger a package failover or to report disk failure events to a target application such as ClusterView. Refer to the manual Using EMS HA Monitors (HP part number B5735-90001) for additional information.

Replacement of Failed Disk Mechanisms

With disk arrays, it is possible to replace a disk while the cluster stays up and the application remains online. Consult your disk array documentation for details.

Root Disk Limitations on Shared Buses

The IODC firmware does not support two or more nodes booting from the same SCSI bus at the same time. For this reason, it is important not to attach more than one root disk per cluster to a single SCSI bus.

For example, Figure 2-7 “Root Disks on Different Shared Buses ” shows a supported configuration in which two nodes share an external SCSI bus and Node A has its primary root disk connected to the bus, but node B has its primary root disk connected to a different bus. (Numbers 0 to 3, 6 and 7 are SCSI addresses on the different buses.)

Figure 2-7 Root Disks on Different Shared Buses

Note that if both nodes had their primary root disks connected to the same bus, you would have an unsupported configuration.

You can put a mirror copy of Node B's root disk on the same SCSI bus as Node A's primary root disk, because three failures would have to occur for both systems to boot at the same time, which is an acceptable risk. In such a scenario, Node B would have to lose its primary root disk and be rebooted, and Node A would have to be rebooted at the same time Node B is, for the IODC firmware to run into a problem. This configuration is shown in Figure 2-8 “Primaries and Mirrors on Different Shared Buses ”.

Figure 2-8 Primaries and Mirrors on Different Shared Buses

Note that you cannot use a disk within a disk array as a root disk if the array is on a shared bus.