HP OpenVMS Systems Documentation

There are many ways to configure a CI (cluster interconnect) OpenVMS Cluster system. This chapter describes how to configure CI OpenVMS Clusters to maximize both availability and performance. This is done by presenting a series of configuration examples of increasing complexity, followed by a comparative analysis of each example. These configurations illustrate basic techniques that can be scaled upward to meet the availability, I/O performance, and storage connectivity needs of very large clusters.

9.1 CI Components

The CI is a radial bus through which OpenVMS Cluster systems communicate with each other and with storage. The CI consists of the following components:

• CI host adapter
• HSJ or HSC storage controller
  An HSJ or HSC storage controller is optional but generally present.
• CI cables
  For each of the CI's two independent paths (called path A and path B), there is a transmit and receive cable pair.
• Star coupler
  This is a passive device that serves as a common connection point for signals between OpenVMS nodes and HSC or HSJ controllers that are connected to the CI. A star coupler consists of two completely independent and electrically isolated "path hubs." Each CI path hub is extremely reliable because it contains only transformers carrying low-power signals.

Availability and performance can both be increased by adding components. Components added for availability need to be configured so that a redundant component is available to assume the work being performed by a failed component. Components added for performance need to be configured so that the additional components can work in parallel with other components.

Frequently, you need to maximize both availability and performance. The techniques presented here are intended to help achieve these dual goals.

9.2 Configuration Assumptions

The configurations shown here are based on the following assumptions:

1. MSCP serving is enabled.
2. Volume Shadowing for OpenVMS is installed.
3. When performance is being discussed:
   1. CI host adapters are CIPCA or CIXCD.
      Older CI adapter models are significantly slower.
   2. CI storage controllers are HSJ50s.
      Compared with HSJ50s, HSJ40s are somewhat slower, and HSC models are significantly slower.

Configuration 1, shown in Figure 9-1, provides no single point of failure. Its I/O performance is limited by the bandwidth of the star coupler.

Figure 9-1 Redundant HSJs and Host CI Adapters Connected to Same CI (Configuration 1)

9.3.1 Components

This configuration offers the following advantages:

• All nodes have direct access to storage.
• Highly expandable.
• CI is inherently dual pathed.
• No single component failure can disable the cluster.
• If a CI adapter fails, or both its paths are disabled, OpenVMS will automatically fail over all I/O and cluster traffic to the other CI adapter.
• Disks are dual ported between HSJ controllers; automatic disk failover to the other controller if an HSJ fails or if an HSJ loses both paths to a star coupler.
• Redundant storage controllers can be used to provide additional performance by dividing disks between the two storage controllers.
  Disks can be assigned to HSJ storage controllers by the OpenVMS Prefer utility supplied in SYS$EXAMPLES, or by issuing a $QIO call with IO$_SETPRFPATH and IO$M_FORCEPATH modifiers, or by using the HSJ SET_PREFERRED command (less desirable; use only for this configuration).
• Critical disks are shadowed with shadow set members on different SCSI buses.
• Read I/Os are automatically load balanced across shadow set members for performance.
• Lowest cost.

This configuration has the following disadvantages:

• Second CI adapter in each host is unlikely to enhance performance.
• Both HSJs have to share the bandwidth of a single CI.
• Failure of a CI path hub or path cable halves the bandwidth available to all CI components that use the failed component.
• Physical damage to a star coupler or associated cables is likely to disable the entire CI, rendering the cluster unusable.
• Physical damage to the StorageWorks cabinet could render the cluster unusable.

This configuration incorporates the following strategies:

• All components are duplicated.
• Redundant storage controllers are included.
• This configuration has no single point of failure.
• Dual porting and volume shadowing provide multiple copies of essential disks across separate HSJ controllers.
• All nodes have shared, direct access to all storage.
• A quorum disk allows other node to continue if one node fails. (Alternatively, at least three nodes could be used.)

The configuration illustrated in Figure 9-2 with redundant HSJs, host CI adapters, and CIs provides no electrical single point of failure. Its two star couplers provide increased I/O performance and availability over configuration 1.

Figure 9-2 Redundant HSJs and Host CI Adapters Connected to Redundant CIs (Configuration 2)

Configuration 2 has the following components:

Configuration 2 offers all the advantages of Configuration 1 plus the following advantages:

• CI is likely to remain fully usable in the event of localized damage to a star coupler cabinet or to the cables connected to it.
• Failure of an HSJ, or an HSJ losing both paths to a star coupler host will result in the other HSJ assuming control of all dual-pathed disks.
• Host with connectivity will automatically MSCP serve unreachable disks to a host that has lost connectivity to an HSJ.
• Dual-pathed disks can be switched to an HSJ with full host connectivity if a host loses connectivity to an HSJ.
  The disks on the HSJ that cannot be reached by a host can be reassigned to the HSJ with full connectivity by operator command, or by a DCL or other program that monitors for connectivity loss. You can assign disks to another storage controller with the Prefer utility supplied in SYS$EXAMPLES, or by issuing a $QIO call with IO$_SETPRFPATH and IO$M_FORCEPATH modifiers, or by powering off the HSJ with reduced connectivity (less desirable).
  The HSJ SET_PREFERRED command is not recommended in this configuration because this command can not be overridden by a host PREFER or IO$_SETPRFPATH modifier. A new SET_PREFERRED command assigning a device to another HSJ will not take effect until the HSJ, to which the device was assigned by a previous SET_PREFERRED command, is power cycled.)

Configuration 2 has the following disadvantages:

• Host CI adapter failure will not cause disks to automatically fail over to an HSJ that still has full host connectivity.
  If a host CI adapter fails and if MSCP serving is enabled, then the other OpenVMS system will begin serving the unreachable HSJ's disks to the host with the failed adapter.
• Physical damage to a star coupler or associated cables is likely to disable the entire CI, rendering the cluster unusable.
• Physical damage to the StorageWorks cabinet could render the cluster unusable.
• Higher cost than configuration 1.

Configuration 2 provides configuration 1 strategies, plus:

• Second CI provides additional redundancy.
• Second CI cables and star coupler are physically separate.

The availability of a CI configuration can be further improved by physically separating the path A and path B CI cables and their associated path hubs. This significantly reduces the probability of a mechanical accident or other localized damage destroying both paths of a CI. This configuration is shown in Figure 9-3.

Figure 9-3 Redundant Components and Path-Separated Star Couplers (Configuration 3)

Configuration 3 is electrically identical to configuration 2. However, the path A and path B cables are physically separate for both CIs. The path A cables for both CI 1 and CI 2 are routed together to star coupler cabinet A, but are connected to different CI path hubs in that cabinet.

Similarly, the path B cables for both CIs go to different CI path hubs in star coupler cabinet B. The path-specific star coupler cabinets and associated path cables should be separated as much as possible. For example, the star coupler cabinets could be installed on opposite sides of a computer room, and the CI cables could be routed so that path A and path B cables follow different paths.

The CI OpenVMS Cluster configuration shown in Figure 9-3 has the following components:

Configuration 3 offers the same individual component advantages as configuration 2, plus:

• Both CIs remain usable in the event of localized damage to a CI path.
• Same cost as configuration 2 with better availability.

Configuration 3 has the following disadvantages:

• Host CI adapter failure will not cause disks to automatically fail over to an HSJ that still has full host connectivity.
  If a host CI adapter fails and if MSCP serving is enabled, the other OpenVMS system will begin serving the unreachable HSJ's disks to the host with the failed adapter.
• Damage to star coupler or associated cables likely to affect the same path of both CIs.
• Damage affecting path n of both CIs can halve the usable bandwidth of both CIs.
• Physical damage to the StorageWorks cabinet could render the cluster unusable.

Configuration 3 provides all the strategies of configuration 2 except for physical separation of CIs. The major advantage over configuration 2 are the path-specific star coupler cabinets. They provide physical isolation of the path A cables and the path A hub from the path B cables and the path B hub.

9.6 Configuration 4

The availability of a CI configuration can be further improved by physically separating shadow set members and their HSJ controllers. This significantly reduces the probability of a mechanical accident or other localized damage that could destroy both members of a shadow set. This configuration is shown in Figure 9-4.

Figure 9-4 Redundant Components, Path-Separated Star Couplers, and Duplicate StorageWorks Cabinets (Configuration 4)

Configuration 4 is similar to configuration 3 except that the shadow set members and their HSJ controllers are mounted in separate StorageWorks cabinets that are located some distance apart.

The StorageWorks cabinets, path-specific star coupler cabinets, and associated path cables should be separated as much as possible. For example, the StorageWorks cabinets and the star coupler cabinets could be installed on opposite sides of a computer room. The CI cables should be routed so that path A and path B cables follow different paths.

The CI OpenVMS Cluster configuration shown in Figure 9-4 has the following components: