Document revision date: 30 March 2001

## Guidelines for OpenVMS Cluster Configurations

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## 8.12 Disaster-Tolerant OpenVMS Cluster Configurations

Disaster-tolerant OpenVMS Cluster configurations make use of Volume Shadowing for OpenVMS, high-speed networks, and specialized management software.

Disaster-tolerant OpenVMS Cluster configurations enable systems at two different geographic sites to be combined into a single, manageable OpenVMS Cluster system. Like the multiple-site cluster discussed in the previous section, these physically separate data centers are connected by FDDI or by a combination of FDDI and ATM, T3, or E3.

The OpenVMS disaster-tolerant product was formerly named the Business Recovery Server (BRS). BRS has been subsumed by a services offering named Disaster Tolerant Cluster Services, which is a system management and software service package. For more information about Disaster Tolerant Cluster Services, contact your Compaq Services representative.

## Chapter 9
Configuring CI OpenVMS Clusters for Availability and Performance

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.

## 9.3 Configuration 1

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

The CI configuration shown in Figure 9-1 has the following components:

Part | Description
--- | ---
Host 1, Host 2 | Dual CI capable OpenVMS Alpha or VAX hosts. Rationale: Either host can fail and the system can continue. The full performance of both hosts is available for application use under normal conditions.
CI 1-1,CI 1-2, CI 2-1,CI 2-2 | Dual CI adapters on each host. Rationale: Either of a host's CI adapters can fail and the host will retain CI connectivity to the other host and to the HSJ storage controllers.
Star Coupler | One star coupler cabinet containing two independent path hubs. The star coupler is redundantly connected to the CI host adapters and HSJ storage controllers by a transmit/receive cable pair per path. Rationale: Either of the path hubs or an attached cable could fail and the other CI path would continue to provide full CI connectivity. When both paths are available, their combined bandwidth is usable for host-to-host and host-to-storage controller data transfer.
HSJ 1, HSJ 2 | Dual HSJ storage controllers in a single StorageWorks cabinet. Rationale: Either storage controller can fail and the other controller can assume control of all disks by means of the SCSI buses shared between the two HSJs. When both controllers are available, each can be assigned to serve a portion of the disks. Thus, both controllers can contribute their I/O-per-second and bandwidth capacity to the cluster.
SCSI 1, SCSI 2 | Shared SCSI buses between HSJ pairs. Rationale: Provide access to each disk on a shared SCSI from either HSJ storage controller. This effectively dual ports the disks on that bus.
Disk 1, Disk 2, . . . Disk n-1, Disk n | Critical disks are dual ported between HSJ pairs by shared SCSI buses. Rationale: Either HSJ can fail, and the other HSJ will assume control of the disks that the failed HSJ was controlling.
Shadow Set 1 through Shadow Set n | Essential disks are shadowed by another disk that is connected on a different shared SCSI. Rationale: A disk or the SCSI bus to which it is connected, or both, can fail, and the other shadow set member will still be available. When both disks are available, their combined READ I/O capacity and READ data bandwidth capacity are available to the cluster.

### 9.3.2 Advantages

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.

### 9.3.3 Disadvantages

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.

### 9.3.4 Key Availability and Performance Strategies

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.)

## 9.4 Configuration 2

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)

### 9.4.1 Components

Configuration 2 has the following components:

Part | Description
--- | ---
Host 1, Host 2 | Dual CI capable OpenVMS Alpha or VAX hosts. Rationale: Either host can fail and the system can continue to run. The full performance of both hosts is available for application use under normal conditions.
CI 1-1,CI 1-2, CI 2-1, CI 2-2 | Dual CI adapters on each host. Adapter CI 1- n is Host 1's CI adapter connected to CI n, and so on. Rationale: Either of a host's CI adapters can fail, and the host will retain CI connectivity to the other host and to the HSJ storage controllers. Each CI adapter on a host is connected to a different star coupler. In the absence of failures, the full data bandwidth and I/O-per-second capacity of both CI adapters is available to the host.
Star Coupler 1, Star Coupler 2 | Two star couplers, each consisting of two independent path hub sections. Each star coupler is redundantly connected to the CI host adapters and HSJ storage controllers by a transmit/receive cable pair per path. Rationale: Any of the path hubs or an attached cable could fail and the other CI path would continue to provide full connectivity for that CI. Loss of a path affects only the bandwidth available to the storage controller and host adapters connected to the failed path. When all paths are available, the combined bandwidth of both CIs is usable.
HSJ 1, HSJ 2 | Dual HSJ storage controllers in a single StorageWorks cabinet. Rationale: Either storage controller can fail and the other controller can control any disks the failed controller was handling by means of the SCSI buses shared between the two HSJs. When both controllers are available, each can be assigned to serve a subset of the disks. Thus, both controllers can contribute their I/O-per-second and bandwidth capacity to the cluster.
SCSI 1, SCSI 2 | Shared SCSI buses connected between HSJ pairs. Rationale: Either of the shared SCSI buses could fail and access would still be provided from the HSJ storage controllers to each disk by means of the remaining shared SCSI bus. This effectively dual ports the disks on that bus.
Disk 1, Disk 2, . . . Disk n-1, Disk n | Critical disks are dual ported between HSJ pairs by shared SCSI buses. Rationale: Either HSJ can fail and the other HSJ will assume control of the disks the failed HSJ was controlling.
Shadow Set 1 through Shadow Set n | Essential disks are shadowed by another disk that is connected on a different shared SCSI. Rationale: A disk or the SCSI bus to which it is connected, or both, can fail and the other shadow set member will still be available. When both disks are available, both can provide their READ I/O capacity and their READ data bandwidth capacity to the cluster.

### 9.4.2 Advantages

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.)

### 9.4.3 Disadvantages

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.

### 9.4.4 Key Availability and Performance Strategies

Configuration 2 provides configuration 1 strategies, plus:

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

## 9.5 Configuration 3

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.

Note The path separation technique illustrated for configuration 3 (Figure 9_3) can also be applied to configuration 1 (Figure 9-1). In this case, each star coupler cabinet would have only one path hub. The CI's path A cables would go to the path hub in Star Coupler A. Similarly, the path B cables would go to Star Coupler B.

### 9.5.1 Components

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

Part | Description
--- | ---
Host 1, Host 2 | Dual CI capable OpenVMS Alpha or VAX hosts. Rationale: Either host can fail and the system can continue. The full performance of both hosts is available for application use under normal conditions.
CI 1-1,CI 1-2, CI 2-1, CI 2-2 | Dual CI adapters on each host. Adapter CI 1- n is Host 1's CI adapter connected to CI n, and so on. Rationale: Either host's CI adapters can fail and the host will retain CI connectivity to the other host and to the HSJ storage controllers. Each CI adapter on a host is connected to a different star coupler. In the absence of failures, the full data bandwidth and I/O-per-second capacity of both CI adapters is available to the host.
Star Coupler A (Path A Hubs), Star Coupler B (Path B Hubs) | Two CI star couplers, each comprising two independent path hubs. Star Coupler A's path hubs are connected to path A cables for both CIs, and Star Coupler B's path hubs are connected to path B cables for both CIs. Rationale: Mechanical or other localized damage to a star coupler or an attached cable would probably not affect the other CI paths. The other paths and star coupler would continue to provide full connectivity for both CIs. Loss of a path affects only the bandwidth available to the storage controllers and host adapters connected to the failed path. When all paths are available, the combined bandwidth of both CIs is usable.
Path A CI Cables, Path B CI Cables | Each path's hub is connected to the CI host adapters and HSJ storage controllers by a transmit/receive cable pair per path. The path A cables of both CIs are routed together, but their routing differs from the routing of the path B cables.
HSJ 1, HSJ 2 | Dual HSJ storage controllers in a single StorageWorks cabinet. Rationale: Either storage controller can fail and the other controller can control any disks the failed controller was handling by means of the SCSI buses shared between the two HSJs. When both controllers are available, each can be assigned to serve a subset of the disks. Thus, both controllers can contribute their I/O-per-second and bandwidth capacity to the cluster.
SCSI 1, SCSI 2 | Shared SCSI buses connected between HSJ pairs. Rationale: Provide access to each disk on a shared SCSI bus from either HSJ storage controller. This effectively dual ports the disks on that bus.
Disk 1, Disk 2, . . . Disk n-1, Disk n | Critical disks are dual ported between HSJ pairs by shared SCSI buses. Rationale: Either HSJ can fail and the other HSJ will assume control of the disks that the failed HSJ was controlling.
Shadow Set 1 through Shadow Set n | Essential disks are shadowed by another disk that is connected on a different shared SCSI. Rationale: A disk, or the SCSI bus to which it is connected, or both, can fail and the other shadow set member will still be available. When both disks are available, their combined READ I/O-per-second capacity and READ data bandwidth capacity is available to the cluster.

### 9.5.2 Advantages

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.

### 9.5.3 Disadvantages

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.

### 9.5.4 Key Availability and Performance Strategies

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.

Note The separate StorageWorks cabinets technique illustrated in configuration 4 (Figure 9-4) can also be applied to configuration 1 (Figure 9_1) and configuration 2 (Figure 9-2).

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