Guidelines for OpenVMS Cluster Configurations

Updated: 11 December 1998

Guidelines for OpenVMS Cluster Configurations

Contents

Index

8.5 Availability in a LAN OpenVMS Cluster

Figure 8-1 shows an optimal configuration for a small-capacity, highly available LAN OpenVMS Cluster system. Figure 8-1 is followed by an analysis of the configuration that includes:

Analysis of its components
Advantages and disadvantages
Key availability strategies implemented

Figure 8-1 LAN OpenVMS Cluster System

8.5.1 Components

The LAN OpenVMS Cluster configuration in Figure 8-1 has the following components:

Part	Description
1	Two Ethernet interconnects. For higher network capacity, use FDDI interconnects instead of Ethernet. Rationale: For redundancy, use at least two LAN interconnects and attach all nodes to all LAN interconnects. A single interconnect would introduce a single point of failure.
2	Three to eight Ethernet-capable OpenVMS nodes. Each node has its own system disk so that it is not dependent on another node. Rationale: Use at least three nodes to maintain quorum. Use fewer than eight nodes to avoid the complexity of managing eight system disks. Alternative 1: If you require satellite nodes, configure one or two nodes as boot servers. Note, however, that the availability of the satellite nodes is dependent on the availability of the server nodes. Alternative 2: For more than eight nodes, use a LAN OpenVMS Cluster configuration as described in Section 8.10.
3	System disks. System disks generally are not shadowed in LAN OpenVMS Clusters because of boot-order dependencies. Alternative 1: Shadow the system disk across two local controllers. Alternative 2: Shadow the system disk across two nodes. The second node mounts the disk as a nonsystem disk. Reference: See Section 11.2.4 for an explanation of boot-order and satellite dependencies.
4	Essential data disks. Use volume shadowing to create multiple copies of all essential data disks. Place shadow set members on at least two nodes to eliminate a single point of failure.

8.5.2 Advantages

This configuration offers the following advantages:

Lowest cost of all the sample configurations shown in this chapter.
Some potential for growth in size and performance.
The LAN interconnect supports the widest choice of nodes.

8.5.3 Disadvantages

This configuration has the following disadvantages:

No shared direct access to storage. The nodes are dependent on an MSCP server for access to shared storage.
Shadowing disks across the LAN nodes causes shadow copies when the nodes boot.
Shadowing the system disks is not practical because of boot-order dependencies.

8.5.4 Key Availability Strategies

The configuration in Figure 8-1 incorporates the following strategies, which are critical to its success:

This configuration has no single point of failure.
Volume shadowing provides multiple copies of essential data disks across separate nodes.
At least three nodes are used for quorum, so the OpenVMS Cluster continues if any one node fails.
Each node has its own system disk; there are no satellite dependencies.

8.6 Configuring Multiple LANs

Follow these guidelines to configure a highly available multiple LAN cluster:

Bridge LAN segments together to form a single extended LAN.
Provide redundant LAN segment bridges for failover support.
Configure LAN bridges to pass the LAN and MOP multicast messages.
Reference: Refer to the documentation for your LAN bridge and to the documentation for RBMS, DECelms, or POLYCENTER Framework for more information about configuring LAN bridges to pass these multicast messages.
Use the Local Area OpenVMS Cluster Network Failure Analysis Program to monitor and maintain network availability. (See OpenVMS Cluster Systems for more information.)
Use the troubleshooting suggestions in OpenVMS Cluster Systems to diagnose performance problems with the SCS layer and the NISCA transport protocol.
Keep LAN average utilization below 50%.

Reference: See Section 10.7.7 for information about extended LANs (ELANs).

When using multiple LAN adapters with multiple LAN segments, distribute the connections to LAN segments that provide MOP service. The distribution allows MOP servers to downline load satellites even when network component failures occur.

It is important to ensure sufficient MOP servers for both VAX and Alpha nodes to provide downline load support for booting satellites. By careful selection of the LAN connection for each MOP server (Alpha or VAX, as appropriate) on the network, you can maintain MOP service in the face of network failures.

8.6.2 Configuring Two LAN Segments

Figure 8-2 shows a sample configuration for an OpenVMS Cluster system connected to two different LAN segments. The configuration includes Alpha and VAX nodes, satellites, and two bridges.

Figure 8-2 Two-LAN Segment OpenVMS Cluster Configuration

The figure illustrates the following points:

Connecting critical nodes to multiple LAN segments provides increased availability in the event of segment or adapter failure. Disk and tape servers can use some of the network bandwidth provided by the additional network connection. Critical satellites can be booted using the other LAN adapter if one LAN adapter fails.
Connecting noncritical satellites to only one LAN segment helps to balance the network load by distributing systems equally among the LAN segments. These systems communicate with satellites on the other LAN segment through one of the bridges.
Only one LAN adapter per node can be used for DECnet and MOP service to prevent duplication of LAN addresses.
LAN adapters providing MOP service (Alpha or VAX, as appropriate) should be distributed among the LAN segments to ensure that LAN failures do not prevent satellite booting.
Using redundant LAN bridges prevents the bridge from being a single point of failure.

8.6.3 Configuring Three LAN Segments

Figure 8-3 shows a sample configuration for an OpenVMS Cluster system connected to three different LAN segments. The configuration also includes both Alpha and VAX nodes and satellites and multiple bridges.

Figure 8-3 Three-LAN Segment OpenVMS Cluster Configuration

The figure illustrates the following points:

Connecting disk and tape servers to two or three LAN segments can help provide higher availability and better I/O throughput.
Connecting critical satellites to two or more LAN segments can also increase availability. If any of the network components fails, these satellites can use the other LAN adapters to boot and still have access to the critical disk servers.
Distributing noncritical satellites equally among the LAN segments can help balance the network load.
A MOP server (Alpha or VAX, as appropriate) is provided for each LAN segment.

Reference: See Section 11.2.4 for more information about boot order and satellite dependencies in a LAN. See OpenVMS Cluster Systems for information about LAN bridge failover.

8.7 Availability in a DSSI OpenVMS Cluster

Figure 8-4 shows an optimal configuration for a medium-capacity, highly available DSSI OpenVMS Cluster system. Figure 8-4 is followed by an analysis of the configuration that includes:

Analysis of its components
Advantages and disadvantages
Key availability strategies implemented

Figure 8-4 DSSI OpenVMS Cluster System

8.7.1 Components

The DSSI OpenVMS Cluster configuration in Figure 8-4 has the following components:

Part	Description
1	Two DSSI interconnects with two DSSI adapters per node. Rationale: For redundancy, use at least two interconnects and attach all nodes to all DSSI interconnects.
2	Two to four DSSI-capable OpenVMS nodes. Rationale: Three nodes are recommended to maintain quorum. A DSSI interconnect can support a maximum of four OpenVMS nodes. Alternative 1: Two-node configurations require a quorum disk to maintain quorum if a node fails. Alternative 2: For more than four nodes, configure two DSSI sets of nodes connected by two LAN interconnects.
3	Two Ethernet interconnects. Rationale: The LAN interconnect is required for DECnet--Plus communication. Use two interconnects for redundancy. For higher network capacity, use FDDI instead of Ethernet.
4	System disk. Shadow the system disk across DSSI interconnects. Rationale: Shadow the system disk across interconnects so that the disk and the interconnect do not become single points of failure.
5	Data disks. Shadow essential data disks across DSSI interconnects. Rationale: Shadow the data disk across interconnects so that the disk and the interconnect do not become single points of failure.

8.7.2 Advantages

The configuration in Figure 8-4 offers the following advantages:

The DSSI interconnect gives all nodes shared, direct access to all storage.
Moderate potential for growth in size and performance.
There is only one system disk to manage.

8.7.3 Disadvantages

This configuration has the following disadvantages:

Applications must be shut down in order to swap DSSI cables. This is referred to as "warm swap." The DSSI cable is warm swappable for the adapter, the cable, and the node.
A node's location on the DSSI affects the recoverability of the node. If the adapter fails on a node located at the end of the DSSI interconnect, the OpenVMS Cluster may become unavailable.

8.7.4 Key Availability Strategies

The configuration in Figure 8-4 incorporates the following strategies, which are critical to its success:

This configuration has no single point of failure.
Volume shadowing provides multiple copies of system and essential data disks across separate DSSI interconnects.
All nodes have shared, direct access to all storage.
At least three nodes are used for quorum, so the OpenVMS Cluster continues if any one node fails.
There are no satellite dependencies.

8.8 Availability in a CI OpenVMS Cluster

Figure 8-5 shows an optimal configuration for a large-capacity, highly available CI OpenVMS Cluster system. Figure 8-5 is followed by an analysis of the configuration that includes:

Analysis of its components
Advantages and disadvantages
Key availability strategies implemented

Figure 8-5 CI OpenVMS Cluster System

8.8.1 Components

The CI OpenVMS Cluster configuration in Figure 8-5 has the following components:

Part Description

1 Two LAN interconnects.
Rationale: The additional use of LAN interconnects is required for DECnet--Plus communication. Having two LAN interconnects---Ethernet or FDDI---increases redundancy. For higher network capacity, use FDDI instead of Ethernet.

2 Two to 16 CI capable OpenVMS nodes.
Rationale: Three nodes are recommended to maintain quorum. A CI interconnect can support a maximum of 16 OpenVMS nodes.
Reference: For more extensive information about the CIPCA, see Appendix C.
Alternative: Two-node configurations require a quorum disk to maintain quorum if a node fails.

3 Two CI interconnects with two star couplers.
Rationale: Use two star couplers to allow for redundant connections to each node.

4 Critical disks are dual ported between CI storage controllers.
Rationale: Connect each disk to two controllers for redundancy. Shadow and dual port system disks between CI storage controllers. Periodically alternate the primary path of dual-ported disks to test hardware.

5 Data disks.
Rationale: Single port nonessential data disks, for which the redundancy provided by dual porting is unnecessary.

6 Essential data disks are shadowed across controllers.
Rationale: Shadow essential disks and place shadow set members on different HSCs to eliminate a single point of failure.

Part	Description
1	Two LAN interconnects. Rationale: The additional use of LAN interconnects is required for DECnet--Plus communication. Having two LAN interconnects---Ethernet or FDDI---increases redundancy. For higher network capacity, use FDDI instead of Ethernet.
2	Two to 16 CI capable OpenVMS nodes. Rationale: Three nodes are recommended to maintain quorum. A CI interconnect can support a maximum of 16 OpenVMS nodes. Reference: For more extensive information about the CIPCA, see Appendix C. Alternative: Two-node configurations require a quorum disk to maintain quorum if a node fails.
3	Two CI interconnects with two star couplers. Rationale: Use two star couplers to allow for redundant connections to each node.
4	Critical disks are dual ported between CI storage controllers. Rationale: Connect each disk to two controllers for redundancy. Shadow and dual port system disks between CI storage controllers. Periodically alternate the primary path of dual-ported disks to test hardware.
5	Data disks. Rationale: Single port nonessential data disks, for which the redundancy provided by dual porting is unnecessary.
6	Essential data disks are shadowed across controllers. Rationale: Shadow essential disks and place shadow set members on different HSCs to eliminate a single point of failure.

8.8.2 Advantages

This configuration offers the following advantages:

All nodes have direct access to all storage.
This configuration has a high growth capacity for processing and storage.
The CI is inherently dual pathed, unlike other interconnects.

8.8.3 Disadvantages

This configuration has the following disadvantage:

Higher cost than the other configurations.

8.8.4 Key Availability Strategies

The configuration in Figure 8-5 incorporates the following strategies, which are critical to its success:

This configuration has no single point of failure.
Dual porting and volume shadowing provides multiple copies of essential disks across separate HSC or HSJ controllers.
All nodes have shared, direct access to all storage.
At least three nodes are used for quorum, so the OpenVMS Cluster continues if any one node fails.
There are no satellite dependencies.
The uninterruptible power supply (UPS) ensures availability in case of a power failure.

8.9 Availability in a MEMORY CHANNEL OpenVMS Cluster

Figure 8-6 shows a highly available MEMORY CHANNEL (MC) cluster configuration. Figure 8-6 is followed by an analysis of the configuration that includes:

Analysis of its components
Advantages and disadvantages
Key availability strategies implemented

Figure 8-6 MEMORY CHANNEL Cluster

8.9.1 Components

The MEMORY CHANNEL configuration shown in Figure 8-6 has the following components:

Part Description

1 Two MEMORY CHANNEL hubs.
Rationale: Having two hubs and multiple connections to the nodes prevents having a single point of failure.

2 Three to eight MEMORY CHANNEL nodes.
Rationale: Three nodes are recommended to maintain quorum. A MEMORY CHANNEL interconnect can support a maximum of eight OpenVMS Alpha nodes.
Alternative: Two-node configurations require a quorum disk to maintain quorum if a node fails.

3 Fast-wide differential (FWD) SCSI bus.
Rationale: Use a FWD SCSI bus to enhance data transfer rates (20 million transfers per second) and because it supports up to two HSZ controllers.

4 Two HSZ controllers.
Rationale: Two HSZ controllers ensure redundancy in case one of the controllers fails. With two controllers, you can connect two single-ended SCSI buses and more storage.

5 Essential system disks and data disks.
Rationale: Shadow essential disks and place shadow set members on different SCSI buses to eliminate a single point of failure.

Part	Description
1	Two MEMORY CHANNEL hubs. Rationale: Having two hubs and multiple connections to the nodes prevents having a single point of failure.
2	Three to eight MEMORY CHANNEL nodes. Rationale: Three nodes are recommended to maintain quorum. A MEMORY CHANNEL interconnect can support a maximum of eight OpenVMS Alpha nodes. Alternative: Two-node configurations require a quorum disk to maintain quorum if a node fails.
3	Fast-wide differential (FWD) SCSI bus. Rationale: Use a FWD SCSI bus to enhance data transfer rates (20 million transfers per second) and because it supports up to two HSZ controllers.
4	Two HSZ controllers. Rationale: Two HSZ controllers ensure redundancy in case one of the controllers fails. With two controllers, you can connect two single-ended SCSI buses and more storage.
5	Essential system disks and data disks. Rationale: Shadow essential disks and place shadow set members on different SCSI buses to eliminate a single point of failure.

8.9.2 Advantages

This configuration offers the following advantages:

All nodes have direct access to all storage.
SCSI storage provides low-cost, commodity hardware with good performance.
The MEMORY CHANNEL interconnect provides high-performance, node-to-node communication at a low price. The SCSI interconnect complements MEMORY CHANNEL by providing low-cost, commodity storage communication.

8.9.3 Disadvantages

This configuration has the following disadvantage:

The fast-wide differential SCSI bus is a single point of failure. One solution is to add a second, fast-wide differential SCSI bus so that if one fails, the nodes can fail over to the other. To use this functionality, the systems must be running OpenVMS Version 7.2 or higher and have multipath support enabled.

8.9.4 Key Availability Strategies

The configuration in Figure 8-6 incorporates the following strategies, which are critical to its success:

Redundant MEMORY CHANNEL hubs and HSZ controllers prevent a single point of hub or controller failure.
Volume shadowing provides multiple copies of essential disks across separate HSZ controllers.
All nodes have shared, direct access to all storage.
At least three nodes are used for quorum, so the OpenVMS Cluster continues if any one node fails.

8.10 Availability in an OpenVMS Cluster with Satellites

Satellites are systems that do not have direct access to a system disk and other OpenVMS Cluster storage. Satellites are usually workstations, but they can be any OpenVMS Cluster node that is served storage by other nodes in the cluster.

Because satellite nodes are highly dependent on server nodes for availability, the sample configurations presented earlier in this chapter do not include satellite nodes. However, because satellite/server configurations provide important advantages, you may decide to trade off some availability to include satellite nodes in your configuration.

Figure 8-7 shows an optimal configuration for a OpenVMS Cluster system with satellites. Figure 8-7 is followed by an analysis of the configuration that includes:

Analysis of its components
Advantages and disadvantages
Key availability strategies implemented

The base configurations in Figure 8-4 and Figure 8-5 could replace the base configuration shown in Figure 8-7. In other words, the FDDI and satallite segments shown in Figure 8-7 could just as easily be attached to the configurations shown in Figure 8-4 and Figure 8-5.

Figure 8-7 OpenVMS Cluster with Satellites

8.10.1 Components

This satellite/server configuration in Figure 8-7 has the following components:

Part Description

1 Base configuration.
The base configuration performs server functions for satellites.

2 Three to 16 OpenVMS server nodes.
Rationale: At least three nodes are recommended to maintain quorum. More than 16 nodes introduces excessive complexity.

3 FDDI ring between base server nodes and satellites.
Rationale: The FDDI ring has increased network capacity over Ethernet, which is slower.
Alternative: Use two Ethernet segments instead of the FDDI ring.

4 Two Ethernet segments from the FDDI ring to attach each critical satellite with two Ethernet adapters. Each of these critical satellites has its own system disk.
Rationale: Having their own boot disks increases the availability of the critical satellites.

5 For noncritical satellites, place a boot server on the Ethernet segment.
Rationale: Noncritical satellites do not need their own boot disks.

6 Limit the satellites to 15 per segment.
Rationale: More than 15 satellites on a segment may cause I/O congestion.

Part	Description
1	Base configuration. The base configuration performs server functions for satellites.
2	Three to 16 OpenVMS server nodes. Rationale: At least three nodes are recommended to maintain quorum. More than 16 nodes introduces excessive complexity.
3	FDDI ring between base server nodes and satellites. Rationale: The FDDI ring has increased network capacity over Ethernet, which is slower. Alternative: Use two Ethernet segments instead of the FDDI ring.
4	Two Ethernet segments from the FDDI ring to attach each critical satellite with two Ethernet adapters. Each of these critical satellites has its own system disk. Rationale: Having their own boot disks increases the availability of the critical satellites.
5	For noncritical satellites, place a boot server on the Ethernet segment. Rationale: Noncritical satellites do not need their own boot disks.
6	Limit the satellites to 15 per segment. Rationale: More than 15 satellites on a segment may cause I/O congestion.

8.10.2 Advantages

This configuration provides the following advantages:

A large number of nodes can be served in one OpenVMS Cluster.
You can spread a large number of nodes over a greater distance.

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