HP OpenVMS Systems Documentation

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.

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 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.

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.

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.

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

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

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.

This configuration has the following disadvantage:

• Higher cost than the other configurations.

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

• Analysis of its components
• Advantages and disadvantages
• Key availability strategies implemented

Figure 8-6 MEMORY CHANNEL Cluster

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

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.

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.

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.

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

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

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.

This configuration has the following disadvantages:

• Satellites with single LAN adapters have a single point of failure that causes cluster transitions if the adapter fails.
• High cost of LAN connectivity for highly available satellites.

The configuration in Figure 8-7 incorporates the following strategies, which are critical to its success:

• This configuration has no single point of failure.
• The FDDI interconnect has sufficient bandwidth to serve satellite nodes from the base server configuration.
• All shared storage is MSCP served from the base configuration, which is appropriately configured to serve a large number of nodes.

Multiple-site OpenVMS Cluster configurations contain nodes that are located at geographically separated sites. Depending on the technology used, the distances between sites can be as great as 150 miles. FDDI, asynchronous transfer mode (ATM), and DS3 are used to connect these separated sites to form one large cluster. Available from most common telephone service carriers, DS3 and ATM services provide long-distance, point-to-point communications for multiple-site clusters.

Figure 8-8 shows a typical configuration for a multiple-site OpenVMS Cluster system. Figure 8-8 is followed by an analysis of the configuration that includes:

• Analysis of components
• Advantages

Figure 8-8 Multiple-Site OpenVMS Cluster Configuration Connected by WAN Link

Although Figure 8-8 does not show all possible configuration combinations, a multiple-site OpenVMS Cluster can include:

• Two data centers with an intersite link (FDDI, ATM, or DS3) connected to a DECconcentrator or GIGAswitch crossbar switch.
• Intersite link performance that is compatible with the applications that are shared by the two sites.
• Up to 96 Alpha and VAX (combined total) nodes. In general, the rules that apply to OpenVMS LAN and extended LAN (ELAN) clusters also apply to multiple-site clusters.
  Reference: For LAN configuration guidelines, see Section 4.12.6. For ELAN configuration guidelines, see Section 10.7.7.

The benefits of a multiple-site OpenVMS Cluster system include the following:

• A few systems can be remotely located at a secondary site and can benefit from centralized system management and other resources at the primary site. For example, a main office data center could be linked to a warehouse or a small manufacturing site that could have a few local nodes with directly attached, site-specific devices. Alternatively, some engineering workstations could be installed in an office park across the city from the primary business site.
• Multiple sites can readily share devices such as high-capacity computers, tape libraries, disk archives, or phototypesetters.
• Backups can be made to archival media at any site in the cluster. A common example would be to use disk or tape at a single site to back up the data for all sites in the multiple-site OpenVMS Cluster. Backups of data from remote sites can be made transparently (that is, without any intervention required at the remote site).
• In general, a multiple-site OpenVMS Cluster provides all of the availability advantages of a LAN OpenVMS Cluster. Additionally, by connecting multiple, geographically separate sites, multiple-site OpenVMS Cluster configurations can increase the availability of a system or elements of a system in a variety of ways:
  • Logical volume/data availability---Volume shadowing or redundant arrays of independent disks (RAID) can be used to create logical volumes with members at both sites. If one of the sites becomes unavailable, data can remain available at the other site.
  • Site failover---By adjusting the VOTES system parameter, you can select a preferred site to continue automatically if the other site fails or if communications with the other site are lost.

Reference: For additional information about multiple-site clusters, see OpenVMS Cluster Systems.

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 HP Services representative.