Document revision date: 30 March 2001

## Guidelines for OpenVMS Cluster Configurations

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### 9.6.1 Components

The CI OpenVMS Cluster configuration shown in Figure 9-4 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 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 are 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 hub sections. Star Coupler A's path hubs are connected to the path A cables for both CIs, and Star Coupler B's path hubs are connected to the 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 the bandwidth available to the storage controllers and host adapters that are 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, each in a separate StorageWorks cabinet. Data is replicated across StorageWorks cabinets using Volume Shadowing for DIGITAL OpenVMS. Rationale: A StorageWorks cabinet can be destroyed, or one storage controller can fail, and the remaining controller located in the other StorageWorks cabinet can control shadow copies of all disks. When both controllers are available, each can be assigned to serve a subset of the disks. Volume shadowing will distribute READ I/Os across the HSJs. Thus, both controllers can contribute their I/O-per-second and bandwidth capacity to the cluster.
SCSI 1, SCSI 2 | Private SCSI buses connected to an HSJ. Rationale: Provide host access to each shadow set member.
Shadow Set | Essential disks are shadowed between HSJ pairs using volume shadowing. Each HSJ and its disks are in a StorageWorks cabinet that is physically separated from the other StorageWorks cabinet. Rationale: An entire StorageWorks cabinet can be destroyed, or a disk, the SCSI bus, or the HSJ to which it is connected can fail, and the other shadow set member will still be available. When both disks are available, they can each provide their READ I/O per second capacity and READ data bandwidth capacity to the cluster.

### 9.6.2 Advantages

Configuration 4 offers most of the individual component advantages of configuration 3, plus:

- Physical damage to one StorageWorks cabinet is not likely to disable the cluster.
- Disk failover to another HSJ is not a problem.

### 9.6.3 Disadvantages

Configuration 4 has the following disadvantages:

- Does not have redundant paths to each disk.
- Write I/Os to shadow sets use twice the CI bandwidth of other alternatives.
- Higher cost than configuration 2 and configuration 3.

### 9.6.4 Key Availability and Performance Strategies

Configuration 4 (Figure 9-4) provides all of the strategies of configuration 3. It also provides shadow set members that are in physically separate StorageWorks cabinets.

## 9.7 Summary

All four configurations illustrate how to obtain both availability and performance by:

- Adding redundant components
- Dual-porting critical disks
- Shadowing essential disks

An advanced technique, separating the CI path A and path B cables and associated hubs, is used in configuration 3 and configuration 4. This technique increases availability and maintains performance with no additional hardware. Configuration 4 provides even greater availability without compromising performance by physically separating shadow set members and their HSJ controllers.

Using these configurations as a guide, you can select the techniques that are appropriate for your computing needs and adapt your environment as conditions change. The techniques illustrated in these configurations can be scaled for larger CI configurations.

## Chapter 10
Configuring OpenVMS Clusters for Scalability

This chapter explains how to maximize scalability in many different kinds of OpenVMS Clusters.

## 10.1 What Is Scalability?

Scalability is the ability to expand an OpenVMS Cluster system in any system, storage, and interconnect dimension and at the same time fully use the initial configuration equipment. Your OpenVMS Cluster system can grow in many dimensions, as shown in Figure 10-1. Each dimension also enables your applications to expand.

Figure 10-1 OpenVMS Cluster Growth Dimensions

### 10.1.1 Scalable Dimensions

Table 10-1 describes the growth dimensions for systems, storage, and interconnects in OpenVMS Clusters.

Table 10-1 Scalable Dimensions in OpenVMS Clusters
This Dimension | Grows by...
--- | ---
Systems
CPU | Implementing SMP within a system. Adding systems to a cluster. Accommodating various processor sizes in a cluster. Adding a bigger system to a cluster. Migrating from VAX to Alpha systems.
Memory | Adding memory to a system.
I/O | Adding interconnects and adapters to a system. Adding MEMORY CHANNEL to a cluster to offload the I/O interconnect.
OpenVMS | Tuning system parameters. Moving to OpenVMS Alpha.
Adapter | Adding storage adapters to a system. Adding CI and DSSI adapters to a system. Adding LAN adapters to a system.
Storage
Media | Adding disks to a cluster. Adding tapes and CD-ROMs to a cluster.
Volume shadowing | Increasing availability by shadowing disks. Shadowing disks across controllers. Shadowing disks across systems.
I/O | Adding solid-state or DECram disks to a cluster. Adding disks and controllers with caches to a cluster. Adding RAID disks to a cluster.
Controller and array | Moving disks and tapes from systems to controllers. Combining disks and tapes in arrays. Adding more controllers and arrays to a cluster.
Interconnect
LAN | Adding Ethernet and FDDI segments. Upgrading from Ethernet to FDDI. Adding redundant segments and bridging segments.
CI, DSSI, Fibre Channel, SCSI, and MEMORY CHANNEL | Adding CI, DSSI, Fibre Channel, SCSI, and MEMORY CHANNEL interconnects to a cluster or adding redundant interconnects to a cluster.
I/O | Adding faster interconnects for capacity. Adding redundant interconnects for capacity and availability.
Distance | Expanding a cluster inside a room or a building. Expanding a cluster across a town or several buildings. Expanding a cluster between two sites (spanning 40 km).

The ability to add to the components listed in Table 10-1 in any way that you choose is an important feature that OpenVMS Clusters provide. You can add hardware and software in a wide variety of combinations by carefully following the suggestions and guidelines offered in this chapter and in the products' documentation and DIGITAL Systems and Options Catalog. When you choose to expand your OpenVMS Cluster in a specific dimension, be aware of the advantages and tradeoffs with regard to the other dimensions. Table 10-2 describes strategies that promote OpenVMS Cluster scalability. Understanding these scalability strategies can help you maintain a higher level of performance and availability as your OpenVMS Cluster grows.

## 10.2 Strategies for Configuring a Highly Scalable OpenVMS Cluster

The hardware that you choose and the way that you configure it has a significant impact on the scalability of your OpenVMS Cluster. This section presents strategies for designing an OpenVMS Cluster configuration that promotes scalability.

### 10.2.1 Scalability Strategies

Table 10-2 lists strategies in order of importance that ensure scalability. This chapter contains many figures that show how these strategies are implemented.

Table 10-2 Scalability Strategies
Strategy | Description
--- | ---
Capacity planning | Running a system above 80% capacity (near performance saturation) limits the amount of future growth possible. Understand whether your business and applications will grow. Try to anticipate future requirements for processor, memory, and I/O.
Shared, direct access to all storage | The ability to scale compute and I/O performance is heavily dependent on whether all of the systems have shared, direct access to all storage. The CI and DSSI OpenVMS Cluster illustrations that follow show many examples of shared, direct access to storage, with no MSCP overhead. Reference: For more information about MSCP overhead, see Section 10.8.1.
Limit node count to between 3 and 16 | Smaller OpenVMS Clusters are simpler to manage and tune for performance and require less OpenVMS Cluster communication overhead than do large OpenVMS Clusters. You can limit node count by upgrading to a more powerful processor and by taking advantage of OpenVMS SMP capability. If your server is becoming a compute bottleneck because it is overloaded, consider whether your application can be split across nodes. If so, add a node; if not, add a processor (SMP).
Remove system bottlenecks | To maximize the capacity of any OpenVMS Cluster function, consider the hardware and software components required to complete the function. Any component that is a bottleneck may prevent other components from achieving their full potential. Identifying bottlenecks and reducing their effects increases the capacity of an OpenVMS Cluster.
Enable the MSCP server | The MSCP server enables you to add satellites to your OpenVMS Cluster so that all nodes can share access to all storage. In addition, the MSCP server provides failover for access to shared storage when an interconnect fails.
Reduce interdependencies and simplify configurations | An OpenVMS Cluster system with one system disk is completely dependent on that disk for the OpenVMS Cluster to continue. If the disk, the node serving the disk, or the interconnects between nodes fail, the entire OpenVMS Cluster system may fail.
Ensure sufficient serving resources | If a small disk server has to serve a large number disks to many satellites, the capacity of the entire OpenVMS Cluster is limited. Do not overload a server because it will become a bottleneck and will be unable to handle failover recovery effectively.
Configure resources and consumers close to each other | Place servers (resources) and satellites (consumers) close to each other. If you need to increase the number of nodes in your OpenVMS Cluster, consider dividing it. See Section 11.2.4 for more information.
Set adequate system parameters | If your OpenVMS Cluster is growing rapidly, important system parameters may be out of date. Run AUTOGEN, which automatically calculates significant system parameters and resizes page, swap, and dump files.

## 10.3 Scalability in CI OpenVMS Clusters

Each CI star coupler can have up to 32 nodes attached; 16 can be systems and the rest can be storage controllers and storage. Figure 10_2 , Figure 10_3 , and Figure 10-4 show a progression from a two-node CI OpenVMS Cluster to a seven-node CI OpenVMS Cluster.

### 10.3.1 Two-Node CI OpenVMS Cluster

In Figure 10-2, two nodes have shared, direct access to storage that includes a quorum disk. The VAX and Alpha systems each have their own system disks.

Figure 10-2 Two-Node CI OpenVMS Cluster

The advantages and disadvantages of the configuration shown in Figure 10-2 include:

Advantages

- All nodes have shared, direct access to all storage.
- As the nodes and storage in this configuration grow, all nodes can still have shared, direct access to storage.
- The MSCP server is enabled for failover to the LAN interconnect in case the CI fails. Enabling the MSCP server also allows you to add satellites.
- This configuration has the lowest cost of the CI configurations shown in this section.

Disadvantage

- The single HSJ/HSC is a potential bottleneck and single point of failure.

An increased need for more storage or processing resources could lead to an OpenVMS Cluster configuration like the one shown in Figure 10-3.

### 10.3.2 Three-Node CI OpenVMS Cluster

In Figure 10-3, three nodes are connected to two HSC controllers by the CI interconnects. The critical system disk is dual ported and shadowed.

Figure 10-3 Three-Node CI OpenVMS Cluster

The advantages and disadvantages of the configuration shown in Figure 10-3 include:

Advantages

- All nodes have shared, direct access to all storage.
- As the nodes and storage in this configuration grow, all nodes can still have shared, direct access to storage.
- The MSCP server is enabled for failover to the LAN interconnect, in case the CI fails. Enabling the MSCP server also allows you to add satellites.
- Volume shadowed, dual-ported disks increase data availability.

Disadvantage

- The HSJs/HSCs are potential bottlenecks.

If the I/O activity exceeds the capacity of the CI interconnect, this could lead to an OpenVMS Cluster configuration like the one shown in Figure 10-4.

### 10.3.3 Seven-Node CI OpenVMS Cluster

In Figure 10-4, seven nodes each have a direct connection to two star couplers and to all storage.

Figure 10-4 Seven-Node CI OpenVMS Cluster

The advantages and disadvantages of the configuration shown in Figure 10-4 include:

Advantages

- All nodes have shared, direct access to all storage.
- This configuration has more than double the storage, processing, and CI interconnect capacity than a configuration like the one shown in Figure 10-3.
- Two CI interconnects between processors and storage provide twice the communication performance of one path.
- Volume shadowed, dual-ported disks increase data availability.

Disadvantage

- This configuration is complex and requires experienced personnel to configure, tune, and manage it properly.

### 10.3.4 Guidelines for CI OpenVMS Clusters

The following guidelines can help you configure your CI OpenVMS Cluster:

- Every system should have shared, direct access to all storage.
- In a CI OpenVMS Cluster larger than four nodes, use a second system disk for increased system performance.
  Reference: For more information on system disks, see Section 11.2.
- Enable your systems, interconnects, and storage to work to their full capacity by eliminating bottlenecks. If any of these components is not able to handle the I/O capacity, none of the other components will work at its best. Ensure that the sum of all the I/O on your nodes is less than or equal to your CI capacity and to your storage capacity. In calculating the sum of the I/O on your nodes, factor in 5--10% extra for lock manager internode communications.
  In general, use the following rules of thumb:
  - The sum of all the I/Os on your nodes plus internode communications should be less than or equal to the sum of your CI capacity.
  - Your CI capacity should be less than or equal to the sum of your storage capacity.

### 10.3.5 Guidelines for Volume Shadowing in CI OpenVMS Clusters

Volume shadowing is intended to enhance availability, not performance. However, the following volume shadowing strategies enable you to utilize availability features while also maximizing I/O capacity. These examples show CI configurations, but they apply to DSSI and SCSI configurations, as well.

Figure 10-5 Volume Shadowing on a Single Controller

Figure 10-5 shows two nodes connected to an HSJ, with a two-member shadow set.

The disadvantage of this strategy is that the controller is a single point of failure. The configuration in Figure 10-6 shows examples of shadowing across controllers, which prevents one controller from being a single point of failure. Shadowing across HSJ and HSC controllers provides optimal scalability and availability within an OpenVMS Cluster system.

Figure 10-6 Volume Shadowing Across Controllers

As Figure 10-6 shows, shadowing across controllers has three variations:

- Strategy A shows each volume in the shadow set attached to separate controllers. This configuration is not optimal because each volume is not attached to each controller.
- Strategy B shows dual-ported devices that have two paths to the volumes through separate controllers. This strategy is an optimal variation because two HSC controllers have direct access to a single storage device.
- Strategy C shows HSJ controllers shadowed across SCSI buses. It also is an optimal variation because two HSJ controllers have direct access to a single storage device.

Figure 10-7 shows an example of shadowing across nodes.

Figure 10-7 Volume Shadowing Across Nodes

As Figure 10-7 shows, shadowing across nodes provides the advantage of flexibility in distance. However, it requires MSCP server overhead for write I/Os. In addition, the failure of one of the nodes and its subsequent return to the OpenVMS Cluster will cause a copy operation.

If you have multiple volumes, shadowing inside a controller and shadowing across controllers are more effective than shadowing across nodes.

Reference: See Volume Shadowing for OpenVMS for more information.

## 10.4 Scalability in DSSI OpenVMS Clusters

Each DSSI interconnect can have up to eight nodes attached; four can be systems and the rest can be storage devices. Figure 10-8, Figure 10_9, and Figure 10-10 show a progression from a two-node DSSI OpenVMS Cluster to a four-node DSSI OpenVMS Cluster.

### 10.4.1 Two-Node DSSI OpenVMS Cluster

In Figure 10-8, two nodes are connected to four disks by a common DSSI interconnect.

Figure 10-8 Two-Node DSSI OpenVMS Cluster

The advantages and disadvantages of the configuration shown in Figure 10-8 include:

Advantages

- Both nodes have shared, direct access to all storage.
- The Ethernet LAN ensures failover capability if the DSSI interconnect fails.

Disadvantages

- The amount of storage that is directly accessible to all nodes is limited.
- A single DSSI interconnect can become a single point of failure.

If the OpenVMS Cluster in Figure 10-8 required more processing power, more storage, and better redundancy, this could lead to a configuration like the one shown in Figure 10-9.

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