HP OpenVMS Systems Documentation

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Guidelines for OpenVMS Cluster Configurations

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10.7.2 Six-Satellite OpenVMS Cluster with Two Boot Nodes

Figure 10-19 shows six satellites and two boot servers connected by Ethernet. Boot server 1 and boot server 2 perform MSCP server dynamic load balancing: they arbitrate and share the work load between them and if one node stops functioning, the other takes over. MSCP dynamic load balancing requires shared access to storage.

Figure 10-19 Six-Satellite LAN OpenVMS Cluster with Two Boot Nodes

The advantages and disadvantages of the configuration shown in Figure 10-19 include:


  • The MSCP server is enabled for adding satellites and allows access to more storage.
  • Two boot servers perform MSCP dynamic load balancing.


  • The Ethernet is a potential bottleneck and a single point of failure.

If the LAN in Figure 10-19 became an OpenVMS Cluster bottleneck, this could lead to a configuration like the one shown in Figure 10-20.

10.7.3 Twelve-Satellite LAN OpenVMS Cluster with Two LAN Segments

Figure 10-20 shows 12 satellites and 2 boot servers connected by two Ethernet segments. These two Ethernet segments are also joined by a LAN bridge. Because each satellite has dual paths to storage, this configuration also features MSCP dynamic load balancing.

Figure 10-20 Twelve-Satellite OpenVMS Cluster with Two LAN Segments

The advantages and disadvantages of the configuration shown in Figure 10-20 include:


  • The MSCP server is enabled for adding satellites and allows access to more storage.
  • Two boot servers perform MSCP dynamic load balancing.
    From the perspective of a satellite on the Ethernet LAN, the dual paths to the Alpha and VAX nodes create the advantage of MSCP load balancing.
  • Two LAN segments provide twice the amount of LAN capacity.


  • This OpenVMS Cluster configuration is limited by the number of satellites that it can support.
  • The single HSJ controller is a potential bottleneck and a single point of failure.

If the OpenVMS Cluster in Figure 10-20 needed to grow beyond its current limits, this could lead to a configuration like the one shown in Figure 10-21.

10.7.4 Forty-Five Satellite OpenVMS Cluster with FDDI Ring

Figure 10-21 shows a large, 51-node OpenVMS Cluster that includes 45 satellite nodes. The three boot servers, Alpha 1, Alpha 2, and Alpha 3, share three disks: a common disk, a page and swap disk, and a system disk. The FDDI ring has three LAN segments attached. Each segment has 15 workstation satellites as well as its own boot node.

Figure 10-21 Forty-Five Satellite OpenVMS Cluster with FDDI Ring

The advantages and disadvantages of the configuration shown in Figure 10-21 include:


  • Decreased boot time, especially for an OpenVMS Cluster with such a high node count.
    Reference: For information about booting an OpenVMS Cluster like the one in Figure 10-21 see Section 11.2.4.
  • The MSCP server is enabled for satellites to access more storage.
  • Each boot server has its own page and swap disk, which reduces I/O activity on the system disks.
  • All of the environment files for the entire OpenVMS Cluster are on the common disk. This frees the satellite boot servers to serve only root information to the satellites.
    Reference: For more information about common disks and page and swap disks, see Section 11.2.
  • The FDDI ring provides 10 times the capacity of one Ethernet interconnect.


  • The satellite boot servers on the Ethernet LAN segments can boot satellites only on their own segments.

10.7.5 High-Powered Workstation OpenVMS Cluster

Figure 10-22 shows an OpenVMS Cluster configuration that provides high performance and high availability on the FDDI ring.

Figure 10-22 High-Powered Workstation Server Configuration

In Figure 10-22, several Alpha workstations, each with its own system disk, are connected to the FDDI ring. Putting Alpha workstations on the FDDI provides high performance because each workstation has direct access to its system disk. In addition, the FDDI bandwidth is higher than that of the Ethernet. Because Alpha workstations have FDDI adapters, putting these workstations on an FDDI is a useful alternative for critical workstation requirements. FDDI is 10 times faster than Ethernet, and Alpha workstations have processing capacity that can take advantage of FDDI's speed.

10.7.6 Guidelines for OpenVMS Clusters with Satellites

The following are guidelines for setting up an OpenVMS Cluster with satellites:

  • Extra memory is required for satellites of large LAN configurations because each node must maintain a connection to every other node.
  • Place only 10 to 20 satellites on each LAN segment.
  • Maximize resources with MSCP dynamic load balancing, as shown in Figure 10-19 and Figure 10-20.
  • Keep the number of nodes that require MSCP serving minimal for good performance.
    Reference: See Section 10.8.1 for more information about MSCP overhead.
  • To save time, ensure that the booting sequence is efficient, particularly when the OpenVMS Cluster is large or has multiple segments. See Section 11.2.4 for more information about how to reduce LAN and system disk activity and how to boot separate groups of nodes in sequence.
  • Use two or more LAN adapters per host (up to four adapters are supported for OpenVMS Cluster communications), and connect to independent LAN paths. This enables simultaneous two-way communication between nodes and allows traffic to multiple nodes to be spread over the available LANs.

10.7.7 Extended LAN Configuration Guidelines

You can use bridges between LAN segments to form an extended LAN (ELAN). This can increase availability, distance, and aggregate bandwidth as compared with a single LAN. However, an ELAN can increase delay and can reduce bandwidth on some paths. Factors such as packet loss, queuing delays, and packet size can also affect ELAN performance. Table 10-3 provides guidelines for ensuring adequate LAN performance when dealing with such factors.

Table 10-3 ELAN Configuration Guidelines
Factor Guidelines
Propagation delay The amount of time it takes a packet to traverse the ELAN depends on the distance it travels and the number of times it is relayed from one link to another by a bridge or a station on the FDDI ring. If responsiveness is critical, then you must control these factors.

When an FDDI is used for OpenVMS Cluster communications, the ring latency when the FDDI ring is idle should not exceed 400 ms. FDDI packets travel at 5.085 microseconds/km and each station causes an approximate 1-ms delay between receiving and transmitting. You can calculate FDDI latency by using the following algorithm:

Latency = (distance in km) * (5.085 ms/km) + (number of stations) * (1 ms/station)

For high-performance applications, limit the number of bridges between nodes to two. For situations in which high performance is not required, you can use up to seven bridges between nodes.

Queuing delay Queuing occurs when the instantaneous arrival rate at bridges and host adapters exceeds the service rate. You can control queuing by:
  • Reducing the number of bridges between nodes that communicate frequently.
  • Using only high-performance bridges and adapters.
  • Reducing traffic bursts in the LAN. In some cases, for example, you can tune applications by combining small I/Os so that a single packet is produced rather than a burst of small ones.
  • Reducing LAN segment and host processor utilization levels by using faster processors and faster LANs, and by using bridges for traffic isolation.
Packet loss Packets that are not delivered by the ELAN require retransmission, which wastes network resources, increases delay, and reduces bandwidth. Bridges and adapters discard packets when they become congested. You can reduce packet loss by controlling queuing, as previously described.

Packets are also discarded when they become damaged in transit. You can control this problem by observing LAN hardware configuration rules, removing sources of electrical interference, and ensuring that all hardware is operating correctly.

Packet loss can also be reduced by using VMS Version 5.5--2 or later, which has PEDRIVER congestion control.

The retransmission timeout rate, which is a symptom of packet loss, must be less than 1 timeout in 1000 transmissions for OpenVMS Cluster traffic from one node to another. ELAN paths that are used for high-performance applications should have a significantly lower rate. Monitor the occurrence of retransmission timeouts in the OpenVMS Cluster.

Reference: For information about monitoring the occurrence of retransmission timeouts, see OpenVMS Cluster Systems.

Bridge recovery delay Choose bridges with fast self-test time and adjust bridges for fast automatic reconfiguration.

Reference: Refer to OpenVMS Cluster Systems for more information about LAN bridge failover.

Bandwidth All LAN paths used for OpenVMS Cluster communication must operate with a nominal bandwidth of at least 10 Mb/s. The average LAN segment utilization should not exceed 60% for any 10-second interval.

Use FDDI exclusively on the communication paths that have the highest performance requirements. Do not put an Ethernet LAN segment between two FDDI segments. FDDI bandwidth is significantly greater, and the Ethernet LAN will become a bottleneck. This strategy is especially ineffective if a server on one FDDI must serve clients on another FDDI with an Ethernet LAN between them. A more appropriate strategy is to put a server on an FDDI and put clients on an Ethernet LAN, as Figure 10-21 shows.

Traffic isolation Use bridges to isolate and localize the traffic between nodes that communicate with each other frequently. For example, use bridges to separate the OpenVMS Cluster from the rest of the ELAN and to separate nodes within an OpenVMS Cluster that communicate frequently from the rest of the OpenVMS Cluster.

Provide independent paths through the ELAN between critical systems that have multiple adapters.

Packet size You can adjust the NISCS_MAX_PKTSZ system parameter to use the full FDDI packet size. Ensure that the ELAN path supports a data field of at least 4474 bytes end to end.

Some failures cause traffic to switch from an ELAN path that supports 4474-byte packets to a path that supports only smaller packets. It is possible to implement automatic detection and recovery from these kinds of failures. This capability requires that the ELAN set the value of the priority field in the FDDI frame-control byte to zero when the packet is delivered on the destination FDDI link. Ethernet-to-FDDI bridges that conform to the IEEE 802.1 bridge specification provide this capability.

10.7.8 System Parameters for OpenVMS Clusters

In an OpenVMS Cluster with satellites and servers, specific system parameters can help you manage your OpenVMS Cluster more efficiently. Table 10-4 gives suggested values for these system parameters.

Table 10-4 OpenVMS Cluster System Parameters
System Parameter Value for
Value for
MSCP_LOAD 0 1 or 2
NPAGEDYN Higher than for standalone node Higher than for satellite node
PAGEDYN Higher than for standalone node Higher than for satellite node
EXPECTED_VOTES Sum of OpenVMS Cluster votes Sum of OpenVMS Cluster votes
RECNXINTERVL 1 Equal on all nodes Equal on all nodes

1Correlate with bridge timers and LAN utilization.

Reference: For a more in-depth description of these parameters, see OpenVMS Cluster Systems.

10.8 Scaling for I/Os

The ability to scale I/Os is an important factor in the growth of your OpenVMS Cluster. Adding more components to your OpenVMS Cluster requires high I/O throughput so that additional components do not create bottlenecks and decrease the performance of the entire OpenVMS Cluster. Many factors can affect I/O throughput:

  • Direct access or MSCP served access to storage
  • File system technologies, such as Files-11
  • Disk technologies, such as magnetic disks, solid-state disks, and DECram
  • Read/write ratio
  • I/O size
  • Caches and cache "hit" rate
  • "Hot file" management
  • RAID striping and host-based striping
  • Volume shadowing

These factors can affect I/O scalability either singly or in combination. The following sections explain these factors and suggest ways to maximize I/O throughput and scalability without having to change in your application.

Additional factors that affect I/O throughput are types of interconnects and types of storage subsystems.

Reference: See Chapter 4 for more information about interconnects and Chapter 5 for more information about types of storage subsystems.

10.8.1 MSCP Served Access to Storage

MSCP server capability provides a major benefit to OpenVMS Clusters: it enables communication between nodes and storage that are not directly connected to each other. However, MSCP served I/O does incur overhead. Figure 10-23 is a simplification of how packets require extra handling by the serving system.

Figure 10-23 Comparison of Direct and MSCP Served Access

In Figure 10-23, an MSCP served packet requires an extra "stop" at another system before reaching its destination. When the MSCP served packet reaches the system associated with the target storage, the packet is handled as if for direct access.

In an OpenVMS Cluster that requires a large amount of MSCP serving, I/O performance is not as efficient and scalability is decreased. The total I/O throughput is approximately 20% less when I/O is MSCP served than when it has direct access. Design your configuration so that a few large nodes are serving many satellites rather than satellites serving their local storage to the entire OpenVMS Cluster.

10.8.2 Disk Technologies

In recent years, the ability of CPUs to process information has far outstripped the ability of I/O subsystems to feed processors with data. The result is an increasing percentage of processor time spent waiting for I/O operations to complete.

Solid-state disks (SSDs), DECram, and RAID level 0 bridge this gap between processing speed and magnetic-disk access speed. Performance of magnetic disks is limited by seek and rotational latencies, while SSDs and DECram use memory, which provides nearly instant access.

RAID level 0 is the technique of spreading (or "striping") a single file across several disk volumes. The objective is to reduce or eliminate a bottleneck at a single disk by partitioning heavily accessed files into stripe sets and storing them on multiple devices. This technique increases parallelism across many disks for a single I/O.

Table 10-5 summarizes disk technologies and their features.

Table 10-5 Disk Technology Summary
Disk Technology Characteristics
Magnetic disk Slowest access time.
Available on multiple interconnects.
Solid-state disk Fastest access of any I/O subsystem device.
Highest throughput for write-intensive files.
Available on multiple interconnects.
DECram Highest throughput for small to medium I/O requests.
Volatile storage; appropriate for temporary read-only files.
Available on any Alpha or VAX system.
RAID level 0 Available on HSJ and HSD controllers.

Note: Shared, direct access to a solid-state disk or to DECram is the fastest alternative for scaling I/Os.

10.8.3 Read/Write Ratio

The read/write ratio of your applications is a key factor in scaling I/O to shadow sets. MSCP writes to a shadow set are duplicated on the interconnect.

Therefore, an application that has 100% (100/0) read activity may benefit from volume shadowing because shadowing causes multiple paths to be used for the I/O activity. An application with a 50/50 ratio will cause more interconnect utilization because write activity requires that an I/O be sent to each shadow member. Delays may be caused by the time required to complete the slowest I/O.

To determine I/O read/write ratios, use the DCL command MONITOR IO.

10.8.4 I/O Size

Each I/O packet incurs processor and memory overhead, so grouping I/Os together in one packet decreases overhead for all I/O activity. You can achieve higher throughput if your application is designed to use bigger packets. Smaller packets incur greater overhead.

10.8.5 Caches

Caching is the technique of storing recently or frequently used data in an area where it can be accessed more easily---in memory, in a controller, or in a disk. Caching complements solid-state disks, DECram, and RAID. Applications automatically benefit from the advantages of caching without any special coding. Caching reduces current and potential I/O bottlenecks within OpenVMS Cluster systems by reducing the number of I/Os between components.

Table 10-6 describes the three types of caching.

Table 10-6 Types of Caching
Caching Type Description
Host based Cache that is resident in the host system's memory and services I/Os from the host.
Controller based Cache that is resident in the storage controller and services data for all hosts.
Disk Cache that is resident in a disk.

Host-based disk caching provides different benefits from controller-based and disk-based caching. In host-based disk caching, the cache itself is not shareable among nodes. Controller-based and disk-based caching are shareable because they are located in the controller or disk, either of which is shareable.

10.8.6 Managing "Hot" Files

A "hot" file is a file in your system on which the most activity occurs. Hot files exist because, in many environments, approximately 80% of all I/O goes to 20% of data. This means that, of equal regions on a disk drive, 80% of the data being transferred goes to one place on a disk, as shown in Figure 10-24.

Figure 10-24 Hot-File Distribution

To increase the scalability of I/Os, focus on hot files, which can become a bottleneck if you do not manage them well. The activity in this area is expressed in I/Os, megabytes transferred, and queue depth.

RAID level 0 balances hot-file activity by spreading a single file over multiple disks. This reduces the performance impact of hot files.

Use the following DCL commands to analyze hot-file activity:

  • MONITOR IO command---Monitors hot disks.
  • MONITOR MSCP command---Monitors MSCP servers.

The MONITOR IO and the MONITOR MSCP commands enable you to find out which disk and which server are hot.

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