HP OpenVMS Systems Documentation

Guidelines for OpenVMS Cluster Configurations

When a Fibre Channel host bus adapter is connected (through a Fibre Channel switch) to an HSG controller, the HSG controller creates an entry in the HSG connection table. There is a separate connection for each host bus adapter, and for each HSG port to which the adapter is connected. (Refer to the HSG CLI command SHOW CONNECTIONS for more information.)

Once an HSG connection exists, you can modify its parameters by using commands that are described in the HSG Array Controller ACS Configuration and CLI Reference Guide. Since a connection can be modified, the HSG does not delete connection information from the table when a host bus adapter is disconnected. Instead, when the user is done with a connection, the user must explicitly delete the connection using a CLI command.

The HSG controller supports a limited number of connections: ACS V8.5 allows a maximum of 64 connections and ACS V8.4 allows a maximum of 32 connections. The connection limit is the same for both single- and dual-redundant controllers. Once the maximum number of connections is reached, then new connections will not be made. When this happens, OpenVMS will not configure disk devices, or certain paths to disk devices, on the HSG.

The solution to this problem is to delete old connections that are no longer needed. However, if your Fibre Channel fabric is large and the number of active connections exceeds the HSG limit, then you must reconfigure the fabric or use FC switch zoning to "hide" some adapters from some HSG ports to reduce the number of connections.

7.10 Using Interrupt Coalescing for I/O Performance Gains (Alpha Only)

Starting with OpenVMS Alpha Version 7.3-1, interrupt coalescing is supported for the KGPSA host adapters and is off by default. Interrupt coalescing can improve performance in environments with high I/O work loads by enabling the adapter to reduce the number of interrupts seen by a host. This feature is implemented in the KGPSA firmware.

You can read and modify the current settings for interrupt coalescing by means of the Fibre Channel Control Program (FC$CP). You must have the CMKRNL privilege to use FC$CP.

If you specify a response count and a delay time (in milliseconds) with FC$CP, the adapter defers interrupting the host until that number of responses is available or until that amount of time has passed, whichever occurs first.

Interrupt coalescing may cause a performance degradation to an application that does synchronous I/O. If no other I/O is going through a given KGPSA, the latency for single writes is an average of 900 microseconds longer with interrupt coalescing enabled (or higher depending on the selected response interval).

Interrupt coalescing is set on a per KGPSA basis. You should have an average of at least 2000 I/Os per second through a given KGPSA before enabling interrupt coalescing.

The format of the command is:

In this format:

• For FGx, the valid range of x is A to Z.
• enable-value is a bit mask, with bit 1 controlling response coalescing and bit 0 controlling interrupt coalescing. The possible decimal values are:

  1=interrupt coalescing
  2=response coalescing
  3=interrupt coalescing and response coalescing

• delay (in milliseconds) can range from 0 to 255 decimal.
• response-count can range from 0 to 63 decimal.
• Any negative value leaves a parameter unchanged.
• Values returned are those that are current after any changes.

OpenVMS recommends the following settings for the FC$CP command:

$  RUN SYS$ETC:FC$CP FGx 2 1 8

Fast Path support was introduced for Fibre Channel in OpenVMS Alpha Version 7.3 and is enabled by default. It is designed for use in a symmetric multiprocessor system (SMP). When Fast Path is enabled, the I/O completion processing can occur on all the processors in the SMP system instead of only on the primary CPU. Fast Path substantially increases the potential I/O throughput on an SMP system, and helps to prevent the primary CPU from becoming saturated.

You can manage Fast Path programmatically using Fast Path system services. You can also manage Fast Path with DCL commands and by using the system parameters FAST_PATH and FAST_PATH_PORTS. For more information about using Fast Path, refer to the HP OpenVMS I/O User's Reference Manual.

7.12 FIBRE_SCAN Utility for Displaying Device Information

FIBRE_SCAN.EXE displays information about all storage devices attached to Fibre Channel on the system; both configured and nonconfigured devices are included. The displayed information includes such data as the Fibre Channel target and LUN values, the vendor and product ID, device type, port and device worldwide identifiers (WWIDs), serial number, firmware revision level, and port login state. While the program primarily describes disk and tape devices, some limited information is also displayed for controller and other generic ($n$GGAn) devices.

FIBRE_SCAN can be invoked in two modes:

$ MCR SYS$ETC:FIBRE_SCAN        ! Scans all ports on the Fibre Channel.
$ MCR SYS$ETC:FIBRE_SCAN  PGx ! Scans only port x on the Fibre Channel.

FIBRE_SCAN requires CMKRNL and LOG_IO privilege.

To capture the FIBRE_SCAN output in a file, use a command such as the following before invoking FIBRE_SCAN:

$ DEFINE/USER SYS$OUTPUT xxx.log

FIBRE_SCAN is a display-only utility and is not capable of loading device drivers nor otherwise configuring devices on the Fibre Channel. To configure devices, use the SYSMAN IO AUTOCONFIGURE command.

Availability is the percentage of time that a computing system provides application service. By taking advantage of OpenVMS Cluster features, you can configure your OpenVMS Cluster system for various levels of availability, including disaster tolerance.

This chapter provides strategies and sample optimal configurations for building a highly available OpenVMS Cluster system. You can use these strategies and examples to help you make choices and tradeoffs that enable you to meet your availability requirements.

8.1 Availability Requirements

You can configure OpenVMS Cluster systems for different levels of availability, depending on your requirements. Most organizations fall into one of the broad (and sometimes overlapping) categories shown in Table 8-1.

OpenVMS Cluster systems offer the following features that provide increased availability:

• A highly integrated environment that allows multiple systems to share access to resources
• Redundancy of major hardware components
• Software support for failover between hardware components
• Software products to support high availability

In an OpenVMS Cluster environment, users and applications on multiple systems can transparently share storage devices and files. When you shut down one system, users can continue to access shared files and devices. You can share storage devices in two ways:

• Direct access
  Connect disk and tape storage subsystems to CI and DSSI interconnects rather than to a node. This gives all nodes attached to the interconnect shared access to the storage system. The shutdown or failure of a system has no effect on the ability of other systems to access storage.
• Served access
  Storage devices attached to a node can be served to other nodes in the OpenVMS Cluster. MSCP and TMSCP server software enable you to make local devices available to all OpenVMS Cluster members. However, the shutdown or failure of the serving node affects the ability of other nodes to access storage.

OpenVMS Cluster systems allow for redundancy of many components, including:

• Systems
• Interconnects
• Adapters
• Storage devices and data

With redundant components, if one component fails, another is available to users and applications.

8.2.3 Failover Mechanisms

OpenVMS Cluster systems provide failover mechanisms that enable recovery from a failure in part of the OpenVMS Cluster. Table 8-2 lists these mechanisms and the levels of recovery that they provide.

Reference: For more information about cluster aliases, generic queues, and autostart queues, refer to the HP OpenVMS Cluster Systems manual.

8.2.4 Related Software Products

Table 8-3 shows a variety of related OpenVMS Cluster software products that HP offers to increase availability.

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

8.3.1 Availability Strategies

Table 8-4 lists strategies for configuring a highly available OpenVMS Cluster. These strategies are listed in order of importance, and many of them are illustrated in the sample optimal configurations shown in this chapter.

Achieving high availability is an ongoing process. How you manage your OpenVMS Cluster system is just as important as how you configure it. This section presents strategies for maintaining availability in your OpenVMS Cluster configuration.

8.4.1 Strategies for Maintaining Availability

After you have set up your initial configuration, follow the strategies listed in Table 8-5 to maintain availability in OpenVMS Cluster system.