HP OpenVMS systems documentation

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:

RUN SYS$ETC:FC$CP FGx enable-value [delay][response-count]

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

7.11 Using Fast Path in Your Configuration

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.

Table 8-1 Availability Requirements

Availability Requirements	Description
Conventional	For business functions that can wait with little or no effect while a system or application is unavailable.
24 x 365	For business functions that require uninterrupted computing services, either during essential time periods or during most hours of the day throughout the year. Minimal down time is acceptable.
Disaster tolerant	For business functions with stringent availability requirements. These businesses need to be immune to disasters like earthquakes, floods, and power failures.

8.2 How OpenVMS Clusters Provide Availability

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.

Table 8-2 Failover Mechanisms

Mechanism	What Happens if a Failure Occurs	Type of Recovery
DECnet--Plus cluster alias	If a node fails, OpenVMS Cluster software automatically distributes new incoming connections among other participating nodes.	Manual. Users who were logged in to the failed node can reconnect to a remaining node. Automatic for appropriately coded applications. Such applications can reinstate a connection to the cluster alias node name, and the connection is directed to one of the remaining nodes.
I/O paths	With redundant paths to storage devices, if one path fails, OpenVMS Cluster software fails over to a working path, if one exists.	Transparent, provided another working path is available.
Interconnect	With redundant or mixed interconnects, OpenVMS Cluster software uses the fastest working path to connect to other OpenVMS Cluster members. If an interconnect path fails, OpenVMS Cluster software fails over to a working path, if one exists.	Transparent.
Boot and disk servers	If you configure at least two nodes as boot and disk servers, satellites can continue to boot and use disks if one of the servers shuts down or fails. Failure of a boot server does not affect nodes that have already booted, providing they have an alternate path to access MSCP served disks.	Automatic.
Terminal servers and LAT software	Attach terminals and printers to terminal servers. If a node fails, the LAT software automatically connects to one of the remaining nodes. In addition, if a user process is disconnected from a LAT terminal session, when the user attempts to reconnect to a LAT session, LAT software can automatically reconnect the user to the disconnected session.	Manual. Terminal users who were logged in to the failed node must log in to a remaining node and restart the application.
Generic batch and print queues	You can set up generic queues to feed jobs to execution queues (where processing occurs) on more than one node. If one node fails, the generic queue can continue to submit jobs to execution queues on remaining nodes. In addition, batch jobs submitted using the /RESTART qualifier are automatically restarted on one of the remaining nodes.	Transparent for jobs waiting to be dispatched. Automatic or manual for jobs executing on the failed node.
Autostart batch and print queues	For maximum availability, you can set up execution queues as autostart queues with a failover list. When a node fails, an autostart execution queue and its jobs automatically fail over to the next logical node in the failover list and continue processing on another node. Autostart queues are especially useful for print queues directed to printers that are attached to terminal servers.	Transparent.

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.

Table 8-3 Products That Increase Availability

Product	Description
Availability Manager	Collects and analyzes data from multiple nodes simultaneously and directs all output to a centralized DECwindows display. The analysis detects availability problems and suggests corrective actions.
RTR	Provides continuous and fault-tolerant transaction delivery services in a distributed environment with scalability and location transparency. In-flight transactions are guaranteed with the two-phase commit protocol, and databases can be distributed worldwide and partitioned for improved performance.
Volume Shadowing for OpenVMS	Makes any disk in an OpenVMS Cluster system a redundant twin of any other same-size disk (same number of physical blocks) in the OpenVMS Cluster.

8.3 Strategies for Configuring Highly Available OpenVMS Clusters

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.

Table 8-4 Availability Strategies

Strategy	Description
Eliminate single points of failure	Make components redundant so that if one component fails, the other is available to take over.
Shadow system disks	The system disk is vital for node operation. Use Volume Shadowing for OpenVMS to make system disks redundant.
Shadow essential data disks	Use Volume Shadowing for OpenVMS to improve data availability by making data disks redundant.
Provide shared, direct access to storage	Where possible, give all nodes shared direct access to storage. This reduces dependency on MSCP server nodes for access to storage.
Minimize environmental risks	Take the following steps to minimize the risk of environmental problems: Provide a generator or uninterruptible power system (UPS) to replace utility power for use during temporary outages. Configure extra air-conditioning equipment so that failure of a single unit does not prevent use of the system equipment.
Configure at least three nodes	OpenVMS Cluster nodes require a quorum to continue operating. An optimal configuration uses a minimum of three nodes so that if one node becomes unavailable, the two remaining nodes maintain quorum and continue processing. Reference: For detailed information on quorum strategies, see Section 11.5 and HP OpenVMS Cluster Systems.
Configure extra capacity	For each component, configure at least one unit more than is necessary to handle capacity. Try to keep component use at 80% of capacity or less. For crucial components, keep resource use sufficiently less than 80% capacity so that if one component fails, the work load can be spread across remaining components without overloading them.
Keep a spare component on standby	For each component, keep one or two spares available and ready to use if a component fails. Be sure to test spare components regularly to make sure they work. More than one or two spare components increases complexity as well as the chance that the spare will not operate correctly when needed.
Use homogeneous nodes	Configure nodes of similar size and performance to avoid capacity overloads in case of failover. If a large node fails, a smaller node may not be able to handle the transferred work load. The resulting bottleneck may decrease OpenVMS Cluster performance.
Use reliable hardware	Consider the probability of a hardware device failing. Check product descriptions for MTBF (mean time between failures). In general, newer technologies are more reliable.

8.4 Strategies for Maintaining Highly Available OpenVMS Clusters

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.

Table 8-5 Strategies for Maintaining Availability

Strategy	Description
Plan a failover strategy	OpenVMS Cluster systems provide software support for failover between hardware components. Be aware of what failover capabilities are available and which can be customized for your needs. Determine which components must recover from failure, and make sure that components are able to handle the additional work load that may result from a failover. Reference: Table 8-2 lists OpenVMS Cluster failover mechanisms and the levels of recovery that they provide.
Code distributed applications	Code applications to run simultaneously on multiple nodes in an OpenVMS Cluster system. If a node fails, the remaining members of the OpenVMS Cluster system are still available and continue to access the disks, tapes, printers, and other peripheral devices that they need.
Minimize change	Assess carefully the need for any hardware or software change before implementing it on a running node. If you must make a change, test it in a noncritical environment before applying it to your production environment.
Reduce size and complexity	After you have achieved redundancy, reduce the number of components and the complexity of the configuration. A simple configuration minimizes the potential for user and operator errors as well as hardware and software errors.
Set polling timers identically on all nodes	Certain system parameters control the polling timers used to maintain an OpenVMS Cluster system. Make sure these system parameter values are set identically on all OpenVMS Cluster member nodes. Reference: For information about these system parameters, refer to HP OpenVMS Cluster Systems.
Manage proactively	The more experience your system managers have, the better. Allow privileges for only those users or operators who need them. Design strict policies for managing and securing the OpenVMS Cluster system.
Use AUTOGEN proactively	With regular AUTOGEN feedback, you can analyze resource usage that may affect system parameter settings.
Reduce dependencies on a single server or disk	Distributing data across several systems and disks prevents one system or disk from being a single point of failure.
Implement a backup strategy	Performing frequent backup procedures on a regular basis guarantees the ability to recover data after failures. None of the strategies listed in this table can take the place of a solid backup strategy.