HP OpenVMS Systems Documentation

One of the benefits of OpenVMS Cluster systems is that multiple computers can simultaneously access storage devices connected to an OpenVMS Cluster storage interconnect. Together, these systems provide high performance and highly available access to storage.

This appendix describes how OpenVMS Cluster systems support the Small Computer Systems Interface (SCSI) as a storage interconnect. Multiple Alpha computers, also referred to as hosts or nodes, can simultaneously access SCSI disks over a SCSI interconnect. Such a configuration is called a SCSI multihost OpenVMS Cluster. A SCSI interconnect, also called a SCSI bus, is an industry-standard interconnect that supports one or more computers, peripheral devices, and interconnecting components.

The discussions in this chapter assume that you already understand the concept of sharing storage resources in an OpenVMS Cluster environment. OpenVMS Cluster concepts and configuration requirements are also described in the following OpenVMS Cluster documentation:

• OpenVMS Cluster Systems
• OpenVMS Cluster Software Software Product Description (SPD 29.78.xx)

This appendix includes two primary parts:

• Section A.1 through Section A.6.6 describe the fundamental procedures and concepts that you would need to plan and implement a SCSI multihost OpenVMS Cluster system.
• Section A.7 and its subsections provide additional technical detail and concepts.

Certain conventions are used throughout this appendix to identify the ANSI Standard and for elements in figures.

A.1.1 SCSI ANSI Standard

OpenVMS Cluster systems configured with the SCSI interconnect must use standard SCSI-2 or SCSI-3 components. The SCSI-2 components must be compliant with the architecture defined in the American National Standards Institute (ANSI) Standard SCSI-2, X3T9.2, Rev. 10L. The SCSI-3 components must be compliant with approved versions of the SCSI-3 Architecture and Command standards. For ease of discussion, this appendix uses the term SCSI to refer to both SCSI-2 and SCSI-3.

A.1.2 Symbols Used in Figures

Figure A-1 is a key to the symbols used in figures throughout this appendix.

Figure A-1 Key to Symbols Used in Figures

In OpenVMS Cluster configurations, multiple VAX and Alpha hosts can directly access SCSI devices in any of the following ways:

• CI interconnect with HSJ or HSC controllers
• Digital Storage Systems Interconnect (DSSI) with HSD controller
• SCSI adapters directly connected to VAX or Alpha systems

You can also access SCSI devices indirectly using the OpenVMS MSCP server.

The following sections describe single-host and multihost access to SCSI storage devices.

A.2.1 Single-Host SCSI Access in OpenVMS Cluster Systems

Prior to OpenVMS Version 6.2, OpenVMS Cluster systems provided support for SCSI storage devices connected to a single host using an embedded SCSI adapter, an optional external SCSI adapter, or a special-purpose RAID (redundant arrays of independent disks) controller. Only one host could be connected to a SCSI bus.

A.2.2 Multihost SCSI Access in OpenVMS Cluster Systems

Beginning with OpenVMS Alpha Version 6.2, multiple Alpha hosts in an OpenVMS Cluster system can be connected to a single SCSI bus to share access to SCSI storage devices directly. This capability allows you to build highly available servers using shared access to SCSI storage.

Figure A-2 shows an OpenVMS Cluster configuration that uses a SCSI interconnect for shared access to SCSI devices. Note that another interconnect (for example, a local area network [LAN]) is required for host-to-host OpenVMS Cluster (System Communications Architecture [SCA]) communications.

Figure A-2 Highly Available Servers for Shared SCSI Access

You can build a three-node OpenVMS Cluster system using the shared SCSI bus as the storage interconnect, or you can include shared SCSI buses within a larger OpenVMS Cluster configuration. A quorum disk can be used on the SCSI bus to improve the availability of two- or three-node configurations. Host-based RAID (including host-based shadowing) and the MSCP server are supported for shared SCSI storage devices.

A.3 Configuration Requirements and Hardware Support

This section lists the configuration requirements and supported hardware for multihost SCSI OpenVMS Cluster systems.

A.3.1 Configuration Requirements

Table A-2 shows the supported hardware components for SCSI OpenVMS Cluster systems; it also lists the minimum required revision for these hardware components. That is, for any component, you must use either the version listed in Table A-2 or a subsequent version. For host support information, go to the HP servers web site:

http://www.hp.com/country/us/eng/prodserv/servers.html

There, you will find documentation for your AlphaServer or AlphaStation system.

For disk support information, refer to the HP storage web site:

http://www.hp.com/country/us/eng/prodserv/storage.html

The SCSI interconnect configuration and all devices on the SCSI interconnect must meet the requirements defined in the ANSI Standard SCSI-2 document, or the SCSI-3 Architecture and Command standards, and the requirements described in this appendix. See also Section A.7.7 for information about other hardware devices that might be used in a SCSI OpenVMS Cluster configuration.

The SCSI standard defines a set of rules governing the interactions between initiators (typically, host systems) and SCSI targets (typically, peripheral devices). This standard allows the host to communicate with SCSI devices (such as disk drives, tape drives, printers, and optical media devices) without having to manage the device-specific characteristics.

The following sections describe the SCSI standard and the default modes of operation. The discussions also describe some optional mechanisms you can implement to enhance the default SCSI capabilities in areas such as capacity, performance, availability, and distance.

A.4.1 Number of Devices

The SCSI bus is an I/O interconnect that can support up to 16 devices. A narrow SCSI bus supports up to 8 devices; a wide SCSI bus support up to 16 devices. The devices can include host adapters, peripheral controllers, and discrete peripheral devices such as disk or tape drives. The devices are addressed by a unique ID number from 0 through 15. You assign the device IDs by entering console commands, or by setting jumpers or switches, or by selecting a slot on a StorageWorks enclosure.

When configuring more devices than the previous limit of eight, make sure that you observe the bus length requirements (see Table A-4).

To configure wide IDs on a BA356 box, refer to the BA356 manual StorageWorks Solutions BA356-SB 16-Bit Shelf User's Guide (order number EK-BA356-UG). Do not configure a narrow device in a BA356 box that has a starting address of 8.

To increase the number of devices on the SCSI interconnect, some devices implement a second level of device addressing using logical unit numbers (LUNs). For each device ID, up to eight LUNs (0--7) can be used to address a single SCSI device as multiple units. The maximum number of LUNs per device ID is eight.

The default mode of operation for all SCSI devices is 8-bit asynchronous mode. This mode, sometimes referred to as narrow mode, transfers 8 bits of data from one device to another. Each data transfer is acknowledged by the device receiving the data. Because the performance of the default mode is limited, the SCSI standard defines optional mechanisms to enhance performance. The following list describes two optional methods for achieving higher performance:

• Increase the amount of data that is transferred in parallel on the interconnect. The 16-bit and 32-bit wide options allow a doubling or quadrupling of the data rate, respectively. Because the 32-bit option is seldom implemented, this appendix discusses only 16-bit operation and refers to it as wide.
• Use synchronous data transfer. In synchronous mode, multiple data transfers can occur in succession, followed by an acknowledgment from the device receiving the data. The standard defines a slow mode (also called standard mode) and a fast mode for synchronous data transfers:
  • In standard mode, the interconnect achieves up to 5 million transfers per second.
  • In fast mode, the interconnect achieves up to 10 million transfers per second.
  • In ultra mode, the interconnect achieves up to 20 million transfers per second.

Because all communications on a SCSI interconnect occur between two devices at a time, each pair of devices must negotiate to determine which of the optional features they will use. Most, if not all, SCSI devices implement one or more of these options.

Table A-3 shows data rates when using 8- and 16-bit transfers with standard, fast, and ultra synchronous modes.

The maximum length of the SCSI interconnect is determined by the signaling method used in the configuration and by the data transfer rate. There are two types of electrical signaling for SCSI interconnects:

• Single-ended signaling
  The single-ended method is the most common and the least expensive. The distance spanned is generally modest.
• Differential signaling
  This method provides higher signal integrity, thereby allowing a SCSI bus to span longer distances.

Table A-4 summarizes how the type of signaling method affects SCSI interconnect distances.

The DWZZA, DWZZB, and DWZZC converters are single-ended to differential converters that you can use to connect single-ended and differential SCSI interconnect segments. The DWZZA is for narrow (8-bit) SCSI buses, the DWZZB is for wide (16-bit) SCSI buses, and the DWZZC is for wide Ultra SCSI buses.

The differential segments are useful for the following:

• Overcoming the distance limitations of the single-ended interconnect
• Allowing communication between single-ended and differential devices

Because the DWZZA, the DWZZB, and the DWZZC are strictly signal converters, you can not assign a SCSI device ID to them. You can configure a maximum of two DWZZA or two DWZZB converters in the path between any two SCSI devices. Refer to the StorageWorks UltraSCSI Configuration Guidelines for information on configuring the DWZZC.

A.4.4 Cabling and Termination

Each single-ended and differential SCSI interconnect must have two terminators, one at each end. The specified maximum interconnect lengths are measured from terminator to terminator.

The interconnect terminators are powered from the SCSI interconnect line called TERMPWR. Each StorageWorks host adapter and enclosure supplies the TERMPWR interconnect line, so that as long as one host or enclosure is powered on, the interconnect remains terminated.

Devices attach to the interconnect by short cables (or etch) called stubs. Stubs must be short in order to maintain the signal integrity of the interconnect. The maximum stub lengths allowed are determined by the type of signaling used by the interconnect, as follows:

• For single-ended interconnects, the maximum stub length is .1 m.
• For differential interconnects, the maximum stub length is .2 m.

Additionally, the minimum distance between stubs on a single-ended interconnect is .3 m. Refer to Figure A-3 for an example of this configuration.

When you are extending the SCSI bus beyond an existing terminator, it is necessary to disable or remove that terminator.

Figure A-3 Maximum Stub Lengths

The hardware configuration that you choose depends on a combination of factors:

• Your computing needs---for example, continuous availability or the ability to disconnect or remove a system from your SCSI OpenVMS Cluster system
• Your environment---for example, the physical attributes of your computing facility
• Your resources---for example, your capital equipment or the available PCI slots

Refer to the OpenVMS Cluster Software Software Product Description (SPD 29.78.xx) for configuration limits.

The following sections provide guidelines for building SCSI configurations and describe potential configurations that might be suitable for various sites.