Document revision date: 30 March 2001

Backup tape storage provides the least expensive storage medium. Tapes are the most common medium for offline storage and provide a range of capacities, cost, and shelf life. In general, tape storage is removable and generally off line.

11.11.1 For More Information

Backup procedures are described in detail in the following manuals:

• OpenVMS System Manager's Manual
• OpenVMS System Management Utilities Reference Manual

With current tape-drive technology, you can initiate a large backup operation that completes without operator intervention (that is, changing tapes). Such unattended backups can save significant time and reduce staffing costs. Cartridge tape loaders with tape magazines, such as the Tx8x7 or the TA91, allow unattended backups of up to nearly 42 GB of online storage. Backups can also be performed on robot-accessible media, such as the StorageTek 4400 ACS through the TC44 interconnect adapter, which provides terabyte capacity for backup archives.

11.11.3 Archive/Backup System for OpenVMS

Archive/Backup System for OpenVMS is a replacement for the Storage Library System (SLS). Archive/Backup provides lower system management costs, reduced equipment costs, and data security. It uses the POLYCENTER Media Library Manager (MLM) and the POLYCENTER Media Robot Manager (MRM) to move data to inexpensive tapes, and allows you to find and restore backed up and archived data easily. POLYCENTER MLM and MRM are the first Compaq products to provide OpenVMS users secure, highly reliable, fully automated access to tape and optical removable media through cost-effective media robots, such as the Odetics 5480 and the Tx8x7 family.

11.11.4 StorageTek 4400 ACS

You can attach the StorageTek 4400 ACS, a storage silo, to either an HSC using the TC44 adapter or directly to the XMI bus of a system using a KCM44 adapter. The StorageTek Silo automates access to a library of IBM 3480 compatible cartridge tapes. The library can contain up to 16 library storage modules. Each module can hold up to 1.2 TB of data in 6000 tape cartridges. A robotic arm can find and mount a requested tape within 45 to 90 seconds. Data movement for tape applications, such as the OpenVMS Backup utility, is performed the same way as with a TA90 tape drive.

11.11.5 Tape-Drive Performance and Capacity

Table 11-8 describes the performance and capacity of various tape drives and the interconnects to which they attach.

Table 11-8 Tape-Drive Performance and Capacity

Interconnect	Description
CI (STI tapes)	The TA92 can transfer at a rate of 2.6 MB/s. Its magazine of IBM 3480 compatible cartridge tapes lets it back up 38 GB unattended. To achieve highest performance, connect the TA92 through a KDM70 controller or configure it with multiple CI adapters, so that the path to the tape drives is separate from the path to the disk drives.
DSSI	The TF867 offers the best tape performance. Its magazine of half-inch cartridge tapes can hold up to 42 GB of data for unattended backup. Its transfer rate is 0.8 MB/s. The TF857 can read TK50 and TK70 tapes, and its magazine can hold up to 18 GB of data.
SCSI	The TSZ07 allows SCSI configurations to access 9-track reel-to-reel tapes. It has a capacity of 140 MB per reel and a 750 KB/s transfer rate. The TZK10 offers a less expensive but slower-performing tape solution for SCSI configurations. It uses a quarter-inch cartridge that holds 525 MB and can transfer at a rate of 200 KB/s.

One of the benefits of OpenVMS Cluster systems is that multiple computers can simultaneously access storage devices connected to a 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-1 Requirements for SCSI Multihost OpenVMS Cluster Configurations

Requirement	Description
Software	All Alpha hosts sharing access to storage on a SCSI interconnect must be running: OpenVMS Alpha Version 6.2 or later OpenVMS Cluster Software for OpenVMS Alpha Version 6.2 or later
Hardware	Table A-2 lists the supported hardware components for SCSI OpenVMS Cluster systems. See also Section A.7.7 for information about other hardware devices that might be used in a SCSI OpenVMS Cluster configuration.
SCSI tape, floppies, and CD-ROM drives	You cannot configure SCSI tape drives, floppy drives, or CD-ROM drives on multihost SCSI interconnects. If your configuration requires SCSI tape, floppy, or CD-ROM drives, configure them on single-host SCSI interconnects. Note that SCSI tape, floppy, or CD-ROM drives may be MSCP or TMSCP served to other hosts in the OpenVMS Cluster configuration.
Maximum hosts on a SCSI bus	You can connect up to three hosts on a multihost SCSI bus. You can configure any mix of the hosts listed in Table A-2 on the same shared SCSI interconnect.
Maximum SCSI buses per host	You can connect each host to a maximum of six multihost SCSI buses. The number of nonshared (single-host) SCSI buses that can be configured is limited only by the number of available slots on the host bus.
Host-to-host communication	All members of the cluster must be connected by an interconnect that can be used for host-to-host (SCA) communication; for example, DSSI, CI, Ethernet, FDDI, or MEMORY CHANNEL.
Host-based RAID (including host-based shadowing)	Supported in SCSI OpenVMS Cluster configurations.
SCSI device naming	The name of each SCSI device must be unique throughout the OpenVMS Cluster system. When configuring devices on systems that include a multihost SCSI bus, adhere to the following requirements: A host can have, at most, one adapter attached to a particular SCSI interconnect. All host controllers attached to a given SCSI interconnect must have the same OpenVMS device name (for example, PKA0), unless port allocation classes are used (see OpenVMS Cluster Systems). Each system attached to a SCSI interconnect must have the a nonzero node disk allocation class value. These node disk allocation class values may differ as long as either of the following conditions is true: The SCSI interconnect has a positive, non-zero port allocation class The only devices attached to the SCSI interconnect are accessed by HSZ70 or HSZ80 controllers that have a non-zero controller allocation class. If you have multiple SCSI interconnects, you must consider all the SCSI interconnects to determine whether you can chose a different value for the node disk allocation class on each system. Note, also, that the addition of a SCSI device to an existing SCSI interconnect requires a revaluation of whether the node disk allocation classes can still be different. Therefore, Compaq recommends that you use the same node disk allocation class value for all systems attached to the same SCSI interconnect. For more information about allocation classes, see OpenVMS Cluster Systems.

A.3.2 Hardware Support

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, refer to the AlphaServer or AlphaStation documentation for your model on the following Compaq web site:

http://www.compaq.com/products/servers/

For disk support information, refer to StorageWorks product documentation on the Compaq web site:

http://www.compaq.com/storage/

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.

Table A-2 Supported Hardware for SCSI OpenVMS Cluster Systems

Component	Supported Item	Minimum Firmware (FW) Version1
Controller	HSZ40--B	2.5 (FW)
	HSZ50
	HSZ70
	HSZ80	8.3 (FW)
Adapters 2	Embedded (NCR-810 based)
	KZPAA (PCI to SCSI)
	KZPSA (PCI to SCSI)	A11 (FW)
	KZPBA-CB (PCI to SCSI)	5.53 (FW)
	KZTSA (TURBOchannel to SCSI)	A10-1 (FW)

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.

Table A-3 Maximum Data Transfer Rates (MB/s)

Mode	Narrow (8-bit)	Wide (16-bit)
Standard	5	10
Fast	10	20
Ultra	20	40

A.4.3 Distance

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.

Table A-4 Maximum SCSI Interconnect Distances

Signaling Technique	Rate of Data Transfer	Maximum Cable Length
Single ended	Standard	6 m 1
Single ended	Fast	3 m
Single ended	Ultra	20.5 m 2
Differential	Standard or fast	25 m
Differential	Ultra	25.5 m 2

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.

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