Document revision date: 19 July 1999

MEMORY CHANNEL technology provides the following features:

• Offers excellent price/performance.
  With several times the CI bandwidth, MEMORY CHANNEL provides a 100 MB/s interconnect with minimal latency. MEMORY CHANNEL architecture is designed for the industry-standard PCI bus.
• Requires no change to existing applications.
  MEMORY CHANNEL works seamlessly with existing cluster software, so that no change is necessary for existing applications. The new MEMORY CHANNEL drivers, PMDRIVER and MCDRIVER, integrate with the System Communications Services layer of OpenVMS Clusters in the same way that existing port drivers do. Higher layers of cluster software are unaffected.
• Offloads CI, DSSI, and the LAN in SCSI clusters.
  You cannot connect storage directly to MEMORY CHANNEL, but you can use it to make maximum use of each interconnect's strength.
  While MEMORY CHANNEL is not a replacement for CI and DSSI, when used in combination with those interconnects, it offloads their node-to-node traffic. This enables them to be dedicated to storage traffic, optimizing communications in the entire cluster.
  When used in a cluster with SCSI and LAN interconnects, MEMORY CHANNEL offloads node-to-node traffic from the LAN, enabling it to handle more TCP/IP or DECnet traffic.
• Provides fail-separately behavior.
  When a system failure occurs, MEMORY CHANNEL nodes behave like any failed node in an OpenVMS Cluster. The rest of the cluster continues to perform until the failed node can rejoin the cluster.

The MEMORY CHANNEL interconnect has a very high maximum throughput of 100 MB/s. If a single MEMORY CHANNEL is not sufficient, up to two interconnects (and two MEMORY CHANNEL hubs) can share throughput.

4.7.3 Supported Adapter

The MEMORY CHANNEL adapter connects to the PCI bus. The newest MEMORY CHANNEL adapter, CCMAA--BA, provides improved performance.

Reference: For complete information about each adapter's features and order numbers, see the DIGITAL Systems and Options Catalog.

To access the most recent DIGITAL Systems and Options Catalog on the World Wide Web, use the following URL:

http://www.digital.com:80/info/soc/

4.8 SCSI Interconnect

The SCSI interconnect is an industry standard interconnect that supports one or more computers, peripheral devices, and interconnecting components. SCSI is a single-path, daisy-chained, multidrop bus. It is a single 8-bit or 16-bit data path with byte parity for error detection. Both inexpensive single-ended and differential signaling for longer distances are available.

In an OpenVMS Cluster, multiple Alpha computers on a single SCSI interconnect can simultaneously access SCSI disks. This type of configuration is called multihost SCSI connectivity. A second type of interconnect is required for node-to-node communication. For multihost access to SCSI storage, the following components are required:

• SCSI host adapter that is supported in a multihost configuration (see Table 4-6)
• SCSI interconnect
• Terminators, one for each end of the SCSI interconnect
• Storage devices that are supported in a multihost configuration (RZnn; refer to the OpenVMS Cluster SPD [29.78.nn])

For larger configurations, the following components are available:

• Storage controllers (HSZnn)
• Bus isolators (DWZZA, DWZZB, or DWZZC) to convert single-ended to differential signaling and to effectively double the SCSI interconnect length

Reference: For a detailed description of how to connect SCSI configurations, see Appendix A.

4.8.1 Advantages

The SCSI interconnect offers the following advantages:

• Lowest cost, shared direct access to storage
  Because SCSI is an industry standard and is used extensively throughout the industry, it is available from many manufacturers at competitive prices.
• Scalable configuration to achieve high performance at a moderate price
  You can choose:
  • Width of SCSI interconnect
    Narrow (8 bits) or wide (16 bits).
  • Transmission mode
    Single-ended signaling, the most common and least expensive, or differential signaling, which provides higher signal integrity and allows a longer SCSI interconnect.
  • Signal speed (standard, fast, or ultra mode)
  • Number of nodes sharing the SCSI bus (two or three)
  • Number of shared SCSI buses to which a node can connect (maximum of six)
  • Storage type and size (RZnn or HSZnn)
  • Computer type and size (AlphaStation or AlphaServer)

Table 4-4 show throughput for the SCSI interconnect.

Table 4-4 Maximum Data Transfer Rates in Megabytes per Second

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

4.8.3 SCSI Interconnect Distances

The maximum length of the SCSI interconnect is determined by the signaling method used in the configuration and, for single-ended signaling, by the data transfer rate.

There are two types of electrical signaling for SCSI interconnects: single ended and differential. Both types can operate in standard mode, fast mode, or ultra mode. For differential signaling, the maximum SCSI cable length possible is the same for standard mode and fast mode.

Table 4-5 summarizes how the type of signaling method affects SCSI interconnect distances.

Table 4-5 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 3

Table 4-6 shows SCSI adapters with the internal buses and computers they support.

Table 4-6 SCSI Adapters

Adapter	Internal Bus	Supported Computers
Embedded (NCR-810 based)/KZPAA 1	PCI	AlphaServer 400
		AlphaServer 1000
		AlphaServer 2000
		AlphaServer 2100
		AlphaStation 200
		AlphaStation 250
		AlphaStation 400
		AlphaStation 600
KZPSA 2	PCI	Supported on all Alpha computers that support KZPSA in single-host configurations. 3
KZTSA 2	TURBOchannel	DEC 3000
KZPBA-CB 4	PCI	Supported on all Alpha computers that support KZPBA in single-host configurations. 3

Reference: For complete information about each adapter's features and order numbers, see the DIGITAL Systems and Options Catalog.

To access the most recent DIGITAL Systems and Options Catalog on the World Wide Web, use the following URL:

http://www.digital.com:80/info/soc/

4.9 CI Interconnect

The CI interconnect is a radial bus through which OpenVMS Cluster systems communicate. It comprises the following components:

• CI host adapter.
• Star coupler---A passive device that serves as a common connection point for signals between OpenVMS nodes and HSC or HSJ controllers that are connected by the CI.
• Optional star coupler expander (CISCE)---Consists of two amplifiers, one for each of its two paths.
• CI cable.

The CI interconnect offers the following advantages:

• High speed
  Suitable for larger processors and I/O-intensive applications.
• Efficient access to large amounts of storage
  HSC and HSJ controllers can connect large numbers of disk and tape drives to the OpenVMS Cluster system, with direct access from all OpenVMS nodes on the CI.
• Minimal CPU overhead for communication
  CI adapters are intelligent interfaces that perform much of the work required for communication among OpenVMS nodes and storage. The CI topology allows all nodes attached to a CI bus to communicate directly with the HSC and HSJ controllers on the same CI bus.
• High availability through redundant, independent data paths
  Each CI adapter is connected with two pairs of CI cables. If a single CI cable connection fails, failover automatically occurs.
• Multiple access paths to disks and tapes
  Dual HSC and HSJ controllers and dual-ported devices create alternative paths to storage devices.

The CI interconnect has a high maximum throughput. CI adapters use high-performance microprocessors that perform many of the processing activities usually performed by the CPU. As a result, they consume minimal CPU processing power.

Because the effective throughput of the CI bus is high, a single CI interconnect is not likely to be a bottleneck in a large OpenVMS Cluster configuration. If a single CI is not sufficient, multiple CI interconnects can increase throughput.

4.9.3 Supported Adapters and Bus Types

The following are CI adapters and internal buses that each supports:

• CIPCA (PCI/EISA)
• CIXCD (XMI)
• CIBCA--B (VAXBI)

Reference: For complete information about each adapter's features and order numbers, see the DIGITAL Systems and Options Catalog.

To access the most recent DIGITAL Systems and Options Catalog on the World Wide Web, use the following URL:

http://www.digital.com:80/info/soc/

4.9.4 Multiple CI Adapters

You can configure multiple CI adapters on some OpenVMS nodes. Multiple star couplers can be used in the same OpenVMS Cluster.

With multiple CI adapters on a node, adapters can share the traffic load. This reduces I/O bottlenecks and increases the total system I/O throughput. Table 4-7 lists the limits for multiple CI adapters per system.

Table 4-7 Maximum CI Adapters per System

System	CIPCA	CIBCA--A	CIBCA--B	CIXCD	Comments
AlphaServer 8400	26	-	-	10	Can use a combination of CIPCA and CIXCD adapters, not to exceed 26. Prior to OpenVMS Version 7.1, the maximum is 10.
AlphaServer 8200	26	-	-	-	Prior to OpenVMS Version 7.1, the maximum is 10.
AlphaServer 4000, 4100	3	-	-	-	When using three CIPCAs, one must be a CIPCA-AA and two must be CIPCA-BA.
AlphaServer 4000 with I/O expansion module	6	-	-	-	When using six CIPCAs, only three can be CIPCA-AA.
AlphaServer 2100A	3	-	-	-	-
AlphaServer 2000, 2100	2	-	-	-	Only one can be a CIPCA-BA.
AlphaServer 1200	2	-	-	-	Only one can be a CIPCA-AA.
DEC 7000/10000	-	-	-	10	-
VAX 6000	-	1	4	4	CIPCA is not supported on a VAX.
VAX 7000, 10000	-	-	-	10	CIPCA is not supported on a VAX. CIBCA is not supported on these models.

Reference: For more extensive information about the CIPCA adapter, see Appendix C.

4.9.5 Configuration Guidelines for CI Clusters

Use the following guidelines when configuring systems in a CI cluster:

• The maximum number of nodes that you can connect to a star coupler is 32. Up to 16 of these nodes can be OpenVMS systems, and the remainder can be HSJ and HSC storage controllers.
• The number of star couplers is limited by the number of CI adapters configured on a system.
• Dual porting of devices between HSJ and HSC controllers is supported as long as they are connected to the same or separate star couplers. Dual porting of devices between HSJ and HSC controllers and local controllers is not supported.
• With the exception of the CIPCA and CIXCD, different types of CI adapters cannot be combined in the same system.
• You can use multiple CI adapters for redundancy and throughput. You can increase throughput by connecting additional CI adapters to separate star couplers; throughput does not increase substantially when you connect a second CI adapter to the same star coupler.

DSSI is a single-path, daisy-chained, multidrop bus. It provides a single, 8-bit parallel data path with both byte parity and packet checksum for error detection.

4.10.1 Advantages

DSSI offers the following advantages:

• High reliability
• Shared, direct access to storage at a lower cost than CI
• Direct communication between systems and storage
• High-performance, intelligent storage controllers with embedded caches

DSSI storage often resides in the same cabinet as the CPUs. For these configurations, the whole system may need to be shut down for service, unlike configurations and interconnects with separately housed systems and storage devices.

4.10.3 Throughput

The maximum throughput is 32 Mb/s.

DSSI has highly intelligent adapters that require minimal CPU processing overhead.

4.10.4 DSSI Adapter Types

There are two types of DSSI adapters:

• Embedded adapter, which is part of the system.
• Optional adapter, which you can purchase separately and add to the system.

The following are DSSI adapters and internal bus that each supports:

• KFESA (EISA)
• KFESB (EISA)
• KFPSA (PCI)
• KFMSA (XMI)---VAX only
• KFMSB (XMI)---Alpha only
• KFQSA (Q-bus)
• N710 (embedded)
• SHAC (embedded)
• EDA640 (embedded)

Reference: For complete information about each adapter's features and order numbers, see the DIGITAL Systems and Options Catalog.

To access the most recent DIGITAL Systems and Options Catalog on the World Wide Web, use the following URL:

http://www.digital.com:80/info/soc/

4.10.6 DSSI-Connected Storage

DSSI configurations use HSD intelligent controllers to connect disk drives to an OpenVMS Cluster. HSD controllers serve the same purpose with DSSI as HSJ controllers serve with CI: they enable you to configure more storage.

Alternatively, DSSI configurations use integrated storage elements (ISEs) connected directly to the DSSI bus. Each ISE contains either a disk and disk controller or a tape and tape controller.

4.10.7 Multiple DSSI Adapters

Multiple DSSI adapters are supported for some systems, enabling higher throughput than with a single DSSI bus.

Table 4-8 lists the limitations for multiple DSSI adapters. You can also refer to the most recent OpenVMS Cluster SPD for the latest information about DSSI adapters.

Table 4-8 DSSI Adapters per System

System	Embedded	KFPSA1	KFQSA2	KFESA	KFESB	KFMSA3	KFMSB3
AlphaServer 8400	-	4	-	-	-	-	12
AlphaServer 8200, 4100	-	4	-	-	-	-	-
AlphaServer 2100	-	4	-	-	-	-	-
AlphaServer 2000, 1000	-	4	-	-	4	-	-
DEC 4000 (embedded N710)	2	-	-	-	-	-	-
DEC 7000/10000	-	-	-	-	-	-	12
MicroVAX II, 3500, 3600, 3800, 3900	-	-	2	-	-	-	-
MicroVAX 3300/3400 (embedded EDA640)	1	-	2	-	-	-	-
VAX 4000 Model 105A (embedded SHAC 4)	1 + 1 4	-	2 5	-	-	-	-
VAX 4000 Model 200 (embedded SHAC 4)	1	-	2	-	-	-	-
VAX 4000 Model 300, 400, 500, 600	2	-	2	-	-	-	-
VAX 4000 Model 505A/705A (embedded SHAC 3)	2 + 2 6	-	2	-	-	-	-
VAX 6000	-	-	-	-	-	6	-
VAX 7000	-	-	-	-	-	12	-

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