HP OpenVMS Systems Documentation
Guidelines for OpenVMS Cluster Configurations
To gain the benefits of disaster tolerance across a multiple-site OpenVMS Cluster, use Disaster Tolerant Cluster Services for OpenVMS, a system management and software package from Compaq.
Consult your Compaq Services representative for more information.
|Remote satellites and nodes||A few systems can be remotely located at a secondary site and can benefit from centralized system management and other resources at the primary site, as shown in Figure D-2. For example, a main office data center could be linked to a warehouse or a small manufacturing site that could have a few local nodes with directly attached site-specific devices. Alternatively, some engineering workstations could be installed in an office park across the city from the primary business site.|
|Data center management consolidation||A single management team can manage nodes located in data centers at multiple sites.|
|Physical resource sharing||Multiple sites can readily share devices such as high-capacity computers, tape libraries, disk archives, or phototypesetters.|
|Remote archiving||Backups can be made to archival media at any site in the cluster. A common example would be to use disk or tape at a single site to back up the data for all sites in the multiple-site OpenVMS Cluster. Backups of data from remote sites can be made transparently (that is, without any intervention required at the remote site).|
In general, a multiple-site OpenVMS Cluster provides all of the
availability advantages of a LAN OpenVMS Cluster. Additionally, by
connecting multiple, geographically separate sites, multiple-site
OpenVMS Cluster configurations can increase the availability of a
system or elements of a system in a variety of ways:
Figure D-2 shows an OpenVMS Cluster system with satellites accessible from a remote site.
Figure D-2 Multiple-Site OpenVMS Cluster Configuration with Remote Satellites
The same configuration rules that apply to OpenVMS Cluster systems on a LAN also apply to a multiple-site OpenVMS Cluster configuration that includes ATM, DS3, or FDDI intersite interconnect. General LAN configuration rules are stated in the following documentation:
Some configuration guidelines are unique to multiple-site OpenVMS
Clusters; these guidelines are described in Section D.3.4.
D.2 Using FDDI to Configure Multiple-Site OpenVMS Cluster Systems
Since VMS Version 5.4--3, FDDI has been the most common method to connect two distant OpenVMS Cluster sites. Using high-speed FDDI fiber-optic cables, you can connect sites with an intersite cable-route distance of up to 25 miles (40 km), the cable route distance between sites.
You can connect sites using these FDDI methods:
Refer to the GIGAswitch/FDDI ATM Linecard Reference Manual for configuration information. Additional OpenVMS Cluster configuration guidelines and system management information can be found in this manual and in OpenVMS Cluster Systems. See the New Features and Documentation Overview Manual for information about ordering the current version of these manuals.
The inherent flexibility of OpenVMS Cluster systems and improved
OpenVMS Cluster LAN protocols also allow you to connect multiple
OpenVMS Cluster sites using the ATM or DS3 or both communications
D.3 Using WAN Services to Configure Multiple-Site OpenVMS Cluster Systems
This section provides an overview of the ATM and DS3 wide area network (WAN) services, describes how you can bridge an FDDI interconnect to the ATM or DS3 or both communications services, and provides guidelines for using these services to configure multiple-site OpenVMS Cluster systems.
The ATM and DS3 services provide long-distance, point-to-point communications that you can configure into your OpenVMS Cluster system to gain WAN connectivity. The ATM and DS3 services are available from most common telephone service carriers and other sources.
DS3 is not available in Europe and some other locations. Also, ATM is a new and evolving standard, and ATM services might not be available in all localities.
ATM and DS3 services are approved for use with the following OpenVMS versions:
|Service||Approved Versions of OpenVMS|
|ATM||OpenVMS Version 6.2 or later|
|DS3||OpenVMS Version 6.1 or later|
The following sections describe the ATM and DS3 communication services
and how to configure these services into multiple-site OpenVMS Cluster
D.3.1 The ATM Communications Service
The ATM communications service that uses the SONET physical layer (ATM/SONET) provides full-duplex communications (that is, the bit rate is available simultaneously in both directions as shown in Figure D-3). ATM/SONET is compatible with multiple standard bit rates. The SONET OC-3 service at 155 Mb/s full-duplex rate is the best match to FDDI's 100 Mb/s bit rate. ATM/SONET OC-3 is a standard service available in most parts of the world. In Europe, ATM/SONET is a high-performance alternative to the older E3 standard.
Figure D-3 ATM/SONET OC-3 Service
To transmit data, ATM frames (packets) are broken into
cells for transmission by the ATM service. Each cell
has 53 bytes, of which 5 bytes are reserved for header information and
48 bytes are available for data. At the destination of the
transmission, the cells are reassembled into ATM frames. The use of
cells permits ATM suppliers to multiplex and demultiplex multiple data
streams efficiently at differing bit rates. This conversion of frames
into cells and back is transparent to higher layers.
D.3.2 The DS3 Communications Service (T3 Communications Service)
The DS3 communications service provides full-duplex communications as shown in Figure D-4. DS3 (also known as T3) provides the T3 standard bit rate of 45 Mb/s. T3 is the standard service available in North America and many other parts of the world.
Figure D-4 DS3 Service
You can use FDDI-to-WAN (for example, FDDI-to-ATM or FDDI-to-DS3 or both) bridges to configure an OpenVMS Cluster with nodes in geographically separate sites, such as the one shown in Figure D-5. In this figure, the OpenVMS Cluster nodes at each site communicate as though the two sites are connected by FDDI. The FDDI-to-WAN bridges make the existence of ATM and DS3 transparent to the OpenVMS Cluster software.
Figure D-5 Multiple-Site OpenVMS Cluster Configuration Connected by DS3
In Figure D-5, the FDDI-to-DS3 bridges and DS3 operate as follows:
Compaq recommends using the GIGAswitch/FDDI system to construct
FDDI-to-WAN bridges. The GIGAswitch/FDDI, combined with the DEFGT WAN
T3/SONET option card, was used during qualification testing of the ATM
and DS3 communications services in multiple-site OpenVMS Cluster
D.3.4 Guidelines for Configuring ATM and DS3 in an OpenVMS Cluster System
When configuring a multiple-site OpenVMS Cluster, you must ensure that
the intersite link's delay, bandwidth, availability, and bit error rate
characteristics meet application needs. This section describes the
requirements and provides recommendations for meeting those
|Maximum intersite link route distance||
The total intersite link cable route distance between members of a
multiple-site OpenVMS Cluster cannot exceed 150 miles (242 km). You can
obtain exact distance measurements from your ATM or DS3 supplier.
This distance restriction can be exceeded when using Disaster Tolerant Cluster Services for OpenVMS, a system management and software package for configuring and managing OpenVMS disaster tolerant clusters.
|Maximum intersite link utilization||Average intersite link utilization in either direction must be less than 80% of the link's bandwidth in that direction for any 10-second interval. Exceeding this utilization is likely to result in intolerable queuing delays or packet loss.|
|Intersite link specifications||The intersite link must meet the OpenVMS Cluster requirements specified in Table D-3.|
|OpenVMS Cluster LAN configuration rules||Apply the configuration rules for OpenVMS Cluster systems on a LAN to a configuration. Documents describing configuration rules are referenced in Section D.1.3.|
When configuring the DS3 interconnect, apply the configuration guidelines for OpenVMS Cluster systems interconnected by LAN that are stated in the OpenVMS Cluster Software SPD (SPD 29.78.nn) and in this manual. OpenVMS Cluster members at each site can include any mix of satellites, systems, and other interconnects, such as CI and DSSI.
This section provides additional recommendations for configuring a multiple-site OpenVMS Cluster system.
The GIGAswitch with the WAN T3/SONET option card provides a full-duplex, 155 Mb/s ATM/SONET link. The entire bandwidth of the link is dedicated to the WAN option card. However, the GIGAswitch/FDDI's internal design is based on full-duplex extensions to FDDI. Thus, the GIGAswitch/FDDI's design limits the ATM/SONET link's capacity to 100 Mb/s in each direction.
The GIGAswitch with the WAN T3/SONET option card provides several protocol options that can be used over a DS3 link. Use the DS3 link in clear channel mode, which dedicates its entire bandwidth to the WAN option card. The DS3 link capacity varies with the protocol option selected. Protocol options are described in Table D-1.
|Protocol Option||Link Capacity|
|ATM 1 AAL--5 2 mode with PLCP 3 disabled.||39 Mb/s|
|ATM AAL--5 mode with PLCP enabled.||33 Mb/s|
|HDLC 4 mode (not currently available).||43 Mb/s|
The intersite bandwidth can limit application locking and I/O performance (including volume shadowing or RAID set copy times) and the performance of the lock manager.
To promote reasonable response time, Compaq recommends that average traffic in either direction over an intersite link not exceed 60% of the link's bandwidth in that direction for any 10-second interval. Otherwise, queuing delays within the FDDI-to-WAN bridges can adversely affect application performance.
Remember to account for both OpenVMS Cluster communications (such as locking and I/O) and network communications (such as TCP/IP, LAT, and DECnet) when calculating link utilization.
An intersite link introduces a one-way delay of up to 1 ms per 100 miles of intersite cable route distance plus the delays through the FDDI-to-WAN bridges at each end. Compaq recommends that you consider the effects of intersite delays on application response time and throughput.
For example, intersite link one-way path delays have the following components:
Calculate the delays for a round trip as follows:
WAN round-trip delay = 2 x (N miles x 0.01 ms per mile + 2 x 0.5 ms per FDDI-WAN bridge)
An I/O write operation that is MSCP served requires a minimum of two round-trip packet exchanges:
WAN I/O write delay = 2 x WAN round-trip delay
Thus, an I/O write over a 100-mile WAN link takes at least 8 ms longer than the same I/O write over a short, local FDDI.
Similarly, a lock operation typically requires a round-trip exchange of packets:
WAN lock operation delay = WAN round-trip delay
An I/O operation with N locks to synchronize it incurs the following delay due to WAN:
WAN locked I/O operation delay = (N x WAN lock operation delay) + WAN I/O delay
The bit error ratio (BER) parameter is an important measure of the frequency that bit errors are likely to occur on the intersite link. You should consider the effects of bit errors on application throughput and responsiveness when configuring a multiple-site OpenVMS Cluster. Intersite link bit errors can result in packets being lost and retransmitted with consequent delays in application I/O response time (see Section D.3.6). You can expect application delays ranging from a few hundred milliseconds to a few seconds each time a bit error causes a packet to be lost.
Interruptions of intersite link service can result in the resources at one or more sites becoming unavailable until connectivity is restored (see Section D.3.5).
Sites with nodes contributing quorum votes should have a local system disk or disks for those nodes.
A large, multiple-site OpenVMS Cluster requires a system management staff trained to support an environment that consists of a large number of diverse systems that are used by many people performing varied tasks.
You can provide portions of a DS3 link with microwave radio equipment.
The specifications in Section D.3.6 apply to any DS3 link. The BER and
availability of microwave radio portions of a DS3 link are affected by
local weather and the length of the microwave portion of the link.
Consider working with a microwave consultant who is familiar with your
local environment if you plan to use microwaves as portions of a DS3
D.3.5 Availability Considerations
If the FDDI-to-WAN bridges and the link that connects multiple sites become temporarily unavailable, the following events could occur:
Many communication service carriers offer availability-enhancing
options, such as path diversity, protective switching, and other
options that can significantly increase the intersite link's
To assist you in communicating your requirements to a WAN service supplier, this section uses WAN specification terminology and definitions commonly used by telecommunications service providers. These requirements and goals are derived from a combination of Bellcore Communications Research specifications and a Digital analysis of error effects on OpenVMS Clusters.
Use the Bellcore and OpenVMS Cluster requirements for ATM/SONET - OC3 and DS3 service error performance (quality) specified in Table D-3 to help you assess the impact of the service supplier's service quality, availability, down time, and service-interruption frequency goals on the system.
To ensure that the OpenVMS Cluster system meets your application response-time requirements, you might need to establish WAN requirements that exceed the Bellcore and OpenVMS Cluster requirements and goals stated in Table D-3.
|Bellcore Communications Research||
Bellcore specifications are the recommended "generic error
performance requirements and objectives" documented in the
Bellcore Technical Reference TR--TSY--000499
TSGR: Common Requirements. These specifications are adopted by
WAN suppliers as their service guarantees. The FCC has also adopted
them for tariffed services between common carriers. However, some
suppliers will contract to provide higher service-quality guarantees at
Other countries have equivalents to the Bellcore specifications and parameters.
|These are the recommended minimum values. Bellcore calls these goals their "objectives" in the TSGR: Common Requirements document.|
In order for Compaq to approve a configuration, parameters must meet or
exceed the values shown in the OpenVMS Cluster Requirements column in
If these values are not met, OpenVMS Cluster performance will probably be unsatisfactory because of interconnect errors/error recovery delays, and VC closures that may produce OpenVMS Cluster state transitions or site failover or both.
If these values are met or exceeded, then interconnect bit error--related recovery delays will not significantly degrade average OpenVMS Cluster throughput. OpenVMS Cluster response time should be generally satisfactory.
Note that if the requirements are only being met, there may be several application pauses per hour. 1
For optimal OpenVMS Cluster operation, all parameters should meet or
exceed the OpenVMS Cluster Goal values.
Note that if these values are met or exceeded, then interconnect bit errors and bit error recovery delays should not significantly degrade average OpenVMS Cluster throughput.
OpenVMS Cluster response time should be generally satisfactory, although there may be brief application pauses a few times per day. 2
|Parameter||Bellcore Requirement||Bellcore Goal||OpenVMS Cluster Requirement1||OpenVMS Cluster Goal1||Units|
|Errored seconds (% ES)||<1.0%||<0.4%||<1.0%||<0.028%||% ES/24 hr|
|The ES parameter can also be expressed as a count of errored seconds, as follows:|
|<864||<345||<864||<24||ES per 24-hr period|
|Burst errored seconds (BES) 2||<= 4||--||<= 4||Bellcore Goal||BES/day|
|Bit error ratio (BER) 3||1 x 10 -9||2 x 10 -10||1 x 10 -9||6 x 10 -12||Errored bits/bit|
|DS3 channel unavailability||None||<= 97 @ 250 miles, linearly decreasing to 24 @ <= 25 miles||None||Bellcore Goal||Min/yr|
|SONET channel unavailability||None||<= 105 @ 250 miles, linearly decreasing to 21 @ <= 50 miles||None||Bellcore Goal||Min/yr|
|Channel-unavailable event 4||None||None||None||1 to 2||Events/year|
" ...unavailability or downtime of a channel begins when the first of 10 [or more ] consecutive Severely Errored Seconds (SESs) occurs, and ends when the first of 10 consecutive non-SESs occurs. " The unavailable time is counted from the first SES in the 10--SES sequence. " The time for the end of unavailable time is counted from the first fault-free second in the [non-SES ] sequence. "