Updated: 11 December 1998

OpenVMS Cluster Systems

Changing a device's allocation class changes the device name. A clusterwide reboot ensures that all nodes see the device under its new name, which in turn means that the normal device and file locks remain consistent.

Rebooting an entire cluster when a device name changes is not mandatory. You may be able to reboot only the nodes that share the SCSI bus, as described in the following steps. The conditions under which you can do this and the results that follow are also described.

1. Dismount the devices whose names have changed from all nodes.
   This is not always possible. In particular, you cannot dismount a disk on nodes where it is the system disk. If the disk is not dismounted, a subsequent attempt to mount the same disk using the new device name will fail with the following error:

   %MOUNT-F-VOLALRMNT, another volume of same label already mounted

   
   Therefore, you must reboot any node that cannot dismount the disk.

2. Reboot all nodes connected to the SCSI bus.
   Before you reboot any of these nodes, make sure the disks on the SCSI bus are dismounted on the nodes not rebooting.

   Note OpenVMS ensures that a node cannot boot if the result is a SCSI bus with naming different from another node already accessing the same bus. (This check is independent of the dismount check in step 1.)

   
   After the nodes that are connected to the SCSI bus reboot, the device exists with its new name.
3. Mount the devices systemwide or clusterwide.
   If no other node has the disk mounted under the old name, you can mount the disk systemwide or clusterwide using its new name. The new device name will be seen on all nodes running compatible software, and these nodes can also mount the disk and access it normally.
   Nodes that have not rebooted still see the old device name as well as the new device name. However, the old device name cannot be used; the device, when accessed by the old name, is off line. The old name persists until the node reboots.

The MSCP server and the TMSCP server make locally connected disks and tapes available to all cluster members. Locally connected disks and tapes are not automatically cluster accessible. Access to these devices is restricted to the local computer unless you explicitly set them up as cluster accessible using the MSCP server for disks or the TMSCP server for tapes.

6.3.1 Enabling Servers

To make a disk or tape accessible to all OpenVMS Cluster computers, the MSCP or TMSCP server must be:

• Loaded on the local computer, as described in Table 6-6
• Made functional by setting the MSCP and TMSCP system parameters, as described in Table 6-7

Table 6-6 MSCP_LOAD and TMSCP_LOAD Parameter Settings

Parameter	Value	Meaning
MSCP_LOAD	0	Do not load the MSCP_SERVER. This is the default.
	1	Load the MSCP server with attributes specified by the MSCP_SERVE_ALL parameter using the default CPU load capacity.
	>1	Load the MSCP server with attributes specified by the MSCP_SERVE_ALL parameter. Use the MSCP_LOAD value as the CPU load capacity.
TMSCP_LOAD	0	Do not load the TMSCP server and do not serve any tapes (default value).
	1	Load the TMSCP server and serve all available tapes, including all local tapes and all multihost tapes with a matching TAPE_ALLOCLASS value.

Table 6-7 summarizes the system parameter values you can specify for MSCP_SERVE_ALL and TMSCP_SERVE_ALL to configure the MSCP and TMSCP servers. Initial values are determined by your responses when you execute the installation or upgrade procedure or when you execute the CLUSTER_CONFIG.COM command procedure described in Chapter 8 to set up your configuration.

Starting with OpenVMS Version 7.2, the serving types are implemented as a bit mask. To specify the type of serving your system will perform, locate the type you want in Table 6-7 and specify its value. For some systems, you may want to specify two serving types, such as serving the system disk and serving locally attached disks. To specify such a combination, add the values of each type, and specify the sum.

Table 6-7 MSCP_SERVE_ALL and TMSCP_SERVE_ALL Parameter Settings

Parameter	Bit	Value When Set	Meaning
MSCP_SERVE_ALL	0	1	Serve all available disks (locally attached and those connected to HS x and DSSI controllers). Disks with allocation classes that differ from the system's allocation class (set by the ALLOCLASS parameter) are also served if bit 3 is not set.
	1	2	Serve locally attached (non-HS x and non-DSSI) disks. The server does not monitor its I/O traffic and does not participate in load balancing.
	2	4	Serve the system disk. This is the default setting. This setting is important when other nodes in the cluster rely on this system being able to serve its system disk. This setting prevents obscure contention problems that can occur when a system attempts to complete I/O to a remote system disk whose system has failed.
	3	8	Restrict the serving specified by bit 0. All disks except those with allocation classes that differ from the system's allocation class (set by the ALLOCLASS parameter) are served. This is pre-Version 7.2 behavior. If your cluster includes systems running Open 7.1- x or earlier, and you want to serve all available disks, you must specify 9, the result of setting this bit and bit 0.
TMSCP_SERVE_ALL	0	1	Serve all available tapes (locally attached and those connected to HS x and DSSI controllers). Tapes with allocation classes that differ from the system's allocation class (set by the ALLOCLASS parameter) are also served if bit 3 is not set.
	1	2	Serve locally attached (non-HS x and non-DSSI) tapes.
	3	8	Restrict the serving specified by bit 0. Serve all tapes except those with allocation classes that differ from the system's allocation class (set by the ALLOCLASS parameter). This is pre-Version 7.2 behavior. If your cluster includes systems running OpenVMS Version 7.1- x or earlier, and you want to serve all available tapes, you must specify 9, the result of setting this bit and bit 0.

Although the serving types are now implemented as a bit mask, the values of 0, 1, and 2, specified by bit 0 and bit 1, retain their original meanings. These values are shown in the following table:

Value	Description
0	Do not serve any disks (tapes). This is the default.
1	Serve all available disks (tapes).
2	Serve only locally attached (non-HS x and non-DSSI) disks (tapes).

6.3.1.1 Serving the System Disk

Setting bit 2 to serve the system disk is important when other nodes in the cluster rely on this system being able to serve its system disk. This setting prevents obscure contention problems that can occur when a system attempts to complete I/O to a remote system disk whose system has failed.

The following sequence of events describes how a contention problem can occur if serving the system disk is disabled (that is, if bit 2 is not set):

• The MSCP_SERVE_ALL setting is changed to disable serving when the system reboots.
• The serving system crashes.
• The client system that was executing I/O to the serving system's system disk is holding locks on resources of that system disk.
• The client system starts mount verification.
• The serving system attempts to boot but cannot because of the locks held on its system disk by the client system.
• The client's mount verification process times out after a period of time set by the MVTIMEOUT system parameter, and the client system releases the locks. The time period could be several hours.
• The serving system is able to reboot.

Use either of the following methods to set these system parameters:

• Specify appropriate values for these parameters in a computer's MODPARAMS.DAT file and then run AUTOGEN.
• Run the CLUSTER_CONFIG.COM or the CLUSTER_CONFIG_LAN.COM procedure, as appropriate, and choose the CHANGE option to perform these operations for disks and tapes.

With either method, the served devices become accessible when the serving computer reboots. Further, the servers automatically serve any suitable device that is added to the system later. For example, if new drives are attached to an HSC subsystem, the devices are dynamically configured.

Note: The SCSI retention command modifier is not supported by the TMSCP server. Retention operations should be performed from the node serving the tape.

6.4 MSCP I/O Load Balancing

MSCP I/O load balancing offers the following advantages:

• Faster I/O response
• Balanced work load among the members of an OpenVMS Cluster

Two types of MSCP I/O load balancing are provided by OpenVMS Cluster software: static and dynamic. Static load balancing occurs on both VAX and Alpha systems; dynamic load balancing occurs only on VAX systems. Both types of load balancing are based on the load capacity ratings of the server systems.

6.4.1 Load Capacity

The load capacity ratings for the VAX and Alpha systems are predetermined by Compaq and are shown in Table 6-8. The ratings assume that the configuration uses one Ethernet adapter (of the fastest supported type) and that disk I/O is not a bottleneck.

These ratings are used in the calculation of the available serving capacity for MSCP static and dynamic load balancing. You can override these default settings by specifying a different load capacity with the MSCP_LOAD parameter.

Table 6-8 MSCP Load-Capacity Ratings Based on Ethernet

CPU Type	Load Capacity (I/O per second)	Comments
Alpha	340	All Alpha CPUs use the same default rating.
VAX	340	All VAX CPUs not listed elsewhere in this table use this default rating.
MicroVAX II	80
MicroVAX 3300, 3400	130	CPU limited (embedded adapter).
MicroVAX 3500, 3600, 3800, 3900	120	DELQA limited.
VAXstation 2000, MicroVAX 2000	20
VAXstation 3100, MicroVAX 3100, VAXstation 3520, VAXstation 3540, VAXft 3000, VAXstation 4000--60, VAX 4000--200	45
VAXstation 4000--90, VAX 4000--300	325	CPU limited.
MicroVAX 3100--90, VAX 4000--100, VAX 4000--400, VAX 4000--600	400	CPU limited.
VAX 11/750	45
VAX 11/780	70
VAX 11/785, VAX 8600, VAX 8650	100	Assume a DELUA.
VAX 8200, VAX 8250, VAX 8300, VAX 8350	60
VAX 85 nn, VAX 8700, VAX 88 nn	340	Assume a DEBNI.
VAX 6000--200, VAX 6000--300	200	CPU limited.
VAX 6000--400, VAX 6000--500, VAX 6000--600, VAX 7000, VAX 9000, VAX 10000	400	Assume a DEMNA.

Note that the MSCP server load-capacity values (either the default value or the value you specify with MSCP_LOAD) are estimates used by the load-balancing feature. They cannot change the actual MSCP serving capacity of a system.

A system's MSCP serving capacity depends on many factors including its power, the performance of its LAN adapter, and the impact of other processing loads. The available serving capacity, which is calculated by each MSCP server as described in Section 6.4.3, is used solely to bias the selection process when a client system (for example, a satellite) chooses which server system to use when accessing a served disk.

6.4.2 Increasing the Load Capacity When FDDI is Used

When FDDI is used instead of Ethernet, the throughput is far greater. To take advantage of this greater throughput, Compaq recommends that you change the server's load-capacity default setting with the MSCP_LOAD parameter. Start with a multiplier of four. For example, the load-capacity rating of any Alpha system connected by FDDI to a disk can be set to 1360 I/O per second (4x340). Depending on your configuration and the software you are running, you may want to increase or decrease this value.

6.4.3 Available Serving Capacity

The load-capacity ratings shown in Table 6-8 (or the value you specified with the MSCP_LOAD parameter) are used by each MSCP server to calculate its available serving capacity.

The available serving capacity is calculated in the following way:

Step	Calculation
1	Each MSCP server counts the read and write requests sent to it and periodically converts this value to requests per second.
2	Each MSCP server subtracts its requests per second from its load capacity (shown in Table 6-8) to compute its available serving capacity.

6.4.4 Static Load Balancing

MSCP servers periodically send their available serving capacities to the MSCP class driver (DUDRIVER). When a disk is mounted or one fails over, DUDRIVER assigns the server with the highest available serving capacity to it. (TMSCP servers do not perform this monitoring function.) This initial assignment is called static load balancing.

6.4.5 Dynamic Load Balancing (VAX Only)

Dynamic load balancing occurs only on VAX systems. MSCP server activity is checked every 5 seconds. If activity to any server is excessive, the serving load automatically shifts to other servers in the cluster.

6.4.6 Overriding MSCP I/O Load Balancing for Special Purposes

In some configurations, you may want to designate one or more systems in your cluster as the primary I/O servers and restrict I/O traffic on other systems. You can accomplish these goals by overriding the default load-capacity ratings used by the MSCP server. For example, if your cluster consists of two Alpha systems and one VAX 6000-400 system and you want to reduce the MSCP served I/O traffic to the VAX, you can assign a low MSCP_LOAD value, such as 50, to the VAX. Because the two Alpha systems each start with a load-capacity rating of 340 and the VAX now starts with a load-capacity rating of 50, the MSCP served satellites will direct most of the I/O traffic to the Alpha systems.

6.5 Managing Cluster Disks With the Mount Utility

For locally connected disks to be accessible to other nodes in the cluster, the MSCP server software must be loaded on the computer to which the disks are connected (see Section 6.3.1). Further, each disk must be mounted with the Mount utility, using the appropriate qualifier: /CLUSTER, /SYSTEM, or /GROUP. Mounting multiple disks can be automated with command procedures; a sample command procedure, MSCPMOUNT.COM, is provided in the SYS$EXAMPLES directory on your system.

The Mount utility also provides other qualifiers that determine whether a disk is automatically rebuilt during a remount operation. Different rebuilding techniques are recommended for data and system disks.

This section describes how to use the Mount utility for these purposes.

6.5.1 Mounting Cluster Disks

To mount disks that are to be shared among all computers, specify the MOUNT command as shown in the following table.

IF...	THEN...
At system startup
The disk is attached to a single system and is to be made available to all other nodes in the cluster.	Use MOUNT/CLUSTER device-name on the computer to which the disk is to be mounted. The disk is mounted on every computer that is active in the cluster at the time the command executes. First, the disk is mounted locally. Then, if the mount operation succeeds, the disk is mounted on other nodes in the cluster.
The computer has no disks directly attached to it.	Use MOUNT/SYSTEM device-name on the computer for each disk the computer needs to access. The disks can be attached to a single system or shared disks that are accessed by an HS x controller. Then, if the mount operation succeeds, the disk is mounted on the computer joining the cluster.
When the system is running
You want to add a disk.	Use MOUNT/CLUSTER device-name on the computer to which the disk is to be mounted. The disk is mounted on every computer that is active in the cluster at the time the command executes. First, the disk is mounted locally. Then, if the mount operation succeeds, the disk is mounted on other nodes in the cluster.

To ensure disks are mounted whenever possible, regardless of the sequence that systems in the cluster boot (or shut down), startup command procedures should use MOUNT/CLUSTER and MOUNT/SYSTEM as described in the preceding table.

Note: Only system or group disks can be mounted across the cluster or on a subset of the cluster members. If you specify MOUNT/CLUSTER without the /SYSTEM or /GROUP qualifier, /SYSTEM is assumed. Also note that each cluster disk mounted with the /SYSTEM or /GROUP qualifier must have a unique volume label.

6.5.2 Examples of Mounting Shared Disks

Suppose you want all the computers in a three-member cluster to share a disk named COMPANYDOCS. To share the disk, one of the three computers can mount COMPANYDOCS using the MOUNT/CLUSTER command, as follows:

$ MOUNT/CLUSTER/NOASSIST $1$DUA4: COMPANYDOCS

If you want just two of the three computers to share the disk, those two computers must both mount the disk with the same MOUNT command, as follows:

$ MOUNT/SYSTEM/NOASSIST $1$DUA4: COMPANYDOCS

To mount the disk at startup time, include the MOUNT command either in a common command procedure that is invoked at startup time or in the computer-specific startup command file.

Note: The /NOASSIST qualifier is used in command procedures that are designed to make several attempts to mount disks. The disks may be temporarily offline or otherwise not available for mounting. If, after several attempts, the disk cannot be mounted, the procedure continues. The /ASSIST qualifier, which is the default, causes a command procedure to stop and query the operator if a disk cannot be mounted immediately.

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