OpenVMS Cluster Systems - OpenVMS Systems - HP

Cluster Storage Devices
6.3 MSCP and TMSCP Served Disks and 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.
6.3.1.2 Setting the MSCP and TMSCP System Parameters
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
6–22 Cluster Storage Devices
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.