Managing Serviceguard Nineteenth Edition, Reprinted June 2011

shared SCSI buses, the practical limit on the number of nodes that can be attached to the same
shared bus is four, because of bus loading and limits on cable length. Even in this case, 16 nodes
could be set up as an administrative unit, and sub-groupings of four could be set up on different
SCSI buses which are attached to different mass storage devices.
In the case of non-shared SCSI connections to an XP series or EMC disk array, the four-node limit
does not apply. Each node can be connected directly to the XP or EMC by means of two SCSI
buses. Packages can be configured to fail over among all sixteen nodes. For more about this type
of configuration, see “Point to Point Connections to Storage Devices ” (page 36).
NOTE: When configuring larger clusters, be aware that cluster and package configuration times
as well as execution times for commands such as cmviewcl will be extended. In the man pages
for some commands, you can find options to help to reduce the time. For example, refer to the
man page for cmquerycl (1m) for options that can reduce the amount of time needed for
probing disks or networks.
Active/Standby Model
You can also create clusters in which there is a standby node. For example, an eight node
configuration in which one node acts as the standby for the other seven could easily be set up by
equipping the backup node with seven shared buses allowing separate connections to each of the
active nodes. This configuration is shown in Figure 8.
Figure 8 Eight-Node Active/Standby Cluster
Point to Point Connections to Storage Devices
Some storage devices allow point-to-point connection to a large number of host nodes without
using a shared SCSI bus. An example is shown in Figure 2-11, a cluster consisting of eight nodes
with a SCSI interconnect. The nodes access shared data on an XP or EMC disk array configured
with 16 SCSI I/O ports. Each node is connected to the array using two separate SCSI channels.
Each channel is a dedicated bus; there is no daisy-chaining of the SCSI bus.
36 Understanding Serviceguard Hardware Configurations