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4.5 Multiple Initiators and Clustering
A multiple-initiator environment is one in which the SCSI bus has two or more host
adapters. To work in a cluster, the host adapters must perform properly in such an
environment—by not resetting the bus without an explicit host command, for
example. Our Ultra2 LVD SCSI Host Adapters are built for multi-initiator
operation and adhere strictly to the SCSI standard.
Figure 4-6 on the next page illustrates the principal components in a cluster's
I/O system. It shows a simple cluster with two servers sharing access to online
storage (the basic model for Microsoft's MSCS clusters). Both servers contain an
host adapter that provides access to the shared extemal I/O bus. Servers can share
disks or arrays attached to this bus. The operating system controls contention for
disk access by either resolving access conflicts with a component called a
distributed lock manager (DLM) or using a so-called "shared-nothing" model that
partitions disks and arrays, giving each server control over a subset of the storage
under normal circumstances. MSCS uses the shared-nothing approach.
In such applications, multiple initiators can be used on a single SCSI bus. The
bus is configured to have more than one host adapter inserted into at least two
distinctly different computer systems sharing peripheral devices. In these
circumstances, it is possible to have one of the two computer systems turned off.
Under these conditions, the host adapter has a jumper that can be enabled
(closed) to set the active on-board termination ON all the time, regardless of
whether power is applied or not (refer to Section 3.4). The host adapter is shipped
without the jumper installed, and this setting need not be changed in most cases.
Before operating such a system, visually inspect the configuration you have
installed to verify that all cables have been inserted correctly.
CHAPTER 4: Installation and Configuration
29
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