User`s guide

2 – System Overview
Configurations
FS0054601-00 E 2-3
Q
In this configuration, connections marked P are preferred paths to a device, and
connections marked A are alternate paths. Alternate paths to devices are typically
hidden from the operating system and file system layers to keep them from being
interpreted as two distinct devices.
2.2.3
SAN or Cluster Configuration
Figure 2-3 shows a simple storage area network (SAN) configuration consisting of
two hosts (X and Y). Each host has an HBA and is connected to a common set of
five target devices (0–4).
Figure 2-3. SAN or Cluster Configuration
If figure 2-3 represents a SAN of unrelated hosts, each host is configured to
recognize a different set of devices. This configuration can be made at the target
device level (called target masking) where host X is configured to see devices 0–1
and mask devices 2–4, and host Y is configured to see devices 2–4 and mask
devices 0–1. Alternatively, this configuration can be made at the logical unit number
(LUN) level (called LUN masking) where hosts X and Y recognize the devices but
each has a different set of LUNs masked on the device.
If figure 2-3 represents a cluster of two hosts, the hardware setup is the same;
however, for the software configuration, each host is configured to recognize the
same set of devices. Cluster software must determine (arbitrate) which host
accesses which devices at any time. For example, both hosts can be configured to
recognize all targets, but cluster software determines that host X is the preferred
host and owns the devices (unless cluster software determines that host X is down
and host Y takes over).