45xx Modular Disk Subsystem Manual for G-Series Releases

Introduction
45xx Modular Disk Subsystem Manual for G-Series Releases—142466
1-14
Fault Tolerance
module, different paths exist to the data stored on the primary and mirror disks, as
shown in Figure 1-8. Therefore, if access to one module fails, the system can still access
the other half of the mirrored volume that is configured in another module, thus
preserving operations.
In Figure 1-8
fault tolerance has been improved because the primary and mirror disks
are in separate modules, but this configuration still has a single point of failure. The
primary and mirror disk drives are dependent on a single adapter for communication.
To eliminate this single point of failure, two more adapters are needed. Four adapters
provides maximum fault tolerance, because the primary disk and its mirror are installed
in separate disk modules and each SAC communicating to the primary and mirror disk
drives (including the backup paths to these disk drives) are installed in separate adapters
(See Figure 1-9
).
Figure 1-8. Two-Adapter Configuration
022CDT .CDD
BEB
BEB
BEB
BEB
SAC 4
SAC 3
SAC 2
SAC 1
6760 Adapter
SAC 4
SAC 3
SAC 2
SAC 1
6760 Adapter
Mirror Backup
Mirror
Primary
Backup
45xx Disk Module
45xx Disk Module