IBM 7133 D40 Service Manual page 76

If, however, disk drive modules are connected into the link between two SSA
attachments that are in the same using system, those disk drive modules become
isolated if both SSA attachments fail, or are held reset, but remain powered on (see
also "Bypass Note" on page 57). In Figure 35, disk drive modules 13 through 16 have
been connected between the SSA attachments in using system 1. If both attachments
fail, or are held reset, but remain powered on, disk drives 1 through 12 can still
communicate with using system 2. Disk drives 13 through 16, however, cannot
communicate with using system 2, because their data paths are through the SSA
attachments in using system 1. When using system 1 is rebooted, disk drives 13
through 16 remain unavailable for a long time.
Disk
Disk
Figure 35. Disk Drive Modules Isolated by Failing Using System
58
7133 Models D40 and T40 Service Guide
Using System 1
A1 A2 1 B1 B2
Disk Disk Disk
16 15 14 13
Disk Disk Disk
1 2 3
3
B2 B1 A2 A1
Using System 2
2
A1 A2
Disk Disk
12 11
Disk Disk
4 5 6
4
B2 B1
B1 B2
Disk Disk
10 9
Disk Disk
7 8
A2 A1