Loops and Data Paths
All devices that are attached to an SSA adapter card are connected through SSA
links. See Figure 3-2 on page 3-60. The SSA links are configured as loops. Data
and commands to a particular device pass through all other devices on the link
between the adapter and the target device.
Data can travel in either direction around a loop. The adapter can, therefore, access
the disk drive modules through two data paths. The using system cannot detect
which data path is being used.
If a disk drive module fails, or is turned off, the loop is broken, and one of the data
paths to a particular module is no longer available. The disk drive modules on the
loop continue to work, but an error is reported to the using system.
Figure 3-3 shows that disk drive module in slot 3 has failed. The disk drive modules
in slots 1 and 2 can communicate with the using system only through connector A1
of the SSA adapter. The disk drive modules in slots 4, 5, and 6 can communicate
only through connector A2 of the SSA adapter.
Figure 3-3. Disk Drive Module in Slot 3 Has Failed
3-62 Service Guide
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