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Cluster Planning 17
SSA subsystems are built up from loops of adapters and disks. A simple
example is shown in Figure 1.
Figure 1. Basic SSA Configuration
Here, a single adapter controls one SSA loop of eight disks. Data can be
transferred around the loop, in either direction, at 20 MBps. Consequently,
the peak transfer rate of the adapter is 80 MBps. The adapter contains two
SSA nodes and can support two SSA loops. Each disk drive also contains a
single SSA node. A node can be either an initiator or a target. An
initiator
issues commands, while a
target
responds with data and status information.
The SSA nodes in the adapter are therefore initiators, while the SSA nodes in
the disk drives are targets.
There are two types of SSA Disk Subsystems for RISC System/6000
available:
• 7131 SSA Multi-Storage Tower Model 405
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HOST
SSA Architecture
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High performance 80 MB/s interface
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Loop architecture with up to 127 nodes per loop
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Up to 25 m (82 ft) between SSA devices with copper cables
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Up to 2.4 km (1.5 mi) between SSA devices with optical extender
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Spatial reuse (multiple simultaneous transmissions)