IBM Midrange System DS4000 Series Hardware Manual page 78

The DS5000 storage subsystem supports eight redundant drive channel pairs on which to
place expansion enclosures. Having many drive channel pairs allows you to achieve high
bandwidth and linear scalability. The FC-AL standard allows you to connect 127 devices
together into a loop. It is important not to have a fully utilized FC-AL to fulfill the linear
performance scalability requirement. The DS5000 storage subsystem allows attachment of
448 drives. Many disk loops bring the DS5000 system to an unprecedented level of
availability, with the ability to spread out disk enclosures over eight back-end drive channels.
For 128 and fewer drives, you can have a dedicated disk loop for each disk enclosure.
Ports 1 and 2 on each controller are grouped together in one drive channel group. Similarly,
ports 3 and 4, 5 and 6, and 7 and 8 are grouped together in other drive channel groups. If you
look at the rear of a properly installed DS5000 storage subsystem, you will see them clearly
labeled. In this case:
Ports 8 and 7 on controller A are channel group 1.
Ports 6 and 5 on controller A are channel group 2.
Ports 4 and 3 on controller A are channel group 3.
Ports 2 and 1 on controller A are channel group 4.
Ports 1 and 2 on controller B are channel group 5.
Ports 3 and 4 on controller B are channel group 6.
Ports 5 and 6 on controller B are channel group 7.
Ports 7 and 8 on controller B are channel group 8.
The two ports on each drive channel group must run at the same speed. There is no blocking
between the two adjacent ports at the drive channel group level. It is best to spread out the
drive-side channel pairs among the channel groups to ensure maximum availability. Each
channel (two ports) is internally connected to a 4 Gbit FC port in Controller A and Controller
B. Each controller has two quad ports disk chips for disk connections. One chip is dedicated
for local ports, with one link per channel. The second one for remote ports (in the second
controller) has one link per channel. Refer to Figure 3-6 on page 32 and Figure 3-10 on
page 36 for architectural details.
A drive-side channel pair is made up of one port from each controller, going left to right. For
example, drive channel pair 1 is composed of controller A, port 8, and controller B, port 1.
Drive channel pair 2 is composed of controller A, port 7, and controller B, port 2, and so on.
The recommended pairing is driven by the connections between FC loop switches within
controllers A and B. For attaching the first four enclosures, we recommend using one port of
each channel. Because of easier management, performance, and availability, we propose
connecting channel pairs in the following order (refer to Figure 3-33 on page 61):
1. Disk port 8 controller A <-> disk port 1 controller B
2. Disk port 6 controller A <-> disk port 3 controller B
3. Disk port 4 controller A <-> disk port 5 controller B
4. Disk port 2 controller A <-> disk port 7 controller B
5. Disk port 7 controller A <-> disk port 2 controller B
6. Disk port 5 controller A <-> disk port 4 controller B
7. Disk port 3 controller A <-> disk port 6 controller B
8. Disk port 1 controller A <-> disk port 8 controller B
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IBM Midrange System Storage Hardware Guide