FW 05.01.00 and SW 07.01.00 HP StorageWorks SAN High Availability Planning Guide (AA-RS2DC-TE, June 2003)

Planning Considerations for Fibre Channel Topologies
71SAN High Availability Planning Guide
Figure 30: Switched mode operation with eight independent looplets
When communication within two or more looplets ceases, a device attached to
one looplet can be switched to communicate with a device attached to another
looplet.
Downstream devices in a looplet are attached to an unmanaged hub; therefore,
each looplet is operating in normal FC-AL loop mode. However, each looplet
attaches to a switched H_Port and ideally should only support connections and
operations for up to 32 FC-AL devices. Therefore, an arbitrated loop can be
constructed that supports the architectural 125-device limit.
Switched mode operation provides the ability to design a complex and
high-performance SAN for the department or workgroup.
Consider the following when planning such a SAN:
■ Connect loop switch H_Ports to multiple unmanaged hubs to provide
additional FC-AL device connectivity in the form of looplets. Cascade the
unmanaged hubs if more than eight hubs are necessary for the configuration.
■ Attach devices that frequently communicate with each other to the same
looplet to take advantage of opportunistic bandwidth sharing (communication
predominately stays within the loop). Switched connections through the loop
switch allow connectivity as necessary to devices attached to other looplets.
■ Each looplet acts as a normal FC-AL loop. Spread multiple servers and high
bandwidth storage devices across several looplets to avoid performance
problems associated with a single looplet.
HubHub Hub Hub Hub Hub HubHub
H_Ports
S1
D1
S2 S3 S4 S5 S6 S7 S8
D2 D3 D4 D5 D6 D7 D8
H_Ports
Loop Switch