FW 07.00.00/HAFM SW 08.06.00 McDATA Products in a SAN Environment Planning Manual (620-000124-500, April 2005)

Planning Considerations for Fibre Channel Topologies

3-35

Planning Considerations for Fibre Channel Topologies

When designing a core-to-edge fabric, servers and storage devices

that support such bandwidth-intensive applications should be

attached to core directors as Tier 1 devices. As a best practices policy

(assuming 1.0625 Gbps ISLs), devices that generate a sustained

output of 35 MBps or higher are candidates for Tier 1 connectivity.

FICON devices also must use Tier 1 connectivity. For additional

information, refer to FCP and FICON in a Single Fabric.

Device Fan-Out Ratio The output of most host devices is bursty in nature, most devices do

not sustain full-bandwidth output, and it is uncommon for the output

of multiple devices to peak simultaneously. These variations are why

multiple hosts can be serviced by a single storage port. This device

sharing leads to the concept of fan-out ratio.

Device fan-out ratio is defined as the storage or array port IOPS

divided by the attached host IOPS, rounded down to the nearest

whole number. A more simplistic definition for device fan out is the

ratio of host ports to a single storage port. Fan-out ratios are typically

device dependent. In general, the maximum device fan-out ratio

supported is 12 to 1. Figure 3-14 illustrates a fan-out ratio of 10 to 1.

Figure 3-14 Device Fan-Out Ratio

Performance Tuning When designing or tuning a fabric for performance, it is critical to

understand application I/O characteristics so that:

Interswitch Link

Fabric Connection

1,000 IOPS

10,000 IOPS

1,000 IOPS

Device Fan-Out Ratio: 10 to 1