Device Fan-Out Ratio; Device Locality - HP StorageWorks 64 - SAN Director Switch Planning Manual

Figure 42: Device locality
Although it is possible to design a SAN that delivers sufficient ISL bandwidth in a
zero-locality environment, it is preferable to design local, one-to-one connectivity
for heavy-bandwidth applications such as video server, seismic data processing, or
medical 3D imaging.
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. IBM FICON devices also must use Tier 1 connectivity. For
additional information, refer to

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.
to 1.
SAN High Availability Planning Guide
High Device Locality
Low Traffic
ISL
Planning Considerations for Fibre Channel Topologies
A
Low Device Locality
High Traffic
"FCP and FICON in a Single
Figure 43 illustrates a fan-out ratio of 10
B
ISL
Fabric" on page 108.
101