HP StorageWorks 64 FW 05.01.00 and SW 07.01.00 HP StorageWorks SAN High Availa - Page 101
Device locality, Device Fan-Out Ratio, FCP and FICON in a Single Fabric,
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Planning Considerations for Fibre Channel Topologies High Device Locality A Low Traffic TM TM ISL Low Device Locality B High Traffic TM TM ISL 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 "FCP and FICON in a Single Fabric" on page 108. 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 43 illustrates a fan-out ratio of 10 to 1. SAN High Availability Planning Guide 101