HP Surestore 64 FW 05.01.00 and SW 07.01.00 HP StorageWorks SAN High Availabil - Page 100
Device Locality, Real-world SANs are
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Planning Considerations for Fibre Channel Topologies Depending on fabric performance requirements and cost, there are several options to solve ISL oversubscription problems, including: ■ Employ device locality - NT server 1 and its associated storage device can be connected through one director. NT server 2 and its associated storage device can be connected through the other director. As a result, minimal traffic flows across the ISL between directors and the congestion problem is mitigated. For additional information, refer to "Device Locality." ■ Install an additional ISL - A second ISL can be installed to balance the traffic load between fabric elements. Two ISLs are sufficient to support the bandwidth of both NT servers operating at peak load. ■ Upgrade the existing ISL - Fabric element software, firmware, and hardware can be upgraded to support a 2.125 Gbps bandwidth traffic load between fabric elements. A 2.125 Gbps ISL is sufficient to support the bandwidth of both NT servers operating at peak load. ■ Deliberately employ ISL oversubscription - Real-world SANs are expected to function well, even with oversubscribed ISLs. Device I/O is typically bursty; few devices operate at peak load for a significant length of time, and device loads seldom peak simultaneously. As a result, ISL bandwidth is usually not fully allocated, even for an oversubscribed link. An enterprise can realize significant cost savings by deliberately designing a SAN with oversubscribed ISLs that provide connectivity for noncritical applications. Device Locality Devices that communicate with each other through the same director or switch have high locality. Devices that must communicate with each other through one or more ISLs have low locality. Part (A) of Figure 42 illustrates high device locality with little ISL traffic. Part (B) of Figure 42 illustrates low device locality. 100 SAN High Availability Planning Guide