HP Surestore 64 FW 05.01.00 and SW 07.01.00 HP StorageWorks SAN High Availabil - Page 99
ISL Oversubscription, ISL oversubscription, serviced
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Planning Considerations for Fibre Channel Topologies Prior to fabric design, application I/O profiles should be estimated or established that classify the application bandwidth requirements. Bandwidth consumption is classified as light, medium, or heavy. These classifications must be considered when planning ISL and device connectivity. For information about application I/O (in Gbps) and fabric performance problems due to ISL connectivity, refer to "ISL Oversubscription." For information about application I/O (in IOPS) and fabric performance problems due to port contention, refer to "Device Fan-Out Ratio" on page 101. ISL Oversubscription ISL oversubscription (or congestion) occurs when multiplexed traffic from several devices is transmitted across a single ISL. When an ISL is oversubscribed, fabric elements use fairness algorithms to interleave data frames from multiple devices, thus giving fractional bandwidth to the affected devices. Although all devices are serviced, ISL and fabric performance is reduced. Figure 41 illustrates ISL oversubscription. Two NT servers, each with maximum I/O of 100 MBps, are contending for the bandwidth of a single ISL operating at 1.0625 Gbps. In addition to data, the ISL must also transmit Class F traffic internal to the fabric. When operating at peak load, each NT server receives less than half the available ISL bandwidth. Storage TM 1 Gbps ISL 200 Bandwidth (MBps) TM 150 100 50 NT Server 1 NT Server 2 (100 MBps Max) (100 MBps Max) 0 15 Figure 41: ISL oversubscription 1 Gbps ISL NT Server 1 NT Server 2 10 15 20 25 Time (Sec) SAN High Availability Planning Guide 99