HP StorageWorks 2/140 FW 08.01.00 McDATA Products in a SAN Environment Plannin - Page 116

Planning Considerations for Fibre Channel Topologies 3 • I/O block size - The third characteristic of application I/O is data block size, which typically ranges from two kilobytes (KB) to over one megabyte (MB). Applications that generate large blocks of data require high bandwidth to the device. 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. 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 3-12 illustrates ISL oversubscription. 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 1 Gbps ISL NT Server 1 NT Server 2 10 15 20 25 Time (Sec) Figure 3-12 ISL Oversubscription 3-32 McDATA Products in a SAN Environment - Planning Manual