HP StorageWorks 2/24 FW 07.00.00/HAFM SW 08.06.00 McDATA Products in a SAN Env - Page 118

Planning Considerations for Fibre Channel Topologies 3 • Read/write mixture - Although application I/O is typically a mixture of read and write operations, some applications are very biased. For example, video server applications are almost 100% read intensive, while real-time video editing applications are mostly write intensive. Read operations typically take less time than write operations, therefore storage devices for a read-intensive application usually wait for data transfer. As a consequence, read-intensive applications typically require high bandwidth to the device. • Type of data access - When an application requires data, access to that data is random or sequential. For example, e-mail server activity is random access, while seismic data processing for the oil and gas industry is sequential access. Sequential data access typically takes less time than random data access, therefore sequential-access applications usually wait for data transfer. As a consequence, sequential-access applications typically require high bandwidth to the device. • 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. 3-32 McDATA Products in a SAN Environment - Planning Manual