FW V06.XX/HAFM SW V08.02.00 HP StorageWorks SAN High Availability Planning Guide (AA-RS2DD-TE, July 2004)

Planning Considerations for Fibre Channel Topologies
100 SAN High Availability Planning Guide
However, when multiple directors or switches are connected through a fabric ISL
that multiplexes traffic from several devices, significant potential for congestion
arises. To minimize congestion, factors such as application I/O profiles, ISL
oversubscription, and device locality must be included in the fabric design.
Application I/O Profiles
Understanding application I/O characteristics is essential to SAN, fabric, and ISL
design. Factors that may affect application I/O include:
■ 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” on page 101. For information about application I/O (in
IOPS) and fabric performance problems due to port contention, refer to “Device
Fan-Out Ratio” on page 103.