FW 05.01.00 and SW 07.01.00 HP StorageWorks SAN High Availability Planning Guide (AA-RS2DC-TE, June 2003)

Planning Considerations for Fibre Channel Topologies
98 SAN High Availability Planning Guide
I/O Requirements
HP directors and switches are designed with non-blocking architecture; therefore,
any two switch ports can communicate at the full Fibre Channel bandwidth of
2.125 Gbps without impact to other switch ports. Because most SAN-attached
devices are not capable of generating I/O traffic at the full bandwidth, there is little
potential for congestion between two devices attached through a single director or
switch.
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.