Specifications
© IBM Copyright, 2012 Version: January 26, 2012
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Summary of Best Practices for Storage Area Networks
one inter-switch link (ISL) between the two switches. Since this configuration is
repeated for multiple storage ports, there can be more and more servers communicating
across a single common ISL. The ratio of total server bandwidth to ISL bandwidth is
known as the ISL oversubscription.
For example, there are eight servers connecting to a common storage port traversing an
ISL. Each server has a 2 Gbps port connection while the ISL is running at 4 Gbps. The
calculated ISL oversubscription would be:
(8 servers * 2 Gbps/server) / 4 Gbps (ISL) = 16 / 4 = 4 for a 4:1 ISL oversubscription
ratio.
Switch vendors typically offer guidelines for ISL oversubscription ratios between 7:1 to
10:1. In cases where the traffic levels and patterns are not well understood, this range
is a good best practices rule of thumb. However, SAN administrators can make use of
higher oversubscription ratios with a good understanding of the traffic pattern utilizing a
particular ISL. Ratios significantly greater than 10:1 can be incorporated into the SAN
environment.
One such example is a ratio which on paper appears to be 20:1; but the servers sharing
the common ISL are evenly divided between servers running heavy database
operations during normal working hours, data warehousing applications during the early
evening and finally back-up operations late evening. As long as these three workloads
have no, or minimal, overlap during the same time of day; the higher oversubscription
ratio is not a true representation of the projected ISL load factor. Thus, the range of 7:1
to 10:1 is a good generic best practice statement in cases of unknown workloads, yet
not a hard and fast rule which can’t be exceeded when the traffic pattern is known.
When expanding a fabric by adding a new switch, or replacing an existing switch, make
sure the new switch does not have any zoning configuration. During the fabric merge
operations, the new switch will learn the fabric’s existing zoning configuration from its
switch neighbors. Attempting to introduce one or a few new zones to the current
zoneset can be problematic including successful merger, switch segmentation,
replacing the established zoneset with the newer (and typically much smaller) zoneset
or other equally disruptive results.