Availability - Brocade Communications Systems SilkWorm 12000 Design, Deployment And Management Manual

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SilkWorm 12000 Based SAN Designs
Scenario "C" (Edge Attach) is the typical case. The number of available paths between the host and
storage is equal to the number of ISLs used to attach the edge to the core. This greatly improves
performance for devices that are located on different switches and that need to communicate with each
other (i.e. zero locality). In addition, the core switch ports are available for increasing the size of the
SAN by adding new edge switches. Further, all edge ports offer equivalent over-subscription, allowing
any device to attach to any edge port. This can ease installation, while maintaining high performance.
Note:
Figure 3-4

Availability

A SAN or application is only as available as the weakest link. To build a highly available SAN-based
computer system it is not sufficient to only have a highly available SAN. It is necessary to account for
availability throughout the entire computer system: dual HBAs, multi-pathing software, highly available
and multi-ported and/or completely duplicated storage subsystems, and clustering software are some of
the components that may make up such a system.
When building a highly available SAN, use highly available and redundant components. One of anything
'is not highly available. Webster's dictionary provides the following definition for the word redundant:
Redundant: serving as a duplicate for preventing failure of an entire system (as a spacecraft) upon
failure of a single component
As the size and scope of SANs increase and businesses integrate SANs into their entire computing
infrastructure, the risk in relying on a single fabric becomes unacceptable. Human error, natural disaster,
software failure, deliberate sabotage, or a combination of unforeseen events can cause the failure of a
single fabric. Using redundant fabrics (see Figure 3-5) increases the level of availability significantly, as
redundant fabrics mitigate human error, software failure, or catastrophe.
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Attaching devices to a core switch limits scalability and can limit performance.
Device Placements Impacts Scalability in a Core/Edge Fabric
Publication Number 53-0000251-02