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A Fibre Channel domain is a collection of switch ports that function as a single entity. A single FC switch or director is an example of a domain with a unique domain ID identifier (DID). FC switches can have up to 256 ports in a single domain with fabric loop ports (FL_Ports) supporting up to 256 additional sub-addresses. Multiple domains (up to 239) can be interconnected using one or more ISLs to create a fabric. Check with your switch and storage vendors to verify how many domains and which domain numbers they support in a single fabric, as well as the maximum number of switches and ISLs supported.
Switches with more than 256 ports overcome the 256 ports per-domain addressing constraint by using logical domains to partition a physical switch into smaller virtual switches and virtual SANs. Dividing a larger switch into multiple logical switches can help to simplify management. But there are still management issues related to supporting the various logical domains and switches even when multiple physical switches are consolidated into logical domains on a single large switch.
With some vendor's switches, it's possible to run different versions of firmware in each partition. This can be helpful for backwards compatibility with older devices when consolidating physical switches. But be aware that simply physically consolidating switches doesn't necessarily result in consolidated management.
Tipping the scales
Vertical scaling by adding port count, and networking switches together into fabrics to scale horizontally, aren't mutually exclusive. A combination of vertical and horizontal scaling can be used to meet your specific application and environment needs. As the cost of FC adapters and ports continues to drop, more environments will be able to improve redundancy by adding secondary paths with extra adapters and switches. Scaling also enables tiered storage access, which is an important part of a flexible data infrastructure for organizations moving toward an information lifecycle management environment. Ultimately, the scaling approach and topology that's best for your needs is one that adapts to your environment with a minimal amount of maintenance and support. As a best practice for high availability and accessibility, deploy storage networks using redundant paths and separate fabrics for servers that need high performance or uninterrupted access to storage resources.
This was first published in February 2005