01-30-2013 08:53 AM - edited 08-29-2017 04:59 AM
On SAN environments there is a growing focus on the resultant infrastructure as a key component of the overall storage solution. There has been a gradual shift from the past paradigm of server-centric storage solutions to a storage-centric model and additionally the more recent network-centric model. Data availability, data integrity, and data consistency requirements are critical to users and also contribute to the increased focus on the SAN infrastructure within an IT organization.
Cisco MDS switches offer VSAN technology, which is a simple and secure way to consolidate many SAN islands into a single physical fabric. Separate fabric services and separate role base management are provided for each VSAN, while providing separation of both the control plane and the data plane.
It is common practice in SAN environments to build two separate, redundant physical fabrics (Fabric A and Fabric B) in case a single physical fabric fails. When designing for large networks, most environments will fall into two types of topologies within a physical fabric:
1. Two-tier: Core-edge design
2. Three-tier: Edge-core-edge design
The Core-Edge fabric topology has all the necessary features of a SAN in terms of resiliency and performance. It provides resiliency and predictable recovery, scalable performance, and scalable port density in a well-structured design. The core-edge model is architected with two or more core switches in the center of the fabric to interconnect two or more edge switches in the periphery of the fabric.
When designing a core-edge topology, a major trade-off is made between three key characteristics of the design. The first trade-off is the overall effective port density that can be used to connect hosts or storage devices. For a given number of switches in a core-edge design, the higher the effective port density, typically the higher ISL oversubscription from the edge layer to the core layer. The second characteristic is the oversubscription of the design. Oversubscription is a natural part of any network topology as the nature of a SAN is to 'fan-out' the connectivity and I/O resources of storage devices. However, the higher the oversubscription of the design, the more likely congestion may occur thereby impacting a wide scope of applications and their I/O patterns. The third characteristic that ties these other two together is cost. The rule-of-thumb suggests the 'higher the oversubscription for a given effective port density, the lower the overall cost of the solution'.
All SAN infrastructures incorporate at least one level of oversubscription. The traffic flow model within a SAN is always between SCSI initiators and SCSI targets, in other words between servers and storage. The number of server connections to the SAN will always outnumber those connected to storage devices simply because of the subsystem high storage capacity. Disk subsystem vendors commonly makes recommendations on a fan-out ratio of storage to server ports that can be upward of 12:1. Therefore it is appropriate to have oversubscription ratios for core-edge designs that approach the recommended 12:1 storage to server fan-out ratio.
The collapsed-core fabric topology is a topology with the features of the core-edge topology but delivers required port densities in a more efficient manner. The collapsed-core topology is enabled by high port density offered by the Cisco MDS.
The salient features of the collapsed-core topology are:
Note: SAN designs should always use two isolated fabrics for high availability, with both hosts and storage connecting to both fabrics. Multipathing software should be deployed on the hosts to manage connectivity between the host and storage so that I/O uses both paths, and there is non-disruptive failover between fabrics in the event of a problem in one fabric. Fabric isolation can be achieved using either VSANs, or dual physical switches. Both provide separation of fabric services, although it could be argued that multiple physical fabrics provide increased physical protection and protection against equipment failure.
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