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Vertical and horizontal scaling

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Vertical scaling is accomplished by replacing components with greater resources, such as switches with higher port counts.

Horizontal scaling builds out a storage network with additional switches, inter-switch links (ISLs) and so forth.
Storage area networks (SANs) must be able to scale as they grow to meet burgeoning storage demands. There are various techniques for scaling SANs from a switch and fabric perspective. The best technique is one that satisfies current storage requirements and accommodates future growth.

The basic building block of a switched Fibre Channel (FC) storage network is the FC switch, which ranges in size from a few ports to hundreds of ports. Similar to Ethernet-based networks, an FC storage network (fabric) can be implemented using a single switch or as a fabric network with multiple switches interconnected using inter-switch links (ISLs). We'll focus on scaling individual FC SAN fabrics to add ports to support more servers and storage, and to improve performance.

While there are various techniques for scaling FC SANs, the basic tools available to storage managers are vertical and horizontal scaling (see Options for scaling Fibre Channel storage networks). Deciding on which approach to use depends on the performance and availability needs of the servers you are attaching to and, to a lesser extent, on the physical distribution of your infrastructure and your ability to manage complexity.

In general, if you have a group of servers and storage located relatively close to each other, they could be attached to a high port-count switch (vertical scaling). For example, using high port-count switches deployed in pairs for redundancy, you could support from 128 to 256 server, storage and ISL ports. Horizontal scaling will likely be more appropriate when dealing with groups of servers at different locations, or when you need more ports than the number a single switch can support. For example, if you have multiple servers and storage located throughout a building, in a campus or across a metropolitan environment, horizontal scaling is more effective.

Vertical Scaling
Vertical scaling--sometimes called "scaling up"--involves increasing resources, such as ports, storage capacity and bandwidth. In its simplest terms, vertical scaling implies making a resource more powerful or larger, such as by adding a switch with greater bandwidth or more ports (see Vertical and horizontal scaling, this page).

Vertical scaling is used to consolidate resources to reduce per-unit costs, simplify management and support, and better utilize resources. Another characteristic of vertically scaled devices is the ability to logically and physically subdivide resources into logical partitions or domains. For example, current generation FC switches from Brocade Communications Systems Inc., Cisco Systems Inc., Computer Network Technology Corp. (recently acquired by McData) and McData Corp. allow for the creation of separate or logical domains.

One benefit of using a vertically scaled large switch is the ability to consolidate ports from many smaller switches into a single, larger device to simplify management, maintenance and configuration, and to reduce the number of ISLs. The downside is the potential lack of resiliency. Consequently, vertically scaled switches should be deployed in pairs in separate fabrics.

Horizontal Scaling
Horizontal scaling generally involves building out a fabric and networking switches together on a local or wide-area basis to increase the number of ports beyond what a single switch offers. Horizontal scaling may also be used to meet the requirements of distributed or geographically dispersed SANs. Horizontally scaled SANs use ISLs to establish links among the various switches.

With horizontal scaling, the high port-count switches provide a high degree of locality; with so many ports adjacent to each other, the need to make hops across an ISL is eliminated. This reduces latency and can improve performance while reducing SAN complexity. But congestion can still occur within a switch due to head-of-line blocking that slows traffic by blocking access to switch ports (see Head-of-line blocking). Congestion might also occur with oversubscribed ports.

Horizontal scaling based on cascade, ring and core/edge topologies use ISLs to provide redundancy, and to scale out to large numbers of ports and higher bandwidth (see Switch topologies). These topologies can be implemented to meet different needs for SAN scaling. They can also be used to connect SAN islands built around individual switches into larger, single fabrics or by using routers to physically connect the switches while logically isolating the fabrics.

This was first published in February 2005

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