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Planning storage area network capacity growth

To address ongoing storage capacity issues clients will want to expand their storage area networks (SANs) -- but how to do that is dependent upon various storage networking factors.

Every customer wants the flexibility to start a storage area network (SAN) small and grow it large as storage capacity needs change. Though that makes sense, the best way to help them expand their storage network is not always clear as users may deploy Fibre Channel, IP storage or both types of network topologies. With some differences existing in best practices for growing these two topologies, here are tips to help you help your customers best architect and grow their SANs.

Before growing either type of storage network, start by understanding what your customer hopes to accomplish. EqualLogic Inc., a provider of iSCSI storage arrays, finds its VARs have the most success helping clients grow their SANs to meet storage capacity needs by first understanding what customers want to accomplish. Examples of questions they may ask customers, and the type of information they gather to make appropriate recommendations, include:

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  • How much do you want to spend?
  • Do you know the amount of data your SAN is transporting? If not, can we gather it using standard Ethernet network sniffers?
  • How fast is your SAN growing? (This is measured by establishing how much storage and how many servers the customer is adding.)
  • Do you want to replicate your data locally, remotely or both?

Quantify your customer's class. EqualLogic generally breaks its customers down into two categories when called upon to help customers grow their SANs: those who initially purchased one storage array that connects to multiple servers and those who purchased multiple storage arrays to connect to multiple servers. In the former case, the customer is usually less sophisticated and uses Cisco 3750 stackable switches in a large percentage of its accounts, which are designed to expand. In this case, they can usually just add more switches to the existing iSCSI SAN and dedicate ports for the inter-switch link.

For the larger multistorage array iSCSI SANs, EqualLogic usually starts customers out with a chassis-based switch such as the Cisco Catalyst 6500 series. These have greater processing power, better levels of availability and reliability, and are easier to grow since they are modular in design coming in three, six, nine and 13-slot chassis configurations. This configuration also eliminates the need to dedicate ports for inter-switch links since all of the slots share the same back-end chassis.

Use three-tier architectures for Ethernet SANs and two-tier architectures for FC SANs. Cisco's Data Center Solutions spokesperson Deepak Munjal finds that customers are more tolerant of a three-tier core, distribution and access or edge-switch architecture in an Ethernet SAN, while in FC SANs users prefer to stay in a two-tier core -- edge architecture. Munjal attributes this to the lower tier and application requirements that typically connect to iSCSI SANs, while in FC SANs storage architects prefer to limit the number of hops and ports to minimize latency between servers and storage, and keep the FC SAN as simple as possible to manage.

Properly growing each of these architectures will then become largely dependent on what type of growth is forecast -- server or storage -- and the application performance requirements. For instance, if more storage is needed in a FC SAN but throughput requirements are to remain the same, it might make sense to buy storage arrays with 4 GB FC ports and replace the existing 1 or 2 GB FC ports on the switch with faster 4 GB switch ports. On the other hand, if one needs to connect a high-performance server to a fully populated FC director, it might make the most sense to identify lower performing servers connected to the FC director, buy an edge switch and move those servers to that edge switch, freeing up the core director for the new server with the higher performance requirements.

About the author: Jerome M. Wendt is the founder and lead analyst of The Datacenter Infrastructure Group, an independent analyst and consulting firm that helps users evaluate the different storage technologies on the market and make the right storage decision for their organization.


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