SAN consolidation strategies


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Storage array consolidation
Storage arrays are the core component of your SAN. They're home to your data and applications, and the purpose of a SAN is to network them into larger, more manageable and scalable storage pools. Array consolidation attempts to combine isolated arrays in SAN islands into fewer, larger physical or virtual arrays under a single management umbrella, creating a central storage pool from which LUNs get assigned to servers.

The size of the LUNs you provision is an important strategy consideration. On one hand, LUNs shouldn't be too large because bigger LUNs decrease the overall storage utilization of your SAN. But if the LUNs are too small, there may be too many LUNs, which can adversely impact array performance and make storage management more complex. To make matters worse, many arrays don't support--or array vendors may not recommend--resizing LUNs, which requires assigning additional LUNs to servers each time server volumes need to be extended. Although it largely depends on the specific environment, the "sweet spot" for the size of assigned LUNs appears to lie between 32GB and 128GB.

The easiest first step to SAN consolidation is to upgrade old arrays that are difficult to expand. Taking array utilization, projected data growth and support costs into account, upgrading may be the most economical option, especially if you have fully appreciated legacy arrays that can be combined into fewer, better performing,

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easier and more economically managed units. When contemplating storage array upgrades, you should also consider the aggregate savings related to the power and rack space gained by squeezing more data into smaller arrays with larger drive sizes.

Among your array consolidation options, you should consider taking advantage of larger drive sizes by replacing smaller sized drives with higher capacity drives and combining high-performance drives with slower high-capacity drives within the arrays. The larger your consolidated SAN grows, the more important it is to have more than one storage tier. In corporate reality, there will always be department heads and project managers who are eager to deploy their own little SAN islands rather than using and paying for centrally allocated storage. For a centralized storage strategy to succeed, being able to offer high performance and low-cost, large-capacity LUNs to your users is imperative. In fact, without the flexibility of multiple storage tiers, small storage islands will proliferate and a centralized corporate storage strategy is more likely to fail.

The University of Minnesota's Follstad was fully aware of this, and decided to offer his users three tiers of storage: Tier-1 storage using EMC Symmetrix DMX arrays with FC drives, Tier-2 storage on Clariion arrays with FC drives and Tier-3 storage on Clariion arrays with SATA drives. With Tier-3 storage internally priced at 25% of Tier-2 storage and one-sixteenth of Tier-1 storage, Follstad can compete with the attractive acquisition cost of isolated SAN islands. "Tier-3 LUNs comprise almost 50% of the available SAN capacity and are popular for archiving, backups and research data," he reports.

This was first published in December 2006

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