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Replacing storage arrays without downtime

The storage deal is done, the storage array is purchased and now for the fun part: Replace your customer's existing storage array with the new array without downtime. This is no simple task, but contributor Jerome Wendt presents several data migration options.

Jerome Wendt

The deal is done and the storage array order is placed. Now comes the tough part: How to replace your customer's existing storage array with your new storage array without downtime. Accomplishing this feat requires the right combination of preparation and skill, but even under the best circumstances you should prepare your customer to expect outages of varying lengths.

Scenarios do exist where you may avoid needing downtime at all. Such is the case when a customer uses a host-based volume manager. For instance, with a tool like Symantec's Volume Manager, once a volume on the new storage array is presented to the host, the tool discovers the new volume and migrates data from that volume on the existing storage array to the volume on the new storage array by performing a volume mirror. Once the migration is completed, it unmounts the storage on the old storage array.

Network-based storage virtualization appliances and clustered storage arrays are two other options that enable to migrate data without downtime. Network-based storage virtualization appliances operate in a manner similar to host-based volume managers in that they manage storage in the network. Clustered storage arrays enable users to treat all data managed by them as one logical pool of storage.

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In both scenarios, you may introduce new storage into a customer environment and use the volume management capabilities of these technologies to migrate data from an old storage array to the new storage. However, the likelihood of encountering either of these technologies is generally small due to their small customer adoption rate, but with these technologies now sufficiently mature, you should encourage customers to adopt them when appropriate.

This will likely leave you with the task of minimizing the cost and downtime associated with the migration of data from the old to the new storage array. Which approach you select will depend on the following:

Do you want and will your customer give you access to each host server that needs data migrated? A number of host-based utilities, such as IBM's TDMF (formerly Softek), work on multiple operating systems and minimize the disruptive nature of the migration of data from old to new storage arrays. It allows you to centrally manage the data migration but you will need to install an agent on each host and take a brief application outage to redirect the application to the new storage array.

If a host-based data migration is not an option, could you take advantage of a network-based migration utility? Products such as DataCore Software's SANSymphony accesses existing storage on old arrays and migrates the data in real time to the new array. This allows the application to continue to run; once all the data is migrated and mirrored on both systems, you stop the application, point it at the new storage where all of the data resides and resume processing. Your customers might still object to introducing an appliance in the data path. If that's the case, for both the host-based and network-based data migration approaches, someone may have to purchase software to complete the migration.

If the above options are not feasible, would a backup software package be a good option? It really depend on the backup software package your customer employs. For instance, if companies are using CommVault's Galaxy backup software, they may use CommVault's QuickRecovery option in conjunction, which would replicate data to the new volume on the new storage array. Once all of the data is synced, you can stop and redirect the application to the volume on the new storage array with minimal application disruption.

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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