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  1. Gather metrics.

jalil falsafi, director of information technology at electronic components distributor Future Electronics Inc. in Montreal, had to migrate data from IBM Corp. DS4100 and DS4300 entry-level arrays to Hewlett-Packard (HP) Co. StorageWorks XP24000 arrays during intervals of relatively slow network traffic over a period of six weeks. That required an in-depth understanding of the capacity of his SAN and when other functions, such as a database backup, would increase network loads.

"You have to scope how many LUNs, or logical disks, you're going to migrate. You have to know their size; you have to know the speed of your array; you have to know the speed of your switch as well as 'hot spots' when traffic loads are very heavy," says Future Electronics' Falsafi. "You need to take the worst-case scenario into consideration, not the average or the minimum."

Falsafi used monitoring tools available in FalconStor Software Inc.'s IPStor network storage server, as well as host- and array-based utilities, to gather those metrics.

"Migration can have a severe impact on overall system performance," says Chris McCall, product marketing

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director at LeftHand Networks Inc. (which is being acquired by HP). "It becomes a fairly nasty issue [with questions such as] 'Is my controller performance maxed out already or close to maxed?'" He warns that overloading a storage or data network with migration traffic can reduce the availability or performance of not only the data being migrated, but all of the data on the network.

Measuring network bandwidth needs before performing a migration is a chore that can be easily overlooked, says Greg Schulz, founder and senior analyst at StorageIO Group, Stillwater, MN. "Unless you know for sure, go out and doublecheck to see what the impact is going to be," he says. Once an administrator is sure how much bandwidth should be allocated to the migration and when it will be available, the bandwidth can be managed with tools such as optimization technologies, replication optimizers and traffic shapers, he adds.

Migration vs. replication
While "migration" AND "replication" are often used interchangeably, their textbook definitions--and the tools required to perform them--are quite different.

Migration means moving data from one platform to another, without leaving the original data in place. It's used when upgrading hardware, moving to a new site, creating a test database, or moving a virtual machine to a new physical server with more processing or network resources.

Replication means creating a second set of data and synchronizing any changes made between the original and the copy so that either set can be used at any time. It's often used for backup and recovery, for continuous data protection (CDP) or in high-availability architectures.

Users may only need their replication tools to support a single vendor's storage arrays. But multivendor support is usually more important for migration tools because the data is often being moved to a different vendor's storage platform.

This was first published in November 2008

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