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  1. Thin provisioning.

thin provisioning helps preserve storage space by only taking up space on a disk when data is actually written to it, not when the volume is first set aside for use by an application or user. This eliminates waste when the application or user doesn't wind up needing the disk space. However, many data migration tools write "from block zero through to the very last block" of a volume on the target system regardless of which blocks are actually being used, nullifying the benefits of the thin provisioning a user had applied on the source array, says Sean Derrington, director of storage management and high availability at Symantec Corp.

File-system utilities or host-based volume managers "that are intelligent enough to figure out if the block is being accessed or not" before deciding to write to it can help circumvent this problem, says GlassHouse Technologies' Nadkarni. Block-level migration techniques that are good for preserving the security around data aren't good for preserving thin provisioning, he says, "because they write to the entire volume."

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Migration toolkit
Migration can be done on the host or on the network, at either the block or file level, or on the array itself at the block level. Users can choose from hundreds of tools ranging from simple utilities supplied with storage arrays (most useful for migrating data among the same vendor's arrays) to open-source software or complex suites that could cost thousands of dollars.

Host-based software tools are often effective at migrating data without downtime. Some support only Windows file systems, while others support multiple operating systems at either the file or block level. Among the host-based file-level tools is the open-source rsync, which synchronizes files across Unix systems. Many operating systems already include host-based, block-level migration tools. Among the network-based, file-level migration tools are virtualization appliances such as EMC Corp.'s Rainfinity. Network-based, block-level migration tools include Brocade's Data Migration Manager, an application that runs on Brocade's DCX Backbone high-end switch and can migrate as many as 128 LUNs in parallel at speeds of up to 5TB per hour, according to the vendor.

Among the relatively few players in the array-based block-level migration tools is Hitachi Data Systems' Universal Replicator software, which can migrate data among Hitachi arrays and those from other vendors.

Many vendors use file systems to mask the complexity of moving data among multiple platforms. Among them is Ibrix Inc.'s Ibrix Fusion FileMigrator, which adds data tiering capabilities to its Ibrix Fusion 4.2 file system. FileMigrator, says the company, allows IT administrators to set policies and move data according to usage patterns. FileMigrator "addresses a huge pain point" by performing data migration "as a background process under the covers based on policies," says Terri McClure, an analyst at Enterprise Strategy Group, Milford, MA.

This was first published in November 2008

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