Data migration: Proceed with caution


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What are your migration objectives?
Host-level, block-based data migration software products vary in functionality and capabilities.

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Knowing which product to choose requires a solid definition of the objectives you're trying to achieve. Here are some questions to ask to select the best product for your environment:

How much downtime is acceptable? Softek Storage Solutions Corp.'s Transparent Data Migration Facility (TDMF) requires no application downtime or quiescence when doing local volume migrations on Unix operating systems. Symantec Corp.'s VolumeManager and Veritas Volume Replicator products may allow users to avoid downtime if VolumeManager is already installed on the server and the synchronous mirror options are turned on. All other block-based migrations require some application downtime, if only for a few seconds.

How old is the operating system? One of the problems with using host-level data migration products is their lack of support for older operating systems.

Do you need or want the option to recover quickly from the source disk? All products don't offer the same degree of ability to fall back to the source volume.

Is a central console needed to manage data migrations across multiple servers? Users who deploy Topio Inc.'s Topio Data Protection Suite (TDPS) will need to ensure that they have a Windows host available to manage migrations on Unix servers.

Do you need to control the data migration from a local server? Only Softek and Symantec provide a command-line interface, and allow users to control and manage the data migrations from the host on which the agent is running.

These products are used to complete the block-level movement of all data from one locally attached storage device to another, or from one server to a remote server. But don't expect them to totally eliminate the need for the application outages or server reboots that data migrations typically require. Differences in how they're installed and configured, and how they move data to the target volume, fall back to the source volume in the event of problems, and manage data flow and integrity during the migration are all factors to consider.

Softek recently conducted a data migration survey of 700 users and found that the most frequent reasons for data migrations were to replace storage or server equipment (38%), storage and server consolidations (17%) and the relocation of data (10%). The study also revealed that 39% of these users perform migrations on a weekly or monthly basis. To accomplish these types of data migrations with few or no application outages and minimal administrative intervention, block-based data migration tools support the following features to varying degrees:

  • Support for different protocols to allow for local and distance data migrations
  • Ability to throttle or control data flows between servers
  • Integrity checking to verify the consistency of the data during migration
  • Ability to fail back to the original storage device
  • A central interface to manage data migrations on different host systems

Installation and configuration
Users who opt for either Symantec's Veritas Storage Foundation or Veritas Volume Replicator to migrate data require an installed version of Symantec's VolumeManager (part of Veritas Storage Foundation). Softek's TDMF and TDMF-IP, and Topio's TDPS install a little differently. All of them require server reboots on Windows systems after installation because the product drivers are similar to antivirus programs, which means they install as filter drivers on Windows systems. However, unlike antivirus filter drivers that reside above the file system, these products sit below the file system and above Windows volume manager to capture and copy write I/Os.

On Unix operating systems, these three products don't require server reboots because they don't make modifications to the Unix kernel. These products take advantage of the better support Unix kernels provide for dynamically loaded drivers. For instance, on HP-UX systems, they use the modload command that's part of HP-UX's Dynamically Loadable Kernel Module (DLKM) to install their drivers without requiring a server reboot.

This was first published in August 2006

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