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.
