In last week's entry I discussed Global Name Spaces as a data mover for moving data to and from a disk-based archive. In addition to a Global Name Space there are other tools to move data to and archive. I find that the other solutions typically fall into one of two camps; Agent-based data movers or crawl-based data movers. There's also another category of monitoring tools that don't actually move the data but tell you what should be moved, leaving the rest up to you. In this entry we'll focus on agent-based data movers.A data-based agent mover, as you might have guessed, installs an agent on each server that is going to be monitored for files that are eligible for movement. This agent then periodically scans the server that it resides on to identify files that need to be moved to the archive. Once archived, they generally leave a stub file behind to provide a road map back to where the original file had been archived. Products in this space include EMC's DiskXtender and CaminoSoft's Managed Server HSM. Others leverage a common metadata database to optimize storage efficiencies like CommVault or Mimosa.

The downside to an agent-based system is that there is a piece of software that needs to be installed on each server that is going to participate in the archive process. While most operating systems now allow hooks that give the applications a safer mode in which to run, there is always concern about agent stability. Typically, there's also a limited amount of OS support from each supplier of these solutions. Agents that work on NAS solutions like NetApp are almost nonexistent.

The big issue with agent-based data movers is their use of stub files. For some reason, these often prove problematic during operation. Users delete them, they still need to be accounted for (or ignored) by the backup application, and they need to be updated if the archived file is moved a second time.

The positive side to agents is that they are instant. The moment a file reaches a certain age or classification, (an MP3 file, for example), action can happen and data can be moved. They often many times do more than just provide data movement capabilities. Some companies can do indexing of the files as they are archived, others can integrate the archive agent with their backup agent. The other unique component of these solutions tends to be support for tape and optical as an archive target. While disk archiving has its advantages, some customers might want to leverage their existing investment in tape or optical media.

George Crump is founder of Storage Switzerland, an analyst firm focused on the virtualization and storage marketplaces. It provides strategic consulting and analysis to storage users, suppliers, and integrators. An industry veteran of more than 25 years, Crump has held engineering and sales positions at various IT industry manufacturers and integrators. Prior to Storage Switzerland, he was CTO at one of the nation's largest integrators.