Sooner or later you're almost certainly going to need tape. For archival storage, tape's combination of capacity, cost and storage life is hard to beat. That doesn't mean, however, that you're going to be backing up directly to tape. Increasingly, backups are going to disks, either in a disk array in a D2D configuration or to an array organized as a virtual tape library (VTL).
The good news in all of this is that while you may still need tape, you don't need nearly as much of it. Not only is the volume of data written to tape much smaller with a VTL in the loop, but the architecture is much less complex (and expensive) and the time requirements are much more flexible. When writing from a VTL to archival tape, you are writing from one data source (the VTL) instead of multiple servers and with an extremely relaxed time frame. This means you also avoid the problems associated with multiplexing data streams from multiple servers into a single tape library.
One popular way to connect a VTL and tape is to hang the tape drive or library off of the VTL as direct-attached storage (DAS). This is cheap, simple and minimizes the load on the network. However, like all DAS, it is inflexible. If you think you may have to support additional VTLs you may want to use an alternate method, such as communicating with the tape unit over your SAN.
Data pruning is a consideration because you seldom need to archive everything you back up. Backups are designed to let you restore your files or system to a known state, almost always less than three months old and usually 30 days old or less. Archival data is data you need to store for the long term -- usually for years at a time and sometimes permanently. Typically, archival data will amount to much less than half of the data you back up.
So, how do you prune the data going from VTL to archival tape? The easiest way is simply not to save certain file extensions to tape. Unfortunately, this is seldom satisfactory, especially in the world of SOX and HIPAA requirements. That means you're going to need some 'storage smarts' in order to enforce business rules on what goes to tape, if you want to prune effectively. Some VTL products, such as those from FalconStor Software and Adaptec Inc., can handle policy-based replication of selected data to tape. Other approaches include having a separate server to handle your archival tape library or using the features of your backup software to monitor and prune the data feeding to tape.
Finally, keep in mind that although VTLs have been around for a while, it is still not a highly standardized technology. Consider VTL integration with tape carefully when choosing a VTL product and understand that not every vendor will offer all the features you want. Sometimes you can't get everything you want in one package.
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About the author: Rick Cook has been writing about mass storage since the days when the term meant an 80 K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last 20 years, he has been a freelance writer specializing in storage and other computer issues.