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Integrated VTL. In this approach, the VTL sits in front of the physical tape library. This is a completely back-end-driven method, with the virtual tape solution invoking a copy-and-export function that makes a replica of the virtual tape on the physical tape media. This is the simplest approach to tape management, but it can also be the most problematic. The advantage of this approach is that it keeps backup data from traversing the primary network. But because it's a completely back-end-driven process, and bar-code consistency between the physical and virtual tape media isn't always enforced (different bar codes may exist so the backup application catalog isn't confused by two cartridges--one physical and one virtual--with the same bar code), the physical tape copy and offsite vaulting aren't reflected in the backup application's media server catalog.
As a result, if a user needs to recover data from a particular volume on the physical tape, the backup app will be completely unaware of the tape's bar code or offsite location. The data would be nearly unrecoverable. Alacritus and FalconStor are two vendors that don't always enforce bar-code consistency.
Copy/export with bar-code consistency. Some virtual tape products preserve the integrity of the media catalog and the ability to recover data from offsite media. The virtual tape system creates
a copy, or export, of the virtual tape cartridge analogous to the approach described earlier, but with one major distinction: The virtual tape control software maintains a unique 1:1 relationship between the bar codes of the virtual and physical tape cartridges. The virtual tape cartridge will always have the same bar code as the physical tape cartridge, and if the physical media is taken offsite, the user can specify the offsite location.
However, a potential problem to this approach is that it puts the "same" cartridge in two different physical locations, which confuses the backup application. The virtual tape solution must therefore keep track of the tape and a disk version of it (if it exists), and preserve consistency between the two, thereby ensuring that the backup application isn't confused.
There are two variations to this approach. In the first, data from the virtual tape cartridge is moved to the physical tape cartridge using the tape export function. Although bar-code consistency is maintained, the backup media server may now incorrectly believe that the tape is no longer in the physical library. As a result, the recovery process for data not on disk involves manual tape reloads. Vendors supporting this type of approach include ADIC, Alacritus, Copan, EMC and FalconStor.
In the second variation, data from the virtual tape cartridge is automatically copied to the physical tape. The virtual tape solution tracks the media bar codes that have been copied and remain in the physical library (whereas in the first variation this media would have been believed to be exported). Because these products view the disk-based "virtual" media and the tape-based "physical" media as one media pool, users can recover data from local tape in an automated manner using a combination of the virtual tape and backup apps. The backup application alone may not be able to recover the data because it may not be aware of the media location. Neartek's VSE2 offers this type of implementation. Users looking to keep backup data off the primary network will find merit in both variations.
This was first published in November 2005