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Standalone. This method relies on the backup media server to move data from virtual to physical tapes--the virtual tape and physical tape are "side by side" and both are addressed by the backup application. The media server mounts the virtual tapes, and then moves the backup data over the network and onto the physical tape drives. This preserves the backup application's control and knowledge of the physical media, eliminating issues that may arise with the export-and-copy approach.

There are a few subtle variations in product capabilities. In some cases, the backup media server will write the backup data set directly to the physical tape library. This is the case for ADIC, Alacritus, Copan, Diligent, EMC, FalconStor, Quantum, Sepaton and Spectra Logic.

In this scenario, the user incurs licensing fees for both sets of virtual and physical tape libraries (as both are written to by the backup application). Some virtual tape architectures, like Neartek's VSE2, allow the backup stream to be directed to the physical tape media via the virtual tape devices. Thus, the backup application sees only the VTLs as its targets, and users avoid the incremental and often substantial licensing expenses of their physical tape devices.

Tape as tape. This is the ability to emulate one tape format with a different underlying physical

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tape infrastructure, an important capability that enables seamless interoperability in tape formats between the backup application and the physical tape.

For example, with tape-as-tape emulation, a backup application that couldn't natively support a particular tape format could be "fooled" into writing to the previously unsupported format. Vendors supporting tape as tape are Alacritus, Copan, EMC, FalconStor, Neartek and Spectra Logic.

A careful assessment of your offsite media management requirements must be conducted when evaluating any virtual tape solution. It's important to look for products that preserve media catalog integrity and data recoverability from offsite media while also providing operational flexibility.

Policy framework
Introducing a new tier of storage into the data protection infrastructure will create some operational challenges for an IT department. Administrators will have to determine how many days of data should be retained on disk, how frequently data should be copied to tape from disk, and the number of data copies that should be retained on disk and tape. Once acceptable standards are established, the execution of these plans can be very labor-intensive.

For this reason, any virtual tape offering must provide a comprehensive and flexible policy framework that allows administrators to specify parameters (such as those above) on a volume-specific basis. A policy for vol1, for example, may stipulate that all virtual media on which vol1 is stored should be mirrored and copied to physical tape on a daily basis starting at 2 a.m. Such policies ensure that backup operations are carried out in a reliable and repeatable manner.

In addition, the ability to define dynamic storage or media pools enables the use of automated migration policies for data movement between tape and disk tiers. Other important aspects of a policy framework include the ability to prioritize tasks or backup jobs. This gives backup jobs some level of quality of service and ensures that they can be completed within a specified backup window. Finally, distributed enterprise environments require effective security measures. The capability to create multiple volume groups or domains, each with its own authorization parameters, must be included in virtual tape policies.

This was first published in November 2005

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